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Details of Grant 

EPSRC Reference: GR/H45094/01
Principal Investigator: Abram, Professor R
Other Investigators:
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Department: Physics
Organisation: Durham, University of
Scheme: Standard Research (Pre-FEC)
Starts: 01 December 1992 Ends: 30 November 1995 Value (£): 93,226
EPSRC Research Topic Classifications:
Electronic Devices & Subsys.
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Summary on Grant Application Form
(i) To develop a two dimensional model of heterojunction bipolar transistors based on Monte Carlo simulation of carrier transport and realistic semiconductor band structure.(ii) To use the model to investigate methods of enhancing the high frequency and high current capabilities of HBTs.Progress:The modelling of the HBT started with the implementation of a 2-D Monte Carlo device simulation program based on previous work carried out in Durham. The material model included both holes (light and heavy bands) and electrons (G, L and c valleys) with phonon scattering between permitted bands. The electric field distribution was obtained in a self-consistent manner using a 2-D finite difference Poisson solver.Initial studies considered an npn AlGaAs/GaAs device of typical dimensions and structure. The steady state response over a range of base-emitter voltages VBE (with VCE fixed) was modelled. As expected the effective base region is pushed out into the collector (Kirk effect) at high injection levels. Despite the heterojunction this push-out also occurs at the base-emitter junction and as this represents a considerable reduction in emitter injection efficiency it was proposed to examine the effect further. To study the frequency response of the HBT, a voltage pulse was applied to the base contact and the total terminal currents (drift plus displacement) were used to determine the cut-off frequency fT. At this stage the software was rewritten to form a subroutine library (SLURPS) and a number of changes were introduced, most significantly the inclusion of additional scattering mechanisms (charge impurity and alloy scattering), the availability of material parameters for a wide range of III-V materials, and the capability to model more complicated device geometries. With this new software, the device structure of the HBT model was modified slightly to coincide more closely with an actual device presently employed at GEC-MMT. The AlGaAs/GaAs materials were retained but pseudopotential bandstructure calculations for ordered InGaP material will permit investigation of an InGaP/GaAs HBT. Future developments include an examination of the onset of avalanche breakdown and the inclusion of effects related to high carrier densities with carrier-carrier scattering and degeneracy being important especially in the base region.
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