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Heterojunction Bipolar Transistor

The HBT is an active bipolar device which incorporates a heterojunction between a wide bandgap semiconductor and narrow bandgap semiconductor, some common materials used are:

  • InP/InGaAs
  • Si/SiGe
  • InAlAs/InGaAs
  • AlGaN/GaN
  • InGaP/GaAs
  • InGaP/InGaAsN
  • AlGaAs/GaAs
  • InP/GaAsSb

The SHBT has a single heterojunction where the emitter is the wide bandgap material, this allows heavy doping of the base for reduced base resistance while the emitter is more lightly doped reducing the capacitance and improving the high frequency performance.
The DHBT (Double Heterojunction Bipolar Transistor) has a wide bandgap collector and emitter, allowing the same advantages of the SHBT, with the additional improvement of increased breakdown voltage and decreased minority carrier injection from the base to the collector in saturation mode.

An additional advantages of the HBT over a conventional BJT (Ref. 33):

  • Graded alloy change along the base allows the incorporation of a built-in field with a greater magnitude than that achievable by grading the doping in a homo-junction BJT.

  • Hot Carrier Injection is possible across the abrupt band discontinuities.

HBTs have been made from many materials (those listed above are some of the most prevalent). The Johnson Figure of Merit may give an idea into the capabilities and limitations of some of the binary compounds.

Johnson Figure of Merit

While this isn't a direct mapping into device performance it does give an indication of what might be expectable for devices of similar dimensions and without quantum effects.

Material Saturation Velocity
  (x106 cm/s)
Breakdown Voltage
(x106 V/cm)
(x1012 V/s)
Silicon 6 .3 1.8
GaAs 10 .4 4
InP .5
GaN 25 5 125
SiC 20 5 100


Diagram of device and v-i characteristics


Some of the information on this page was compiled from the following sources:

  • P Asbeck, "Heterojunction Bipolar Transistors" From Handbook of Thin Film Devices, Vol. 1: "Hetero-Structure for High Performance Devices", CEC Wood - Editor, (2000), Academic Press
  • UV Bhapkar, MS Shur, Journal of Applied Physics, 82, 1649, (1997)
  • Ioffe Institute Semionductor materials properties sheet - online.
  • M. E. Levinshtein, S. L. Rumyantsev, and M. S. Shur, Editors, "Properties of Advanced Semiconductor Materials: GaN, AlN, InN, BN, SiC, and SiGe", John Wiley and Sons, ISBN 0-471-35827-4, New York (2001)


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