The effect of base grading on the gain and high-frequency performance of AlGaAs/GaAs heterojunction bipolar transistors

TitleThe effect of base grading on the gain and high-frequency performance of AlGaAs/GaAs heterojunction bipolar transistors
Publication TypeJournal Article
Year of Publication1989
AuthorsHo, S. C. M., and D. L. Pulfrey
JournalElectron Devices, IEEE Transactions on
Volume36
Pagination2173 -2182
Date Publishedoct.
ISSN0018-9383
KeywordsAl mole fraction, AlGaAs-GaAs, aluminium compounds, Auger recombination, base current, base grading, base transit time, base-collector boundary, base-emitter boundary, current gain, cutoff frequency, gallium arsenide, heterojunction bipolar transistors, high-frequency performance, III-V semiconductors, neutral regions, one-dimensional analytical model, pyramidal structure, radiative recombination, semiconductor device models, Shockley Read Hall recombination, space charge region
Abstract

A comprehensive one-dimensional analytical model of the graded-base AlxGa1-xAs/GaAs heterojunction bipolar transistor is presented and used to examine the influence of base grading on the current gain and the high-frequency performance of a device with a conventional pyramidal structure. Grading is achieved by varying the Al mole fraction x linearly across the base to a value of zero at the base-collector boundary. Recombination in the space-charge and neutral regions of the device is modeled by considering Schockley-Read-Hall, Auger, and radiative processes. Owing to the different dependencies on base grading of the currents associated with these recombination mechanisms, the base current is minimized, and hence the gain reaches a maximum value, at a moderate level of base grading ( x=0.1 at the base-emitter boundary). The maximum improvement in gain, with respect to the ungraded base case, is about fourfold. It is shown that the reduction in base transit time due to increased base grading leads to a 30% improvement in fT in the most pronounced case of base grading studied (x=0.3 at the base-emitter boundary). The implications this has for improving f max via increases in base width and base doping density are briefly examined

URLhttp://dx.doi.org/10.1109/16.40897
DOI10.1109/16.40897

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