HOME

TheInfoList



OR:

A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
which rectifies the
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
radio signal. It was employed as a
detector A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
( demodulator) to extract the audio modulation signal from the modulated carrier, to produce the sound in the earphones. It was the first type of
semiconductor diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...
, and one of the first semiconductor electronic devices. The most common type was the so-called cat's whisker detector, which consisted of a piece of crystalline mineral, usually galena (
lead sulfide Lead sulfide refers to two compounds containing lead and sulfur: * Lead(II) sulfide, PbS, containing lead in the +2 oxidation state, naturally occurring as the mineral galena *Lead(IV) sulfide Lead(IV) sulfide is a chemical compound with the formu ...
), with a fine wire touching its surface. Greenleaf Whittier Pickard, ''Detector for Wireless Telegraphy and Telephony'', filed: 21 June 1911, granted: 21 July 1914 The "asymmetric conduction" of electric current across electrical contacts between a crystal and a metal was discovered in 1874 by Karl Ferdinand Braun. Crystals were first used as radio wave detectors in 1894 by Jagadish Chandra Bose in his
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
experiments.although at the microwave frequencies he used these detectors did not function as rectifying semiconductor diodes like later crystal detectors, but as a thermal detector called a bolometer. Bose first patented a crystal detector in 1901. Jagadis Chunder Bose, ''Detector for Electrical Disturbances'', filed: 30 September 1901, granted 29 March 1904 The crystal detector was developed into a practical radio component mainly by G. W. Pickard, Greenleaf Whittier Pickard, ''Means for Receiving Intelligence Communicated by Electric Waves'', filed: 30 August 1906, granted: 20 November 1906 archived
part1part2part3part4
/ref> who began research on detector materials in 1902 and found hundreds of substances that could be used in forming rectifying junctions. The physical principles by which they worked were not understood at the time they were used, but subsequent research into these primitive point contact semiconductor junctions in the 1930s and 1940s led to the development of modern
semiconductor electronics A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivity li ...
. The unamplified radio receivers that used crystal detectors are called crystal radios. The crystal radio was the first type of radio receiver that was used by the general public, and became the most widely used type of radio until the 1920s."...crystal detectors have been used n receiversin greater numbers than any other ype of detectorsince about 1907." It became obsolete with the development of
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as ...
receivers around 1920, but continued to be used until World War II and remains a common educational project today thanks to its simple design.


Operation

The contact between two dissimilar materials at the surface of the detector's semiconducting crystal forms a crude
semiconductor diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...
, which acts as a rectifier, conducting electric current well in only one direction and resisting current flowing in the other direction. In a crystal radio, it was connected between the
tuned circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can a ...
, which passed on the oscillating current induced in the antenna from the desired radio station, and the earphone. Its function was to act as a demodulator, rectifying the radio signal, converting it from
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
to a pulsing
direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...
, to extract the audio signal ( modulation) from the
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the ...
carrier wave In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...
. An AM demodulator which works in this way, by rectifying the modulated carrier, is called an envelope detector. The
audio frequency An audio frequency or audible frequency (AF) is a periodic vibration whose frequency is audible to the average human. The SI unit of frequency is the hertz (Hz). It is the property of sound that most determines pitch. The generally accepted ...
current produced by the detector passed through the earphone causing the earphone's diaphragm to vibrate, pushing on the air to create sound waves. The earphone was typically a piezoelectric crystal type, so sensitive that the radio receiver could operate without an electrical power supply, using only energy from the incident radio wave to drive the earphone directly, with no electronic amplification. This diagram shows a simplified explanation of how it works: :''(A)'' This graph shows the amplitude modulated radio signal from the receiver's tuned circuit, which is applied as a voltage across the detector's contacts. The rapid oscillations are the
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the ...
carrier wave In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...
. The audio signal (the sound) is contained in the slow variations ( modulation) of the size of the waves. If this signal were applied directly to the earphone, it could not be converted to sound, because the audio excursions are the same on both sides of the axis, averaging out to zero, which would result in no net motion of the earphone's diaphragm. :''(B)'' This graph shows the current through the crystal detector which is applied to the earphone and bypass capacitor. The crystal conducts current in only one direction, stripping off the oscillations on one side of the signal, leaving a pulsing direct current whose amplitude does not average zero but varies with the audio signal. :''(C)'' This graph shows the current which passes through the earphone. A bypass
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
across the earphone terminals, in combination with the intrinsic forward resistance of the diode, creates a
low-pass filter A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
that smooths the waveform by removing the radio frequency carrier pulses and leaving the audio signal. When this varying current passes through the earphone piezoelectric crystal, it causes the crystal to deform (flex), deflecting the earphone diaphragm; the varying deflections of the diaphragm cause it to vibrate and produce sound waves ( acoustic waves). If instead a voice-coil type headphone is used, the varying current from the low-pass filter flows through the voice coil, generating a varying magnetic field which pulls and pushes the earphone diaphragm, again causing it to vibrate and produce sound. Crystal radios had no amplifying components to increase the loudness of the radio signal; the sound power produced by the earphone came solely from the radio waves of the radio station being received, intercepted by the antenna. Therefore, the sensitivity of the detector was a major factor determining the sensitivity and reception range of the receiver, motivating much research into finding sensitive detectors. In addition to its main use in crystal radios, crystal detectors were also used as radio wave detectors in scientific experiments, in which the DC output current of the detector was registered by a sensitive
galvanometer A galvanometer is an electromechanical measuring instrument for electric current. Early galvanometers were uncalibrated, but improved versions, called ammeters, were calibrated and could measure the flow of current more precisely. A galvan ...
, and in test instruments such as wavemeters used to calibrate the frequency of
radio transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
s.


Types

The crystal detector consisted of an electrical contact between the surface of a
semiconducting A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
and either a metal or another crystal. Since at the time they were developed no one knew how they worked, crystal detectors evolved by trial and error. The construction of the detector depended on the type of crystal used, as it was found different minerals varied in how much contact area and pressure on the crystal surface was needed to make a sensitive rectifying contact. Crystals that required a light pressure like galena were used with the wire cat whisker contact;
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
was used with a heavier point contact, while
silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal s ...
( carborundum) could tolerate the heaviest pressure. Another type used two crystals of different minerals with their surfaces touching, the most common being the "Perikon" detector. Since the detector would only function when the contact was made at certain spots on the crystal surface, the contact point was almost always made adjustable. Below are the major categories of crystal detectors used during the early 20th century:


Cat whisker detector

Patented by Karl Ferdinand Braun and
Greenleaf Whittier Pickard Greenleaf Whittier Pickard (February 14, 1877, Portland, Maine – January 8, 1956, Newton, Massachusetts) was a United States radio pioneer. Pickard was a researcher in the early days of wireless. While not the earliest discoverer of the rectifyi ...
in 1906, this was the most common type of crystal detector, mainly used with galena but also other crystals. It consisted of a pea-size piece of crystalline mineral in a metal holder, with its surface touched by a fine metal wire or needle (the "cat whisker"). The contact between the tip of the wire and the surface of the crystal formed a crude unstable point-contact
metal–semiconductor junction In solid-state physics, a metal–semiconductor (M–S) junction is a type of electrical junction in which a metal comes in close contact with a semiconductor material. It is the oldest practical semiconductor device. M–S junctions can either ...
, forming a Schottky barrier diode."''The cat’s-whisker detector is a primitive point-contact diode. A point-contact junction is the simplest implementation of a Schottky diode, which is a majority-carrier device formed by a metal-semiconductor junction.''" The wire whisker is the
anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic ...
, and the crystal is the
cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...
; current can flow from the wire into the crystal but not in the other direction. Only certain sites on the crystal surface functioned as rectifying junctions. The device was very sensitive to the exact geometry and pressure of contact between wire and crystal, and the contact could be disrupted by the slightest vibration. Therefore, a usable point of contact had to be found by trial and error before each use. The wire was suspended from a moveable arm and was dragged across the crystal face by the user until the device began functioning. In a crystal radio, the user would tune the radio to a strong local station if possible and then adjust the cat whisker until the station or
radio noise In radio reception, radio noise is unwanted random radio frequency electrical signals, fluctuating voltages, always present in a radio receiver in addition to the desired radio signal. Radio noise near in frequency to the radio signal being receive ...
(a static hissing noise) was heard in the radio's earphones. This required some skill and a lot of patience. An alternative method of adjustment was to use a battery-operated " buzzer" connected to the radio's ground wire or inductively coupled to the tuning coil, to generate a test signal. The spark produced by the buzzer's contacts functioned as a weak
radio transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
whose radio waves could be received by the detector, so when a rectifying spot had been found on the crystal the buzz could be heard in the earphones, at which time the buzzer was turned off. The detector consisted of two parts mounted next to each other on a flat nonconductive base: a
crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
forming the semiconductor side of the junction, and a "cat whisker", a springy piece of thin metal wire, forming the metal side of the junction The most common crystal used was galena (
lead sulfide Lead sulfide refers to two compounds containing lead and sulfur: * Lead(II) sulfide, PbS, containing lead in the +2 oxidation state, naturally occurring as the mineral galena *Lead(IV) sulfide Lead(IV) sulfide is a chemical compound with the formu ...
, PbS, varieties were sold under the names "Lenzite" and "Hertzite"), a widely occurring ore of
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
. Other crystalline minerals were also used, the more common ones were iron pyrite (iron sulfide, FeS2, "fool's gold", also sold under the trade names "Pyron" and "Ferron"),
molybdenite Molybdenite is a mineral of molybdenum disulfide, Mo S2. Similar in appearance and feel to graphite, molybdenite has a lubricating effect that is a consequence of its layered structure. The atomic structure consists of a sheet of molybdenum ato ...
( molybdenum disulfide, MoS2), and cerussite (
lead carbonate Lead(II) carbonate is the chemical compound with the chemical formula . It is a white solid with several practical uses, despite its toxicity. It occurs naturally as the mineral cerussite. Structure Like all metal carbonates, lead(II) carbonate a ...
, PbCO3) Not all specimens of a crystal would function in a detector, often several crystal pieces had to be tried to find an active one. Galena with good detecting properties was rare and had no reliable visual characteristics distinguishing it from galena samples with poor detecting properties. A rough pebble of detecting mineral about the size of a pea was mounted in a metal cup, which formed one side of the circuit. The electrical contact between the cup and the crystal had to be good, because this contact must ''not'' act as a second rectifying junction, creating two back-to-back diodes which would prevent the device from conducting at all. To make good contact with the crystal, it was either clamped with setscrews or embedded in
solder Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...
. Because the relatively high melting temperature of tin-lead
solder Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...
can damage many crystals, a fusible alloy with a low melting point, well under , such as Wood's metal was used. One surface was left exposed to allow contact with the cat-whisker wire.
Phosphor bronze Phosphor bronze is a member of the family of copper alloys. It is composed of copper that is alloyed with 0.5–11% of tin and 0.01–0.35% phosphorus, and may contain other elements to confer specific properties (e.g. lead at 0.5–3.0% to form ...
wire of about 30 AWG / 0.25 mm diameter was commonly used as a cat whisker because it had the right amount of springiness. It was mounted on an adjustable arm with an insulated handle so that the entire exposed surface of the crystal could be probed from many directions to find the most sensitive spot. Cat whiskers in homemade detectors usually had a simple curved shape, but most professional cat whiskers had a coiled section in the middle that served as a spring. The crystal required just the right gentle pressure by the wire; too much pressure caused the device to conduct in both directions. Precision detectors made for radiotelegraphy stations often used a metal needle instead of a "cat's whisker", mounted on a thumbscrew-operated leaf spring to adjust the pressure applied. Gold or silver needles were used with some crystals.


Carborundum detector

Invented in 1906 by Henry H. C. Dunwoody, Henry H. C. Dunwoody, ''Wireless Telegraph System'', filed: 23 March 1906, granted: 4 December 1906 this consisted of a piece of
silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal s ...
(SiC, then known by the trade name ''carborundum''), either clamped between two flat metal contacts, or mounted in fusible alloy in a metal cup with a contact consisting of a hardened steel point pressed firmly against it with a spring. Carborundum, an artificial product of electric furnaces produced in 1893, required a heavier pressure than the cat whisker contact. The carborundum detector was popular because its sturdy contact did not require readjustment each time it was used, like the delicate cat whisker devices. Some carborundum detectors were adjusted at the factory and then sealed and did not require adjustment by the user. It was not sensitive to vibration and so was used in shipboard wireless stations where the ship was rocked by waves, and military stations where vibration from gunfire could be expected. Another advantage was that it was tolerant of high currents, and could not be "burned out" by atmospheric electricity from the antenna. Therefore, it was the most common type used in commercial radiotelegraphy stations. Silicon carbide is a semiconductor with a wide
band gap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference ( ...
of 3 eV, so to make the detector more sensitive a forward
bias Bias is a disproportionate weight ''in favor of'' or ''against'' an idea or thing, usually in a way that is closed-minded, prejudicial, or unfair. Biases can be innate or learned. People may develop biases for or against an individual, a group ...
voltage of several volts was usually applied across the junction by a battery and
potentiometer A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. The measuring instrum ...
. The voltage was adjusted with the potentiometer until the sound was loudest in the earphone. The bias moved the operating point to the curved "knee" of the device's current–voltage curve, which produced the largest rectified current.


Silicon detector

Patented and first manufactured in 1906 by Pickard, this was the first type of crystal detector to be commercially produced. Silicon required more pressure than the cat whisker contact, although not as much as carborundum. A flat piece of
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
was embedded in fusible alloy in a metal cup, and a metal point, usually
brass Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other wit ...
or
gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...
, was pressed against it with a spring. The surface of the silicon was usually ground flat and polished. Silicon was also used with
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient t ...
and
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...
contacts. The silicon detector was popular because it had much the same advantages as carborundum; its firm contact could not be jarred loose by vibration and it did not require a bias battery, so it saw wide use in commercial and military radiotelegraphy stations.


Crystal-to-crystal detectors

Another category was detectors which used two different crystals with their surfaces touching, forming a crystal-to-crystal contact. The "Perikon" detector, invented 1908 by Pickard Greenleaf Whittier Pickard, ''Oscillation receiver'', filed: 15 September 1908, granted: 16 February 1909 was the most common. ''Perikon'' stood for "PERfect pIcKard cONtact". It consisted of two crystals in metal holders, mounted face to face. One crystal was
zincite Zincite is the mineral form of zinc oxide ( Zn O). Its crystal form is rare in nature; a notable exception to this is at the Franklin and Sterling Hill Mines in New Jersey, an area also famed for its many fluorescent minerals. It has a hexagon ...
(
zinc oxide Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement ...
, ZnO), the other was a copper iron sulfide, either bornite (Cu5FeS4) or
chalcopyrite Chalcopyrite ( ) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mo ...
(CuFeS2). In Pickard's commercial detector ''(see picture)'', multiple zincite crystals were mounted in a fusible alloy in a round cup ''(on right)'', while the chalcopyrite crystal was mounted in a cup on an adjustable arm facing it ''(on left)''. The chalcopyrite crystal was moved forward until it touched the surface of one of the zincite crystals. When a sensitive spot was located, the arm was locked in place with the setscrew. Multiple zincite pieces were provided because the fragile zincite crystal could be damaged by excessive currents and tended to "burn out" due to atmospheric electricity from the wire antenna or currents leaking into the receiver from the powerful spark transmitters used at the time. This detector was also sometimes used with a small forward bias voltage of around 0.2V from a battery to make it more sensitive. Although the zincite-chalcopyrite "Perikon" was the most widely used crystal-to-crystal detector, other crystal pairs were also used. Zincite was used with carbon, galena, and
tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionall ...
. Silicon was used with
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...
,
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient t ...
and
tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionall ...
crystals.


History

During the first three decades of radio, from 1888 to 1918, called the ''
wireless telegraphy Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
'' or "spark" era, primitive
radio transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
s called
spark gap transmitter A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the main type use ...
s were used, which generated radio waves by an
electric spark An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an ionized, electrically conductive channel through a normally-insulating medium, often air or other gases or gas mixtures. Michael F ...
. These transmitters were unable to produce the continuous sinusoidal waves which are used to transmit audio (sound) in modern AM or FM radio transmission. Instead spark gap transmitters transmitted information by
wireless telegraphy Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
; the user turned the transmitter on and off rapidly by tapping on a telegraph key, producing pulses of radio waves which spelled out text messages in Morse code. Therefore, the radio receivers of this era did not have to
demodulate Demodulation is extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated c ...
the radio wave, extract an audio signal from it as modern receivers do, they merely had to detect the presence or absence of the radio waves, to make a sound in the earphone when the radio wave was present to represent the "dots" and "dashes" of Morse code. The device which did this was called a ''
detector A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
''. The crystal detector was the most successful of many detector devices invented during this era. The crystal detector evolved from an earlier device, the first primitive radio wave detector, called a '' coherer'', developed in 1890 by
Édouard Branly Édouard Eugène Désiré Branly (23 October 1844 – 24 March 1940) was a French inventor, physicist and professor at the Institut Catholique de Paris. He is primarily known for his early involvement in wireless telegraphy and his invention of the ...
and used in the first radio receivers in 1894–96 by Marconi and
Oliver Lodge Sir Oliver Joseph Lodge, (12 June 1851 – 22 August 1940) was a British physicist and writer involved in the development of, and holder of key patents for, radio. He identified electromagnetic radiation independent of Hertz's proof and at his ...
. Made in many forms, the coherer consisted of a high resistance electrical contact, composed of conductors touching with a thin resistive surface film, usually oxidation, between them. Radio waves changed the resistance of the contact, causing it to conduct a DC current. The most common form consisted of a glass tube with electrodes at each end, containing loose metal filings in contact with the electrodes. Before a radio wave was applied, this device had a high
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paralle ...
, in the megohm range. When a radio wave from the antenna was applied across the electrodes it caused the filings to "cohere" or clump together and the coherer's resistance fell, causing a DC current from a battery to pass through it, which rang a bell or produced a mark on a paper tape representing the "dots" and "dashes" of Morse code. Most coherers had to be tapped mechanically between each pulse of radio waves to return them to a nonconductive state. The coherer was a very poor detector, motivating much research to find better detectors. It worked by complicated thin film surface effects, so scientists of the time didn't understand how it worked, except for a vague idea that radio wave detection depended on some mysterious property of "imperfect" electrical contacts. Researchers investigating the effect of radio waves on various types of "imperfect" contacts to develop better coherers, invented crystal detectors.


Braun's experiments

The "unilateral conduction" of crystals was discovered by Karl Ferdinand Braun, a German physicist, in 1874 at the
University of Würzburg The Julius Maximilian University of Würzburg (also referred to as the University of Würzburg, in German ''Julius-Maximilians-Universität Würzburg'') is a public research university in Würzburg, Germany. The University of Würzburg is one of ...
. He studied copper pyrite (Cu5FeS4), iron pyrite (iron sulfide, FeS2), galena (PbS) and copper antimony sulfide (Cu3SbS4). This was before radio waves had been discovered, and Braun did not apply these devices practically but was interested in the nonlinear
current–voltage characteristic A current–voltage characteristic or I–V curve (current–voltage curve) is a relationship, typically represented as a chart or graph, between the electric current through a circuit, device, or material, and the corresponding voltage, or ...
that these sulfides exhibited. Graphing the current as a function of voltage across a contact made by a piece of mineral touched by a wire cat whisker, he found the result was a line that was flat for current in one direction but curved upward for current in the other direction, instead of a straight line, showing that these substances did not obey Ohm's law. Due to this characteristic, some crystals had up to twice as much resistance to current in one direction as they did to current in the other. In 1877 and 1878 he reported further experiments with psilomelane, . Braun did investigations which ruled out several possible causes of asymmetric conduction, such as electrolytic action and some types of thermoelectric effects. Thirty years after these discoveries, after Bose's experiments, Braun began experimenting with his crystalline contacts as radio wave detectors. In 1906 he obtained a German patent on a galena cat whisker detector, but was too late to obtain patents in other countries.


Bose's experiments

The first person to use crystals for radio wave detection was Indian physicist Jagadish Chandra Bose of the
University of Calcutta The University of Calcutta (informally known as Calcutta University; CU) is a public collegiate state university in India, located in Kolkata, West Bengal, India. Considered one of best state research university all over India every year, ...
in his landmark 60 GHz
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
optics experiments from 1894 to 1900. also reprinted o
IndianDefense
/ref>Sarkar, Tapan K.; Sengupta, Dipak L. "An appreciation of J. C. Bose's pioneering work in millimeter and microwaves" in Like other scientists since Hertz, Bose was investigating the similarity between radio waves and light by duplicating classic
optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...
experiments with radio waves. Sarkar, et al (2006) ''History of Wireless'', pp. 477–483
/ref> For a receiver he first used a coherer consisting of a steel spring pressing against a metal surface with a current passing through it. Dissatisfied with this detector, around 1897 Bose measured the change in resistivity of dozens of metals and metal compounds exposed to microwaves. He experimented with many substances as contact detectors, focusing on galena. His detectors consisted of a small galena crystal with a metal point contact pressed against it with a thumbscrew, mounted inside a closed
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
ending in a horn antenna to collect the microwaves. Bose passed a current from a battery through the crystal, and used a
galvanometer A galvanometer is an electromechanical measuring instrument for electric current. Early galvanometers were uncalibrated, but improved versions, called ammeters, were calibrated and could measure the flow of current more precisely. A galvan ...
to measure it. When microwaves struck the crystal the galvanometer registered a drop in resistance of the detector. At the time scientists thought that radio wave detectors functioned by some mechanism analogous to the way the eye detected light, and Bose found his detector was also sensitive to visible light and ultraviolet, leading him to call it an ''artificial retina''. He patented the detector 30 September 1901. This is often considered the first patent on a semiconductor device.


Pickard: first commercial detectors

Greenleaf Whittier Pickard Greenleaf Whittier Pickard (February 14, 1877, Portland, Maine – January 8, 1956, Newton, Massachusetts) was a United States radio pioneer. Pickard was a researcher in the early days of wireless. While not the earliest discoverer of the rectifyi ...
may be the person most responsible for making the crystal detector a practical device. Pickard, an engineer with the American Wireless Telephone and Telegraph Co. invented the rectifying contact detector, discovering rectification of radio waves in 1902 while experimenting with a coherer detector consisting of a steel needle resting across two carbon blocks. On 29 May 1902 he was operating this device, listening to a radiotelegraphy station. Coherers required an external current source to operate, so he had the coherer and telephone earphone connected in series with a 3 cell battery to provide power to operate the earphone. Annoyed by background "frying" noise caused by the current through the carbon, he reached over to cut two of the battery cells out of the circuit to reduce the current The generation of an audio signal without a DC bias battery made Pickard realize the device was acting as a rectifier. During the next four years, Pickard conducted an exhaustive search to find which substances formed the most sensitive detecting contacts, eventually testing thousands of minerals, and discovered about 250 rectifying crystals. In 1906 he obtained a sample of fused
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
, an artificial product recently synthesized in electric furnaces, and it outperformed all other substances. He patented the silicon detector 30 August 1906. In 1907 he formed a company to manufacture his detectors, Wireless Specialty Products Co., and the silicon detector was the first crystal detector to be sold commercially. Pickard went on to produce other detectors using the crystals he had discovered; the more popular being the iron pyrite "Pyron" detector and the
zincite Zincite is the mineral form of zinc oxide ( Zn O). Its crystal form is rare in nature; a notable exception to this is at the Franklin and Sterling Hill Mines in New Jersey, an area also famed for its many fluorescent minerals. It has a hexagon ...
chalcopyrite Chalcopyrite ( ) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mo ...
crystal-to-crystal "Perikon" detector in 1908, which stood for "PERfect pIcKard cONtact".


Use during the wireless telegraphy era

Guglielmo Marconi developed the first practical wireless telegraphy transmitters and receivers in 1896, and radio began to be used for communication around 1899. The coherer was used as detector for the first 10 years, until around 1906. During the
wireless telegraphy Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
era prior to 1920, there was virtually no
broadcasting Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum ( radio waves), in a one-to-many model. Broadcasting beg ...
; radio served as a point-to-point text messaging service. Until the
triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's ...
vacuum tube began to be used around
World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
, radio receivers had no amplification and were powered only by the radio waves picked up by their antennae. Long distance radio communication depended on high power transmitters (up to 1 MW), huge wire antennas, and a receiver with a sensitive detector. Crystal detectors were invented by several researchers at about the same time. Braun began to experiment with crystal detectors around 1899, around when Bose patented his galena detector. Pickard invented his silicon detector in 1906. Also in 1906 Henry Harrison Chase Dunwoody, Some biographical information on General Henry H.C. Dunwoody is available a
Arlington National Cemetery
a retired general in the U.S. Army Signal Corps, patented the
silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal s ...
( carborundum) detector, Braun patented a galena cat whisker detector in Germany, German patent 178871 Karl Ferdinand Braun,
Wellenempfindliche Kontaktstel
', filed: 18 February 1906, granted: 22 October 1906
and L. W. Austin invented a silicon–tellurium detector. Around 1907 crystal detectors replaced the coherer and
electrolytic detector The electrolytic detector, or liquid barretter, was a type of detector (demodulator) used in early radio receivers. First used by Canadian radio researcher Reginald Fessenden in 1903, it was used until about 1913, after which it was superseded ...
to become the most widely used form of radio detector.The 1911 edition of the US Navy's manual of radio stated: "''There are but two types of detectors now in use: crystal or rectifying detectors and the electrolytic. Coherers and microphones '' nother type of coherer detector' are practically obsolete, and comparatively few of the magnetic and Audion or valve '' riode' detectors have been installed.''" Until the triode vacuum tube began to be used during World War I, crystals were the best radio reception technology, used in sophisticated receivers in wireless telegraphy stations, as well as in homemade crystal radios.The 1913 edition of the US Navy's manual of radio stated: "''Only one type of detector is now in use: the crystal. Coherers and microphones are practically obsolete, and comparatively few magnetic and Audion or valve '' riode' detectors have been installed.''" In transoceanic radiotelegraphy stations elaborate inductively coupled crystal receivers fed by mile long wire antennas were used to receive transatlantic telegram traffic.Marconi used carborundum detectors beginning around 1907 in his first commercial transatlantic wireless link between Newfoundland, Canada and Clifton, Ireland. Much research went into finding better detectors and many types of crystals were tried. The goal of researchers was to find rectifying crystals that were less fragile and sensitive to vibration than galena and pyrite. Another desired property was tolerance of high currents; many crystals would become insensitive when subjected to discharges of atmospheric electricity from the outdoor wire antenna, or current from the powerful spark transmitter leaking into the receiver. Carborundum proved to be the best of these; it could rectify when clamped firmly between flat contacts. Therefore, carborundum detectors were used in shipboard wireless stations where waves caused the floor to rock, and military stations where gunfire was expected. In 1907–1909, George Washington Pierce at Harvard conducted research into how crystal detectors worked. Using an oscilloscope made with Braun's new cathode ray tube, he produced the first pictures of the waveforms in a working detector, proving that it did rectify the radio wave. During this era, before modern solid-state physics, most scientists believed that crystal detectors operated by some thermoelectric effect. Although Pierce didn't discover the mechanism by which it worked, he did prove that the existing theories were wrong; his oscilloscope waveforms showed there was no phase delay between the voltage and current in the detector, ruling out thermal mechanisms. Pierce originated the name ''crystal rectifier''. Between about 1905 and 1915 new types of radio transmitters were developed which produced continuous sinusoidal waves: the
arc converter The arc converter, sometimes called the arc transmitter, or Poulsen arc after Danish engineer Valdemar Poulsen who invented it in 1903, was a variety of spark transmitter used in early wireless telegraphy. The arc converter used an electric arc ...
(Poulsen arc) and the Alexanderson alternator. These slowly replaced the old
damped wave Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples i ...
spark transmitters. Besides having a longer transmission range, these transmitters could be
modulated In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
with an audio signal to transmit sound by amplitude modulation (AM). It was found that, unlike the coherer, the rectifying action of the crystal detector allowed it to
demodulate Demodulation is extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated c ...
an AM radio signal, producing audio (sound). Although other detectors used at the time, the
electrolytic detector The electrolytic detector, or liquid barretter, was a type of detector (demodulator) used in early radio receivers. First used by Canadian radio researcher Reginald Fessenden in 1903, it was used until about 1913, after which it was superseded ...
, Fleming valve and the triode could also rectify AM signals, crystals were the simplest, cheapest AM detector. As more and more radio stations began experimenting with transmitting sound after World War I, a growing community of radio listeners built or bought crystal radios to listen to them. Use continued to grow until the 1920s when vacuum tube radios replaced them.


Crystodyne: negative resistance diodes

Some semiconductor diodes have a property called ''
negative resistance In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it. This is in contrast to an ordina ...
'' which means the current through them decreases as the voltage increases over a part of their I–V curve. This allows a diode, normally a
passive Passive may refer to: * Passive voice, a grammatical voice common in many languages, see also Pseudopassive * Passive language, a language from which an interpreter works * Passivity (behavior), the condition of submitting to the influence of o ...
device, to function as an
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...
or oscillator. For example, when connected to a resonant circuit and biased with a DC voltage, the negative resistance of the diode can cancel the positive resistance of the circuit, creating a circuit with zero AC resistance, in which spontaneous oscillating currents arise. This property was first observed in crystal detectors around 1909 by William Henry Eccles and Pickard. They noticed that when their detectors were biased with a DC voltage to improve their sensitivity, they would sometimes break into spontaneous oscillations. However these researchers just published brief accounts and didn't pursue the effect. The first person to exploit negative resistance practically was self-taught Russian physicist Oleg Losev, who devoted his career to the study of crystal detectors. In 1922 working at the new
Nizhny Novgorod Radio Laboratory Nizhny Novgorod Radio Laboratory (russian: Нижегородская радиолаборатория, НРЛ) was the first Soviet science laboratory in the field of radio electronics. It was based in 1918 in Nizhny Novgorod. In 1928, the laborat ...
he discovered negative resistance in biased
zincite Zincite is the mineral form of zinc oxide ( Zn O). Its crystal form is rare in nature; a notable exception to this is at the Franklin and Sterling Hill Mines in New Jersey, an area also famed for its many fluorescent minerals. It has a hexagon ...
(
zinc oxide Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement ...
) point contact junctions. Lee, Thomas H. (2004) The Design of CMOS Radio-Frequency Integrated Circuits, 2nd Ed., p. 20
/ref> He realized that amplifying crystals could be an alternative to the fragile, expensive, energy-wasting vacuum tube. He used biased negative resistance crystal junctions to build solid-state
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...
s, oscillators, and amplifying and regenerative radio receivers, 25 years before the invention of the transistor. an
"The Crystodyne Principle"
''Radio News'', September 1924, pages 294-295, 431.
Later he even built a superheterodyne receiver. However his achievements were overlooked because of the success of vacuum tubes. His technology was dubbed "Crystodyne" by science publisher
Hugo Gernsback Hugo Gernsback (; born Hugo Gernsbacher, August 16, 1884 – August 19, 1967) was a Luxembourgish–American editor and magazine publisher, whose publications including the first science fiction magazine. His contributions to the genre as pub ...
one of the few people in the West who paid attention to it. After ten years he abandoned research into this technology and it was forgotten. The negative resistance diode was rediscovered with the invention of the tunnel diode in 1957, for which
Leo Esaki Reona Esaki (江崎 玲於奈 ''Esaki Reona'', born March 12, 1925), also known as Leo Esaki, is a Japanese physicist who shared the Nobel Prize in Physics in 1973 with Ivar Giaever and Brian David Josephson for his work in electron tunneling i ...
won the 1973
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
. Today, negative resistance diodes such as the
Gunn diode A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a two-terminal semiconductor electronic component, with negative resistance, used in high-frequency electronics. It is based on the "Gunn effect" discovered in 1 ...
and
IMPATT diode An IMPATT diode (impact ionization avalanche transit-time diode) is a form of high-power semiconductor diode used in high-frequency microwave electronics devices. They have negative resistance and are used as oscillators and amplifiers at microw ...
are widely used as
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
oscillators in such devices as radar speed guns and garage door openers.


Discovery of the light emitting diode (LED)

In 1907 British Marconi engineer Henry Joseph Round noticed that when direct current was passed through a
silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal s ...
(carborundum) point contact junction, a spot of greenish, bluish, or yellowish light was given off at the contact point. Round had constructed a light emitting diode (LED). However he just published a brief two paragraph note about it and did no further research. While investigating crystal detectors in the mid-1920s at Nizhny Novgorod, Oleg Losev independently discovered that biased carborundum and zincite junctions emitted light. Losev was the first to analyze this device, investigate the source of the light, propose a theory of how it worked, and envision practical applications. He published his experiments in 1927 in a Russian journal, English version published as and the 16 papers he published on LEDs between 1924 and 1930 constitute a comprehensive study of this device. Losev did extensive research into the mechanism of light emission. He measured rates of evaporation of benzine from the crystal surface and found it was not accelerated when light was emitted, concluding that the luminescence was a "cold" light not caused by thermal effects. He theorized correctly that the explanation of the light emission was in the new science of
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistr ...
, speculating that it was the inverse of the
photoelectric effect The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...
discovered by
Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
in 1905. He wrote to Einstein about it, but did not receive a reply. Losev designed practical carborundum electroluminescent lights, but found no one interested in commercially producing these weak light sources. Losev died in World War II. Due partly to the fact that his papers were published in Russian and German, and partly to his lack of reputation (his upper class birth barred him from a college education or career advancement in
Soviet The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nation ...
society, so he never held an official position higher than technician) his work is not well known in the West.


Use during the broadcast era

In the 1920s, the amplifying
triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's ...
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as ...
, invented in 1907 by Lee De Forest, replaced earlier technology in both radio transmitters and receivers.The 1918 edition of the US Navy's manual of radio stated: "''There are two types of detectors now in use: the Audion '' riode' and the crystal or rectifying detector. Coherers and microphones '' nother type of coherer detector' are practically obsolete... but the use of Audions...is increasing.''" AM
radio broadcasting Radio broadcasting is transmission of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based radio ...
spontaneously arose around 1920, and radio listening exploded to become a hugely popular pastime. The initial listening audience for the new broadcasting stations was probably largely owners of crystal radios. But lacking amplification, crystal radios had to be listened to with earphones, and could only receive nearby local stations. The amplifying vacuum tube radios which began to be mass-produced in 1921 had greater reception range, did not require the fussy adjustment of a cat whisker, and produced enough audio output power to drive
loudspeaker A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or ...
s, allowing the entire family to listen comfortably together, or dance to Jazz Age music. So during the 1920s vacuum tube receivers replaced crystal radios in all except poor households.The 1920 "British Admiralty Handbook of Wireless Telegraphy" stated that: "''Crystal detectors are being replaced by '' riode' valve detectors which are more stable, easier to adjust, and generally more satisfactory''". The 1925 edition said valves were "''replacing the crystal for all ordinary purposes''" Commercial and military wireless telegraphy stations had already switched to more sensitive vacuum tube receivers. Vacuum tubes temporarily put an end to crystal detector research. The temperamental, unreliable action of the crystal detector had always been a barrier to its acceptance as a standard component in commercial radio equipment and was one reason for its rapid replacement. Frederick Seitz, an early semiconductor researcher, wrote: The crystal radio became a cheap alternative receiver used in emergencies and by people who couldn't afford tube radios: teenagers, the poor, and those in developing countries. Building a crystal set remained a popular educational project to introduce people to radio, used by organizations like the Boy Scouts. The galena detector, the most widely used type among amateurs, became virtually the only detector used in crystal radios from this point on. The carborundum junction saw some use as a detector in early vacuum tube radios because it was more sensitive than the triode
grid-leak detector A grid leak detector is an electronic circuit that demodulates an amplitude modulated alternating current and amplifies the recovered modulating voltage. The circuit utilizes the non-linear cathode to control grid conduction characteristic and ...
. Crystal radios were kept as emergency backup radios on ships. During
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
in Nazi-occupied Europe the radio saw use as an easily constructed, easily concealed clandestine radio by Resistance groups. After World War II, the development of modern semiconductor diodes finally made the galena cat whisker detector obsolete.


Development of the theory of semiconductor rectification

Semiconductor devices like the crystal detector work by
quantum mechanical Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, qua ...
principles; their operation cannot be explained by classical physics. The birth of
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistr ...
in the 1920s was the necessary foundation for the development of semiconductor physics in the 1930s, during which physicists arrived at an understanding of how the crystal detector worked. The German word ''halbleiter'', translated into English as "
semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
", was first used in 1911 to describe substances whose conductivity fell between conductors and insulators, such as the crystals in crystal detectors.
Felix Bloch Felix Bloch (23 October 1905 – 10 September 1983) was a Swiss-American physicist and Nobel physics laureate who worked mainly in the U.S. He and Edward Mills Purcell were awarded the 1952 Nobel Prize for Physics for "their development of new ...
and
Rudolf Peierls Sir Rudolf Ernst Peierls, (; ; 5 June 1907 – 19 September 1995) was a German-born British physicist who played a major role in Tube Alloys, Britain's nuclear weapon programme, as well as the subsequent Manhattan Project, the combined Allie ...
around 1930 applied quantum mechanics to create a theory of how electrons move through a crystal. In 1931, Alan Wilson created quantum band theory which explains the electrical conductivity of solids.
Werner Heisenberg Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a breakthrough paper. In the subsequent serie ...
conceived the idea of a ''
hole A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...
'', a vacancy in a crystal lattice where an electron should be, which can move about the lattice like a positive particle; both electrons and holes conduct current in semiconductors. A breakthrough came when it was realized that the rectifying action of crystalline semiconductors was not due to the crystal alone but to the presence of impurity atoms in the crystal lattice. Michael Riordan, Lillian Hoddeson (1998) ''Crystal Fire: The Invention of the Transistor and the Birth of the Information Age'', p. 89-93
/ref> In 1930 Bernhard Gudden and Wilson established that electrical conduction in semiconductors was due to trace impurities in the crystal, a "pure" semiconductor did not act as a semiconductor, but as an insulator (at low temperatures). The maddeningly variable activity of different pieces of crystal when used in a detector, and the presence of "active sites" on the surface, was due to natural variations in the concentration of these impurities throughout the crystal. Nobel Laureate
Walter Brattain Walter Houser Brattain (; February 10, 1902 – October 13, 1987) was an American physicist at Bell Labs who, along with fellow scientists John Bardeen and William Shockley, invented the point-contact transistor in December 1947. They shared the ...
, coinventor of the transistor, noted: The "metallurgical purity" chemicals used by scientists to make synthetic experimental detector crystals had about 1% impurities which were responsible for such inconsistent results. During the 1930s progressively better refining methods were developed, allowing scientists to create ultrapure semiconductor crystals into which they introduced precisely controlled amounts of trace elements (called doping). This for the first time created semiconductor junctions with reliable, repeatable characteristics, allowing scientists to test their theories, and later making manufacture of modern diodes possible. The theory of rectification in a metal-semiconductor junction, the type used in a cat whisker detector, was developed in 1938 independently by Walter Schottky Schottky, W. "Halbleitertheorie der Sperrsschicht." Naturwissenschaften Vol. 26 (1938) pp. 843. Abstract in English as "Semiconductor Theory of the Blocking Layer" in Sze, S.M. Semiconductor Devices: Pioneering Papers. (World Scientific Publishing Co., 1991) pp. 381 at
Siemens & Halske Siemens & Halske AG (or Siemens-Halske) was a German electrical engineering company that later became part of Siemens. It was founded on 12 October 1847 as ''Telegraphen-Bauanstalt von Siemens & Halske'' by Werner von Siemens and Johann Ge ...
research laboratory in Germany and Nevill Mott reprinted in at Bristol University, UK. Mott received the 1977
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
. In 1949 at
Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial Research and development, research and scientific developm ...
William Shockley William Bradford Shockley Jr. (February 13, 1910 – August 12, 1989) was an American physicist and inventor. He was the manager of a research group at Bell Labs that included John Bardeen and Walter Brattain. The three scientists were jointl ...
derived the
Shockley diode equation The ''Shockley diode equation'' or the ''diode law'', named after transistor co-inventor William Shockley of Bell Telephone Laboratories, gives the I–V (current-voltage) characteristic of an idealized diode in either forward or reverse bias (appl ...
which gives the nonlinear exponential current–voltage curve of a crystal detector, observed by scientists since Braun and Bose, which is responsible for rectification .


The first modern diodes

The development of
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
technology during the 1930s run up to
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
for use in military
radar Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, we ...
led to the resurrection of the point contact crystal detector. Microwave radar receivers required a nonlinear device that could act as a mixer, to mix the incoming microwave signal with a
local oscillator In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the frequenc ...
signal, to shift the microwave signal down to a lower intermediate frequency (IF) at which it could be amplified. The vacuum tubes used as mixers at lower frequencies in superheterodyne receivers could not function at microwave frequencies due to excessive capacitance. In the mid-1930s George Southworth at
Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial Research and development, research and scientific developm ...
, working on this problem, bought an old cat whisker detector and found it worked at microwave frequencies. Hans Hollmann in Germany made the same discovery. The
MIT Radiation Laboratory The Radiation Laboratory, commonly called the Rad Lab, was a microwave and radar research laboratory located at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts. It was first created in October 1940 and operated until 31 ...
launched a project to develop microwave detector diodes, focusing on silicon, which had the best detecting properties. By about 1942 point-contact silicon crystal detectors for radar receivers such as the 1N21 and 1N23 were being mass-produced, consisting of a slice of boron-doped silicon crystal with a
tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...
wire point pressed firmly against it. The cat whisker contact did not require adjustment, and these were sealed units. A second parallel development program at
Purdue University Purdue University is a public land-grant research university in West Lafayette, Indiana, and the flagship campus of the Purdue University system. The university was founded in 1869 after Lafayette businessman John Purdue donated land and mone ...
produced germanium diodes. Such point-contact diodes are still being manufactured, and may be considered the first modern diodes. After the war,
germanium diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...
s replaced galena cat whisker detectors in the few crystal radios being made. Germanium diodes are more sensitive than silicon diodes as detectors, because germanium has a lower forward voltage drop than silicon (0.4 vs 0.7 volts). Today a few galena cat whisker detectors are still being made, but only for antique replica crystal radios or devices for science education.


See also

* Coherer * Barretter detector *
Electrolytic detector The electrolytic detector, or liquid barretter, was a type of detector (demodulator) used in early radio receivers. First used by Canadian radio researcher Reginald Fessenden in 1903, it was used until about 1913, after which it was superseded ...
*
Magnetic detector The magnetic detector or Marconi magnetic detector, sometimes called the "Maggie", was an early radio wave detector used in some of the first radio receivers to receive Morse code messages during the wireless telegraphy era around the turn of the ...
* List of historic technological nomenclature * Point-contact transistor *
Reginald Fessenden Reginald Aubrey Fessenden (October 6, 1866 – July 22, 1932) was a Canadian-born inventor, who did a majority of his work in the United States and also claimed U.S. citizenship through his American-born father. During his life he received hundre ...


References


External links


Crystal and Solid Contact Rectifiers
1909 publication describes experiments to determine the means of rectification ( PDF file).
Radio Detector Development
from 1917 '' The Electrical Experimenter''
The Crystal Experimenters Handbook
1922 London publication devoted to point-contact diode detectors (PDF file courtesy of Lorne Clark via earlywireless.com) * ;Patents * - ''Oscillation detector'' (multiple metallic sulfide detectors), Clifford D. Babcock, 1908 * - ''Oscillation detector and rectifier'' ("plated" silicon carbide detector with DC bias), G.W. Pickard, 1909 * - ''Oscillation receiver'' (fractured surface red zinc oxide (zincite) detector), G.W. Pickard, 1909 * - ''Oscillation device'' (iron pyrite detector), G.W. Pickard, 1909 * - ''Oscillation detectors'' (paired dissimilar minerals), G.W. Pickard, 1914 {{Authority control History of radio Radio electronics Diodes Detectors Bangladeshi inventions Indian inventions