High-frequency characterization of heterojunction bipolar transistors using numerical simulation

The high-frequency performance of semiconductor devices is estimated using a small-signal numerical calculation based on drift-diffusion equations. In particular, unity current gain frequency in the common-emitter configuration (f/sub T/) and maximum frequency of oscillation (f/sub max/) are calculated for a heterojunction bipolar transistor. f/sub max/ is calculated from numerically obtained y parameters using formulas for maximum available gain, Mason´s invariant (U), and a passivity criterion. They all give the same value for f/sub max/. The influence of extrinsic and intrinsic base resistance on f/sub max/ is investigated for one device design. It is also found that a frequency used approximation formula for f/sub max/ is inaccurate, especially at higher current levels.