A study of base grading effects on the steady-state current gain of heterojunction bipolar transistors

An analytical model is developed for the common-emitter DC current gain of heterojunction bipolar transistors with graded base for charge transport enhancement. Relevant device physics such as drift mechanism, current-induced base widening, and velocity overshoot are considered. It is shown that although base grading will often improve the device performance, it is sometimes disadvantageous for the current gain because the base grading may decrease the valence-band barrier height and thus increase the base current, resulting in a reduction in the current gain. Numerical simulations and measured dependencies published in the literature are included in support of the model