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/sub /spl pi//, C/sub diff/, C/sub depl/, and g/sub m/. It shows that nonlinearities of r/sub /spl pi// and C/sub diff/ are almost completely canceled by g/sub m/ nonlinearity at all frequencies. The residual g/sub m/ nonlinearity is highly degenerated by input circuit impedances. Therefore, r/sub /spl pi//, C/sub diff/, C/sub depl/, and g/sub m/ nonlinearities generate less harmonics than C/sub bc/ nonlinearity. If C/sub bc/ is linearized, g/sub m/ is the main nonlinear source of HBT, and C/sub depl/ becomes very important at a high frequency. The degeneration resistor R/sub E/ is more effective than R/sub B/ for reducing g/sub 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/sub S/,/sub 2/spl omega/2/ = 0 and Z/sub S/,/sub /spl omega/2-/spl omega/1/ = 0.