MONTE-CARLO STUDIES OF THE EFFECT OF EMITTER JUNCTION GRADING ON THE ELECTRON-TRANSPORT IN INALAS/INGAAS HETEROJUNCTION BIPOLAR-TRANSISTORS

InAlAs/InGaAs HBT's 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 (tau(ec)) of 1.18 ps is predicted for an In(Ga1-xAlx)As grading with x = 0.6 at the E-B interface and J(C) = 0.7 x 10(5) A/cm2. 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 x0 approximately 0.7 but increases for larger values. A cutoff frequency (f(T)) of as high as 270 GHz was observed for x0 = 0.7, indicating that best transport can be achieved from intermediately graded rather than abrupt E-B junction designs.