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The cut-off frequency of base-graded and junction-graded Al<sub>x</sub>Ga<sub>1-x</sub>As DHBTs

Publication Type:

Journal Article


Solid-State Electron. (UK), Volume 34, Number 12, p.1325 - 8 (1991)



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


A detailed model of AlxGa1-xAs double-heterojunction bipolar transistors is used to examine the effects on the cut-off frequency of varying both the degree of base grading in abrupt-junction devices, and the amount of base-collector junction grading in uniform-base devices. It is shown that, in the graded-base case, there is an optimum degree of base grading, stemming from the trade-off between the strength of the aiding field for minority carrier transport across the base and the height of the barrier at the base-collector junction, which inhibits charge flow to the collector. In the uniform-base case, it is shown that a small amount of base-collector junction grading is sufficient to effectively remove the blocking action of the conduction band discontinuity, and so allow the base transit time to attain its diffusion-limited value


DHBT;cut-off frequency;model;double-heterojunction bipolar transistors;base grading;abrupt-junction devices;base-collector junction grading;uniform-base devices;minority carrier transport;conduction band discontinuity;base transit time;AlxGa1-xAs;