Physical modeling of degenerately doped compound semiconductors for high-performance HBT design

Abstract

Numerous research groups are currently developing high-performance InP HBTs with fT and fMAX greater than 400GHz. However, the heavily degenerate doping concentrations used in these devices present new challenges to numerical device simulation. This work focuses on three physical phenomena in InP and In0.53Ga0.47As and their implementation in physics based device simulators. First, the use of the parabolic band approximation yields a constant DOS effective mass, but this results in an erroneously deep Fermi-level under heavily degenerate donor concentrations. An empirical model is presented and shown to have good agreement with previously published simulations and experimental data. Second, bandgap narrowing parameters and a table based model are used as a more generic model for compound semiconductors. Third, calculated parameters to address the Mott transition are used to obtain the proper free-carrier concentrations throughout the HBT. The improper calibration or neglect of these three physical phenomena is shown to alter HBT band profiles at thermal equilibrium by as much as 400meV; the turn-on voltage by approximately 50mV; and the fT dependence on JC by approximately 18%.