The principal difference between the BJT and HBT is in the use of differing semiconductor materials for the emitter and base regions, 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 device is measured by the Kroemer factor, after Herbert Kroemer who received a Nobel Prize for his work in this field in 2000 at the University of California, Santa Barbara.
Materials used for the substrate include silicon, gallium arsenide, and indium phosphide, while silicon / silicon-germanium alloys, aluminium gallium arsenide / gallium arsenide, and indium phosphide / indium gallium arsenide 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.
Due to the need to manufacture HBT devices with extremely high-doped 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 and etching. The contact layer underneath the collector is, named subcollector, is an active part of the transistor.
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
Metalorganic Chemical Vapor Deposition
This process is used to manufacture compound semiconductor devices, which consist of thin films of gallium arsenide, indium phosphide and other alloys of the group III and V elements of the Periodic Table. Compound semiconductors are used in a vast array of electronic and photonic devices, such as in solid-state lasers, light-emitting diodes, space solar cells, and high-speed transistors. These are critically needed components in both optical and wireless telecommunications systems.
Compound Semiconductor devices are used for the solar panels and the RF transmitters and receivers in communications satelites (pictured is a DirecTV satelite by Hughes Electronics).
In the metalorganic chemical vapor deposition (MOCVD) process, volatile precursors, e.g., trimethylindium, trimethylgallium and phosphine, are fed to the reactor in hydrogen carrier gas. When these molecules flow over the hot substrate, they decompose and deposit a thin film, e.g., InGaP. By depositing a compound that is lattice matched to the substrate (e.g., GaAs (001)), an epitaxial single crystal is grown. A device is produced by varying the composition and doping in the layers, while maintaining lattice matching at all times.
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