Second order distortion modeling in HBTs

There is a great deal of interest in operating semiconductor devices in large-signal, high frequency mode and the Heterojunction Bipolar Transistor (HBT) possesses some unique properties that make it suitable for such applications. Unexpectedly, it has very good linear properties. In this paper we examine the fundamental reasons for such good performance of HBTs, compare various models and the influence of different parameters, such as emitter resistance, base-collector capacitance and time delay of current gain. In particular, we examine the cancellation of second harmonic currents in the base emitter junction in great detail. The method of nonlinear currents was used to develop a set of analytical expressions for equivalent circuits of varying complexity. It was found that the emitter resistance R/sub ee/ linearizes the HBT at low frequencies by providing negative feedback and by bringing the phase difference of the second order currents closer to 180/spl deg/. At high frequencies the base collector capacitance C/sub bc/ dominates. The inherent nonlinearity of C/sub bc/ degrades the cancellation of second order currents in the base emitter junction. We have also found a connection between two different current gain models that makes them equivalent under certain conditions.