Abstract
This paper presents a two-dimensional numerical analysis to investigate the effects of the graded layer, setback layer and self-heating on the current gain and cutoff frequency of AlGaAs GaAs heterojunction bipolar transistors (HBTs). In addition to the standard drift-diffusion set of equations, the free carrier energy balance equations are also used in simulation to solve self-consistently the free carrier transport in the HBT. The results suggest that the elevated lattice temperature in the HBT resulting from the self-heating effect is a key factor limiting the HBT d.c. and a.c. performance at high current levels. Furthermore, it is found that the HBT with both the graded and setback layers has higher peak current gain and cutoff frequency than the abrupt HBT, graded HBT or abrupt HBT with a setback layer. Experimental data measured from abrupt and graded HBTs are included in support of the simulation results.
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