MODELING OF JUNCTION CAPACITANCES OF GRADED BASE HETEROJUNCTION BIPOLAR-TRANSISTORS

Capacitance models for emitter-base and collector-base junctions of graded base heterojunction bipolar transistors (HBTs) are developed. Effects of base grading on potential barriers, space-charge region (SCR) boundaries, and emitter-base and collector-base junction capacitances are investigated. The emitter-base capacitance is divided into depletion capacitance and capacitance resulting from the mobile carriers in the SCR. To obtain a closed-form solution for the capacitance, intrinsic potential is assumed to be piecewise linear. For the collector-base capacitance, depletion approximation is employed, since the junction is usually reverse-biased in normal operation. It is demonstrated that the present model is quite consistent with the measured data.