I. Introduction
InP-based double heterojuction bipolar transistors (DHBTs) have attracted much attention during last decade. Benefited from high electron mobility, current density and breakdown voltage, InP HBT is suitable for millimeter wave and teraherz power applications. The mainstream models of HBTs are Gummel-Poon, VBIC, MEXTRAM, HICUM, UCSD, AgilentHBT and FBH [1]–[4]. Among them, UCSD, AgilentHBT and FBH are designed for III-V HBTs. The UCSD HBT model was improved over the GP model by considering junction barrier effect with an ideality factor that varies with bias, separating base and collector resistances into extrinsic and intrinsic portions to formulate high speed HBT characteristics accurately. AgilentHBT model is based on the general concepts from the UCSD HBT model with improvements in modeling the hetero-junction effects, the soft-knee, dynamic self-heating and flexible collector transit time. FBH model also is an extended GP model, taking account for partition of intrinsic and extrinsic base-collector diode, non-ideal base currents and base-emitter and base-collector break-down. Observed from recent publications, Thomas B. Reed of UCSB uses AgilentHBT model to design a 220 GHz power amplifier with output power 90 mW [5], which indicate that AgilentHBT model shows bright prospect for millimeter and teraherz circuits design.