A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons. The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The package may include lenses or optical filters. Devices designed for use specially as a photodiode use a PIN junction rather than a p–n junction, to increase the speed of response. Photodiodes usually have a slower response time as their surface area increases. A photodiode is designed to operate in reverse bias. A

solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.solar power Solar power is the conversion of energy from sunlight into electricity, either directly using photovoltaics (PV) or indirectly using concentrated solar power. Photovoltaic cells convert light into an electric current using the photovoltaic ef ...

is a large area photodiode. Photodiodes are used in scientific and industrial instruments to measure light intensity, either for its own sake or as a measure of some other property (density of smoke, for example). A photodiode can be used as the receiver of data encoded on an infrared beam, as in household remote controls. Photodiodes can be used to form an optocoupler, allowing transmission of signals between circuits without a direct metallic connection between them, allowing isolation from high voltage differences.

Principle of operation

A photodiode is a PIN structure or p–n junction. When a

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are Massless particle, massless ...

of sufficient energy strikes the diode, it creates an

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary partic ...

–

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 ...

pair. This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward 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 mnemoni ...

, and electrons toward 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 i ...

, and a photocurrent is produced. The total current through the photodiode is the sum of the dark current (current that is generated in the absence of light) and the photocurrent, so the dark current must be minimized to maximize the sensitivity of the device. To first order, for a given spectral distribution, the photocurrent is linearly proportional to the irradiance.

Photovoltaic mode

In photovoltaic mode (zero bias), photocurrent flows into the anode through a short circuit to the cathode. If the circuit is opened or has a load impedance, restricting the photocurrent out of the device, a voltage builds up in the direction that forward biases the diode, that is, anode positive with respect to cathode. If the circuit is shorted or the impedance is low, a forward current will consume all or some of the photocurrent. This mode exploits the photovoltaic effect, which is the basis for

solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.reverse biased, that is, with the cathode driven positive with respect to the anode. This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction's

capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized a ...

and increases the region with an electric field that will cause electrons to be quickly collected. The reverse bias also creates dark current without much change in the photocurrent. Although this mode is faster, the photoconductive mode can exhibit more electronic noise due to dark current or avalanche effects. The leakage current of a good PIN diode is so low (<1 nA) that the Johnson–Nyquist noise of the load resistance in a typical circuit often dominates.

Related devices

Avalanche photodiode An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode detector that exploits the photoelectric effect to convert light into electricity. From a functional standpoint, they can be regarded as the semiconductor analog of ...

s are photodiodes with structure optimized for operating with high reverse bias, approaching the reverse breakdown voltage. This allows each ''photo-generated'' carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective ''responsivity'' of the device. IEEE 315-1975 (1993) 8

A phototransistor is a light-sensitive transistor. A common type of phototransistor, the ''bipolar phototransistor'', is in essence a bipolar transistor encased in a transparent case so that

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

can reach the ''base–collector junction''. It was invented by Dr. John N. Shive (more famous for his wave machine) at Bell Labs in 1948 but it was not announced until 1950. The electrons that are generated by photons in the base–collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain β (or h_fe). If the base and collector leads are used and the emitter is left unconnected, the phototransistor becomes a photodiode. While phototransistors have a higher responsivity for light they are not able to detect low levels of light any better than photodiodes. Phototransistors also have significantly longer response times. Another type of phototransistor, the ''field-effect phototransistor'' (also known as photoFET), is a light-sensitive field-effect transistor. Unlike photobipolar transistors, photoFETs control drain-source current by creating a gate voltage. A solaristor is a two-terminal gate-less phototransistor. A compact class of two-terminal phototransistors or solaristors have been demonstrated in 2018 by ICN2 researchers. The novel concept is a two-in-one power source plus transistor device that runs on solar energy by exploiting a memresistive effect in the flow of photogenerated carriers.

Materials

The material used to make a photodiode is critical to defining its properties, because only

s with sufficient energy to excite

s across the material's bandgap will produce significant photocurrents. Materials commonly used to produce photodiodes are listed in the table below. Because of their greater bandgap, silicon-based photodiodes generate less noise than germanium-based photodiodes. Binary materials, such as MoS₂, and graphene emerged as new materials for the production of photodiodes.

Unwanted and wanted photodiode effects

Any p–n junction, if illuminated, is potentially a photodiode. Semiconductor devices such as diodes, transistors and ICs contain p–n junctions, and will not function correctly if they are illuminated by unwanted electromagnetic radiation (light) of wavelength suitable to produce a photocurrent. This is avoided by encapsulating devices in opaque housings. If these housings are not completely opaque to high-energy radiation (ultraviolet, X-rays, gamma rays), diodes, transistors and ICs can malfunction due to induced photo-currents. Background radiation from the packaging is also significant. Radiation hardening mitigates these effects. In some cases, the effect is actually wanted, for example to use LEDs as light-sensitive devices (see

LED as light sensor In electronics, an LED circuit or LED driver is an electrical circuit used to power a light-emitting diode (LED). The circuit must provide sufficient current to light the LED at the required brightness, but must limit the current to prevent damag ...

) or even for

energy harvesting Energy harvesting (EH, also known as power harvesting or energy scavenging or ambient power) is the process by which energy is derived from external sources (e.g., solar power, thermal energy, wind energy, salinity gradients, and kinetic ...

, then sometimes called ''light-emitting and light-absorbing diodes'' (LEADs).

Features

Critical performance parameters of a photodiode include spectral responsivity, dark current, response time and noise-equivalent power. ; Spectral responsivity: The spectral responsivity is a ratio of the generated photocurrent to incident light power, expressed in A/ W when used in photoconductive mode. The wavelength-dependence may also be expressed as a '' quantum efficiency'' or the ratio of the number of photogenerated carriers to incident photons which is a unitless quantity. ; Dark current: The dark current is the current through the photodiode in the absence of light, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. Dark current must be accounted for by calibration if a photodiode is used to make an accurate optical power measurement, and it is also a source of noise when a photodiode is used in an optical communication system. ;

Response time Response time may refer to: *The time lag between an electronic input and the output signal which depends upon the value of passive components used. *Responsiveness, how quickly an interactive system responds to user input *Response time (biology) ...

: The response time is the time required for the detector to respond to an optical input. A photon absorbed by the semiconducting material will generate an electron–hole pair which will in turn start moving in the material under the effect of the electric field and thus generate a current. The finite duration of this current is known as the transit-time spread and can be evaluated by using Ramo's theorem. One can also show with this theorem that the total charge generated in the external circuit is e and not 2e as one might expect by the presence of the two carriers. Indeed, the

integral In mathematics, an integral assigns numbers to functions in a way that describes displacement, area, volume, and other concepts that arise by combining infinitesimal data. The process of finding integrals is called integration. Along with ...

of the current due to both electron and hole over time must be equal to e. The resistance and capacitance of the photodiode and the external circuitry give rise to another response time known as RC time constant (

\tau=RC

). This combination of R and C integrates the photoresponse over time and thus lengthens the impulse response of the photodiode. When used in an optical communication system, the response time determines the bandwidth available for signal modulation and thus data transmission. ; Noise-equivalent power: Noise-equivalent power (NEP) is the minimum input optical power to generate photocurrent, equal to the rms noise current in a 1

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one her ...

bandwidth. NEP is essentially the minimum detectable power. The related ''characteristic detectivity'' (

D

) is the inverse of NEP (1/NEP) and the '' specific detectivity'' (

D^\star

) is the detectivity multiplied by the square root of the area (

A

) of the photodetector (

D^\star=D\sqrt

) for a 1 Hz bandwidth. The specific detectivity allows different systems to be compared independent of sensor area and system bandwidth; a higher detectivity value indicates a low-noise device or system. Although it is traditional to give (

D^\star

) in many catalogues as a measure of the diode's quality, in practice, it is hardly ever the key parameter. When a photodiode is used in an optical communication system, all these parameters contribute to the ''

sensitivity Sensitivity may refer to: Science and technology Natural sciences * Sensitivity (physiology), the ability of an organism or organ to respond to external stimuli ** Sensory processing sensitivity in humans * Sensitivity and specificity, statisti ...

'' of the optical receiver which is the minimum input power required for the receiver to achieve a specified '' bit error rate''.

Applications

P–n photodiodes are used in similar applications to other photodetectors, such as photoconductors,

charge-coupled device A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are ...

s (CCD), and photomultiplier tubes. They may be used to generate an output which is dependent upon the illumination (analog for measurement), or to change the state of circuitry (digital, either for control and switching or for digital signal processing). Photodiodes are used in

consumer electronics Consumer electronics or home electronics are Electronics, electronic (Analogue electronics, analog or digital electronics, digital) equipment intended for everyday use, typically in private homes. Consumer electronics include devices used for ...

devices such as

compact disc The compact disc (CD) is a digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the first compact disc was manufactured. It was then released in Octo ...

players, smoke detectors, medical devicesE. Aguilar Pelaez et al., "LED power reduction trade-offs for ambulatory pulse oximetry," 2007 29th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Lyon, 2007, pp. 2296-2299. doi: 10.1109/IEMBS.2007.4352784, URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4352784&isnumber=4352185 and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications either photodiodes or photoconductors may be used. Either type of photosensor may be used for light measurement, as in

camera A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with a ...

light meters, or to respond to light levels, as in switching on street lighting after dark. Photosensors of all types may be used to respond to incident light or to a source of light which is part of the same circuit or system. A photodiode is often combined into a single component with an emitter of light, usually a

light-emitting diode A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light ( ...

(LED), either to detect the presence of a mechanical obstruction to the beam ( slotted optical switch) or to couple two digital or analog circuits while maintaining extremely high electrical isolation between them, often for safety ( optocoupler). The combination of LED and photodiode is also used in many sensor systems to characterize different types of products based on their optical absorbance. Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors. They are also widely used in various medical applications, such as detectors for

computed tomography A computed tomography scan (CT scan; formerly called computed axial tomography scan or CAT scan) is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers ...

(coupled with

scintillator A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbe ...

s), instruments to analyze samples ( immunoassay), and pulse oximeters. PIN diodes are much faster and more sensitive than p–n junction diodes, and hence are often used for optical communications and in lighting regulation. P–n photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed,

avalanche photodiode An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode detector that exploits the photoelectric effect to convert light into electricity. From a functional standpoint, they can be regarded as the semiconductor analog of ...

s, intensified charge-coupled devices or photomultiplier tubes are used for applications such as

astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...

, spectroscopy,

night vision equipment A night-vision device (NVD), also known as a night optical/observation device (NOD), night-vision goggle (NVG), is an optoelectronic device that allows visualization of images in low levels of light, improving the user's night vision. The dev ...

and

laser rangefinding A laser rangefinder, also known as a laser telemeter, is a rangefinder that uses a laser beam to determine the distance to an object. The most common form of laser rangefinder operates on the time of flight principle by sending a laser pulse in ...

Comparison with photomultipliers

Advantages compared to photomultipliers: # Excellent linearity of output current as a function of incident light # Spectral response from 190 nm to 1100 nm (

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 ...

), longer

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

s with other semiconductor materials # Low noise # Ruggedized to mechanical stress # Low cost # Compact and light weight # Long lifetime # High quantum efficiency, typically 60–80% # No high voltage required Disadvantages compared to photomultipliers: # Small area # No internal gain (except

s, but their gain is typically 10²–10³ compared to 10⁵-10⁸ for the photomultiplier) # Much lower overall sensitivity # Photon counting only possible with specially designed, usually cooled photodiodes, with special electronic circuits # Response time for many designs is slower # Latent effect

Pinned photodiode

The pinned photodiode (PPD) has a shallow implant (P+ or N+) in N-type or P-type diffusion layer, respectively, over a P-type or N-type (respectively) substrate layer, such that the intermediate diffusion layer can be fully depleted of majority carriers, like the base region of a bipolar junction transistor. The PPD (usually PNP) is used in CMOS active-pixel sensors; a precursor NPN variant with a clocked top N layer was invented at Sony in 1975 for use in CCD image sensors. Early charge-coupled device image sensors suffered from shutter lag. This was largely resolved with the invention of the pinned photodiode. It was invented by Nobukazu Teranishi, Hiromitsu Shiraki and Yasuo Ishihara at NEC in 1980. They recognized that lag can be eliminated if the signal carriers could be transferred from the photodiode to the CCD. This led to their invention of the pinned photodiode, a photodetector structure with low lag, low noise, high quantum efficiency and low dark current. It was first publicly reported by Teranishi and Ishihara with A. Kohono, E. Oda and K. Arai in 1982, with the addition of an anti-blooming structure. The new photodetector structure invented at NEC was given the name "pinned photodiode" (PPD) by B.C. Burkey at Kodak in 1984. In 1987, the PPD began to be incorporated into most CCD sensors, becoming a fixture in

consumer electronic Consumer electronics or home electronics are electronic (analog or digital) equipment intended for everyday use, typically in private homes. Consumer electronics include devices used for entertainment, communications and recreation. Usually ...

video cameras and then digital still cameras. In 1994, Eric Fossum, while working at

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedi ...

Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA ...

(JPL), proposed an improvement to the CMOS sensor: the integration of the pinned photodiode. A CMOS sensor with PPD technology was first fabricated in 1995 by a joint JPL and Kodak team that included Fossum along with P.P.K. Lee, R.C. Gee, R.M. Guidash and T.H. Lee. Since then, the PPD has been used in nearly all CMOS sensors. The CMOS sensor with PPD technology was further advanced and refined by R.M. Guidash in 1997, K. Yonemoto and H. Sumi in 2000, and I. Inoue in 2003. This led to CMOS sensors achieve imaging performance on par with CCD sensors, and later exceeding CCD sensors.

Photodiode array

A one-dimensional array of hundreds or thousands of photodiodes can be used as a position sensor, for example as part of an angle sensor. A two-dimensional array is used in image sensors and optical mice. In some applications, photodiode arrays allow for high-speed parallel readout, as opposed to integrating scanning electronics as in a

(CCD) or CMOS sensor. The optical mouse chip shown in the photo has parallel (not multiplexed) access to all 16 photodiodes in its 4 × 4 array.

Passive-pixel image sensor

The passive-pixel sensor (PPS) is a type of photodiode array. It was the precursor to the active-pixel sensor (APS). A passive-pixel sensor consists of passive pixels which are read out without amplification, with each pixel consisting of a photodiode and a MOSFET switch. In a photodiode array, pixels contain a p–n junction, integrated

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 a ...

, and MOSFETs as selection transistors. A photodiode array was proposed by G. Weckler in 1968, predating the CCD. This was the basis for the PPS. The noise of photodiode arrays is sometimes a limitation to performance. It was not possible to fabricate active pixel sensors with a practical pixel size in the 1970s, due to limited microlithography technology at the time.

References

External links

Photodiode I–V characteristics

Using the Photodiode to convert the PC to a Light Intensity Logger

Design Fundamentals for Phototransistor Circuits
(archived on February 5, 2005)

Excelitas Application Notes on Pacer Website
(archived on March 4, 2016) {{Authority control Optical diodes Photodetectors Silicon photonics devices