Physical model of the C_bc for the linearity characteristics of AlGaAs/GaAs HBTs

It is well-known that C_bc is the dominant nonlinear element in HBTs. To study its behavior, we have developed an analytical nonlinear HBT equivalent circuit model. The present model includes the effect of the ionized donor charge in the depleted collector region being compensated for by the injected mobile charge. The model-based simulation shows that, at a small signal range, the IM3 of the normal HBT has the normal 3 : 1 gain slope generated by the nonlinearity of Cbc. At a large signal level, the load line passes through some regions with constant C_bc because its collector is fully depleted by the injected free carriers, and the growth rate of IM3 is decreased. The punch-through collector HBT has constant C_bc during the whole RF cycle, and the IM3, which is generated by g_m nonlinearity, has the normal 3 : 1 gain slope for all input signal levels. Therefore, the IM3 level is significantly lower for the punch-through HBT at a low power level, but the IM3s of both devices are comparable at a high power level. The experiment supports our proposed model.