Modeling of Current-gain Collapse in Multi-finger HBT ’s

This paper presents an equivalent circuit model for common-emitter current-gain collapse in multi-finger HBTs.The collapse of current-gain is an undesirable phenomenon,which occurs in multi-finger HBTs when they are operated at high power density.The collapse manifests itself by a distinct abrupt decrease of collector current in the output characteristics of the HBT.Until now,there hasn ’t been an equivalent circuit model to simulate the current collapse behavior of multi-finger HBTs.In this paper,for the first time,the large-signal Ebers-Moll model is modified to include the effect of current-gain collapse.Hence,the model presented here is a big step towards predicting the behavior of multi-finger HBTs under high power and high frequency operating con- ditions.Also,the self-heating effect which gives rise to the negative output resistance is included in the equivalent circuit without the use of an external thermal feedback circuit.Hence,the equivalent circuit is much simpler to implement and easier to analyze. Moreover,it includes the phenomenon of current-gain collapse.The model is applied to an InGaAs/GaAs multi-finger heterojunction bipo- lar transistor (HBT)and the simulated and the measured characteristics are compared in order to validate the model.Both the small signal and large signal models are implemented as user-defined models in the commercial circuit simulator,LIBRA