Abstract

To understand the linear characteristics of HBT more accurately, an analytical nonlinear HBT model using Volterra Series analysis is developed. The model considers four nonlinear components: rπ, Cdiff , Cdepl, and gm. It shows that nonlinearities of rπ and Cdiff are almost completely cancelled by gm nonlinearity at all frequencies. The residual gm nonlinearity are highly degenerated by the input impedances. Therefore, rπ, Cπ and gm nonlinearities generate less IM3 than Cbc. If Cbc is linearized, Cdepl and gm are the main nonlinear sources of HBT, and Cdepl becomes very important at a high frequency. It was also found that the degeneration resistor, RE, is more effective than RB for reducing gm nonlinearity. This analysis also provides the dependency of the source second harmonic impedance on the linearity of HBT. The IM3 of HBT is significantly reduced by setting the second harmonic impedance of ZS,2ω2 = 0 and ZS,ω2-ω1 = 0.