Analysis of nonlinear behavior of power HBTs

To accurately understand the linear characteristics of a heterojunction bipolar transistor (HBT), we developed an analytical nonlinear HBT model using Volterra-series analysis. The model considers four nonlinear components: r_π, C_diff, C_depl, and g_m. It shows that nonlinearities of r _π and C_diff are almost completely canceled by g _m nonlinearity at all frequencies. The residual g_m nonlinearity is highly degenerated by input circuit impedances. Therefore, r_π, C_diff, C_depl, and g _m nonlinearities generate less harmonics than C_bc nonlinearity. If C_bc is linearized, g_m is the main nonlinear source of HBT, and C_depl becomes very important at a high frequency. The degeneration resistor R_E is more effective than R_B for reducing g_m nonlinearity. This analysis also shows the dependency of the third-order intermodulation (IM3) on the terminations of the source second harmonic impedances. The IM3 of HBT is significantly reduced by setting the second harmonic impedances of Z_S,_2ω2 = 0 and Z_S,_ω2-ω1 = 0