Modeling The Steady-State Collector Current Of Algaas/Gaas Heterojunction Bipolar Transistors Using A Generalized Drift-Diffusion Equation

Fabrication of submicrometer heterjunction bipolar transistors (HBTs) is now possible because of the recent emergence of molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) technologies. The small geometry of HBTs, however, leads to non-negligible transient transport effects such as velocity overshoot and thus invalidates the use of the drift-diffusion model for charge transport. To avoid this difficulty and because of the band discontinuity (spike) at the interface of the heterojunctions, the thermionic emission theory is conventionally employed for modelling the charge transport at the heterointerface. This note treats the carrier transport, and thus the current, at the heterointerface of HBTs using a one-dimensional generalized drift-diffusion current equation developed by H.K. Thornber. For illustration, an n-p-n Al03Ga0.7As/GaAs/GaAs HBT fabricated by Texas Instruments having an emitter doping concentration of 2×1017 cm-3, a base doping concentration of 2×1018 cm-3, a collector doping concentration of 2×10$/16 cm-3, and an emitter-base junction area of 49 μm2 are considered