The heterojunction bipolar transistor (HBT) is a type of

bipolar junction transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipola ...

(BJT) which uses differing semiconductor materials for the emitter and base regions, creating a

heterojunction A heterojunction is an interface between two layers or regions of dissimilar semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction. It is often advantageous to engineer the electronic energy bands in ma ...

. The HBT improves on the BJT in that it can handle signals of very high frequencies, up to several hundred

GHz 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 he ...

. It is commonly used in modern ultrafast circuits, mostly

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 up ...

(RF) systems, and in applications requiring a high power efficiency, such as RF power amplifiers in

cellular phones A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link while ...

. The idea of employing a heterojunction is as old as the conventional BJT, dating back to a patent from 1951. Detailed theory of heterojunction bipolar transistor was developed by Herbert Kroemer in 1957.

Materials

The principal difference between the BJT and HBT is in the use of differing semiconductor materials for the emitter-base junction and the base-collector junction, creating a heterojunction. The effect is to limit the injection of holes from the base into the emitter region, since the potential barrier in the valence band is higher than in the conduction band. Unlike BJT technology, this allows a high doping density to be used in the base, reducing the base resistance while maintaining gain. The efficiency of the heterojunction is measured by the Kroemer factor. Kroemer was awarded a

Nobel Prize The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...

in 2000 for his work in this field at the University of California, Santa Barbara. Materials used for the substrate include silicon, gallium arsenide, and

indium phosphide Indium phosphide (InP) is a binary semiconductor composed of indium and phosphorus. It has a face-centered cubic (" zincblende") crystal structure, identical to that of GaAs and most of the III-V semiconductors. Manufacturing Indium phosphide ...

, while silicon / silicon-germanium alloys,

aluminum gallium arsenide Aluminium gallium arsenide (also gallium aluminium arsenide) ( Alx Ga1−x As) is a semiconductor material with very nearly the same lattice constant as GaAs, but a larger bandgap. The ''x'' in the formula above is a number between 0 and 1 - this ...

/ gallium arsenide, and

indium gallium arsenide Indium gallium arsenide (InGaAs) (alternatively gallium indium arsenide, GaInAs) is a ternary alloy ( chemical compound) of indium arsenide (InAs) and gallium arsenide (GaAs). Indium and gallium are ( group III) elements of the periodic table w ...

are used for the epitaxial layers. Wide-

bandgap 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 (i ...

semiconductors such as

gallium nitride Gallium nitride () is a binary III/ V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV affords ...

and

indium gallium nitride Indium gallium nitride (InGaN, ) is a semiconductor material made of a mix of gallium nitride (GaN) and indium nitride (InN). It is a ternary group III/ group V direct bandgap semiconductor. Its bandgap can be tuned by varying the amount of i ...

are especially promising. In SiGe graded heterostructure transistors, the amount of germanium in the base is graded, making the bandgap narrower at the collector than at the emitter. That tapering of the bandgap leads to a field-assisted transport in the base, which speeds transport through the base and increases frequency response.

Fabrication

Due to the need to manufacture HBT devices with extremely high-doped thin base layers,

molecular beam epitaxy Molecular-beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. MBE is widely used in the manufacture of semiconductor devices, including transistors, and it is considered one of the fundamental tools for the dev ...

is principally employed. In addition to base, emitter and collector layers, highly doped layers are deposited on either side of collector and emitter to facilitate an

ohmic contact An ohmic contact is a non-rectifying electrical junction: a junction between two conductors that has a linear current–voltage (I–V) curve as with Ohm's law. Low-resistance ohmic contacts are used to allow charge to flow easily in both direct ...

, which are placed on the contact layers after exposure by

photolithography In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer, to protec ...

and etching. The contact layer underneath the collector, named subcollector, is an active part of the transistor. Other techniques are used depending on the material system. IBM and others use ultra-high vacuum chemical vapor deposition (UHVCVD) for SiGe; other techniques used include

MOVPE Metalorganic vapour-phase epitaxy (MOVPE), also known as organometallic vapour-phase epitaxy (OMVPE) or metalorganic chemical vapour deposition (MOCVD), is a chemical vapour deposition method used to produce single- or polycrystalline thin films. ...

for

III-V Semiconductor materials are nominally small band gap insulators. The defining property of a semiconductor material is that it can be compromised by doping it with impurities that alter its electronic properties in a controllable way. Because o ...

systems. Normally the epitaxial layers are lattice matched (which restricts the choice of bandgap etc.). If they are near-lattice-matched the device is pseudomorphic, and if the layers are unmatched (often separated by a thin buffer layer) it is metamorphic.

Limits

A pseudomorphic heterojunction bipolar transistor developed at the

University of Illinois at Urbana-Champaign The University of Illinois Urbana-Champaign (U of I, Illinois, University of Illinois, or UIUC) is a public land-grant research university in Illinois in the twin cities of Champaign and Urbana. It is the flagship institution of the Un ...

, built from

and

and designed with compositionally graded collector, base and emitter, was demonstrated to cut off at a speed of 710 GHz.Indium Phosphide: Transcending frequency and integration limits. Semiconductor Today. Vol 1 Issue 3. Sept 2006
/ref> Besides being record breakers in terms of speed, HBTs made of

InP Indium phosphide (InP) is a binary semiconductor composed of indium and phosphorus. It has a face-centered cubic (" zincblende") crystal structure, identical to that of GaAs and most of the III-V semiconductors. Manufacturing Indium phosphide c ...

/ InGaAs are ideal for monolithic optoelectronic integrated circuits. A PIN-type photo detector is formed by the base-collector-subcollector layers. The

of InGaAs works well for detecting 1550 nm-

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 ...

infrared laser signals used in optical communication systems. Biasing the HBT to obtain an active device, a photo transistor with high internal gain is obtained. Among other HBT applications are mixed signal circuits such as analog-to-digital and digital-to-analog converters.

References

External links

*
HBT Optoelectronic Circuits developed in the Technion
(15Mb, 230p)

604 GHz Early 2005 {{DEFAULTSORT:Heterojunction Bipolar Transistor Microwave technology Terahertz technology Transistor types

Materials

Fabrication

Limits

See also

References

External links