Current transient in polyimide-passivated InP/InGaAs heterojunction bipolar transistors: systematic experiments and physical model

We present the first comprehensive experimental investigation of base current transient in InP/InGaAs heterojunction bipolar transistors (HBTs) induced by polyimide passivation, and its influence on the device characteristics. We have confirmed that the trapping and detrapping of static charge in the polyimide at the polyimide/semiconductor interface is the root cause for the current transient effect. Experimental and theoretical evidence indicates that the base current transient is caused by the change of the base-emitter junction surface potential as a result of the decrease in the number of trapped electrons in the polyimide film. A physical model accounting for the time-dependence of surface recombination current induced by electron detrapping in the polyimide is derived and predicts the current transient behavior. The physical understanding is helpful for further optimization of polyimide passivation process for HBTs.