Monte Carlo studies of the effect of emitter junction grading on the electron transport in InAlAs/InGaAs heterojunction bipolar transistors

InAlAs/InGaAs HBTs with various emitter junction gradings are simulated using a self-consistent Monte Carlo simulator. The effects of the emitter junction grading and the shift of the emitter-base p-n junction into the emitter depletion region due to diffusion of the base dopant are investigated. A minimum transit time of 1.18 ps is predicted for an In(Ga/sub 1-x/Al/sub x/)As grading with x=0.6 at the E-B interface and J/sub C/=0.7*10/sup 5/ A/cm/sup 2/. Graded-base designs do not offer any transit time performance improvement compared with the graded E-B approach. For transient performance, the device switching time is found to remain constant at about 2.2 ps up to x/sub 0/ approximately 0.7 but increases for larger values. A cutoff frequency as high as 270 GHz was observed for x/sub 0/=0.7, indicating that the best transport can be achieved from intermediately graded rather than abrupt E-B junction designs.<
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