A MODEL-BASED COMPARISON OF ALINAS/GAINAS AND INP/GAINAS HBTS - A MONTE-CARLO STUDY

The high-speed performances of AlInAs/GaInAs and InP/GaInAs heterojunction bipolar transistors (HBT’s) have been thoroughly investigated using a one-dimensional self-consistent particle simulator. Optimum alloy compositions for a graded-gap base structure have been obtained for both transistors through the tradeoff between the emitter charging time and the base transit time. The saturation velocity in the GaInAs n-type collector has been found to be smaller than that in InP, which has been attributed to the diffusion of a large number of “hot” back-scattered Γ-valley electrons in the GaInAs collector. The difference in the collector transit time in p-type collectors has been found to be trivial, since the maximum electron velocity was restricted to below 1.2 × 108 cm/s due to a strong non-parabolicity effect. The cutoff frequency has been estimated to be twice for the former and 1.5 times for the latter higher than that for Al-GaAs/GaAs HBT’s. These results have been attributed to a larger bandgap difference between the emitter and base to yield a high base built-in field, rather than a larger Γ-L band separation energy in the collector to enhance the velocity overshoot effect. © 1990 IEEE