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_1-xAl_x)As grading with x=0.6 at the E-B interface and J_C=0.7×10⁵ 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 x₀~0.7 but increases for larger values. A cutoff frequency as high as 270 GHz was observed for x₀=0.7, indicating that the best transport can be achieved from intermediately graded rather than abrupt E-B junction designs