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

Publication Type:

Journal Article


IEEE Trans. Electron Devices (USA), Volume 36, Number 10, p.2173 - 82 (1989)



aluminium compounds;gallium arsenide;heterojunction bipolar transistors;III-V semiconductors;semiconductor device models;


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 fmax via increases in base width and base doping density are briefly examined


space charge region;Shockley Read Hall recombination;Auger recombination;radiative recombination;cutoff frequency;base grading;high-frequency performance;heterojunction bipolar transistors;one-dimensional analytical model;current gain;pyramidal structure;Al mole fraction;base-collector boundary;neutral regions;base current;base-emitter boundary;base transit time;AlGaAs-GaAs;