Heterojunction bipolar transistor

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The heterojunction bipolar transistor (HBT) is an improvement of the bipolar junction transistor (BJT) that can handle signals of very high frequencies up to several hundred GHz. It is common in modern ultrafast circuits, mostly radio-frequency (RF) systems.

1 Materials
2 Fabrication
3 Limits
4 See also
5 External links

[edit] Materials

Bands in graded heterojunction npn bipolar transistor. Barriers indicated for electrons to move from emitter to base, and for holes to be injected backward from base to emitter; Also, grading of bandgap in base assists electron transport in base region; Light colors indicate depleted regions

The principal difference between the BJT and HBT is the use of differing semiconductor materials for the emitter and base regions, creating a heterojunction. The effect is to limit the injection of holes into the base region, since the potential barrier in the valence band is so large. Unlike BJT technology, this allows high doping to be used in the base, creating higher electron mobility while maintaining gain. The efficiency of the device is measured by the Kroemer factor, after Herbert Kroemer who received a Nobel Prize for his work in this field in 2000.

Materials used for the substrate include silicon-germanium alloys and gallium arsenide, while aluminium gallium arsenide, indium phosphide and indium gallium phosphide are used for the epitaxial layers. Wide-bandgap semiconductors are especially promising, eg. gallium nitride and indium gallium nitride.

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.

[edit] Fabrication

Due to the need to manufacture HBT devices with extremely thin base layers, molecular beam epitaxy 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, which are placed on the contact layers after exposure by photolithography.

Other techniques are used depending on the material system. IBM and others use UHV CVD for SiGe; other techniques used include MOVPE for III-V systems.

[edit] Limits

A pseudomorphic heterojunction bipolar transistor developed at the University of Illinois at Urbana-Champaign, built from indium phosphide and indium gallium arsenide and designed with compositionally graded collector, base and emitter, was demonstrated to cut off at a speed of 710 gigahertz.

Besides being record breakers in terms of speed, HBTs made of InP/InGaAs are ideal for monolithic optoelectronic integrated circuits. The bandgap of InGaAs fits for detection of 1.55μm-wavelength signal used in optical communication systems. Among other HBT applications are mixed signal circuits such as analog-to-digital and digital-to-analog converters.