Analysis of device parameters for pnp-type AlGaAs/GaAs HBTs including high-injection using new direct parameter extraction

Device parameters of the small-signal T equivalent circuit for pnp-type AlGaAs/GaAs heterojunction bipolar transistors (HBTs) are obtained using a new direct parameter extraction technique. These parameters are analyzed not only under the low-current conditions but also under high-current conditions so as to understand the RF-performance fall-off after base pushout occurs. In this analysis, the intrinsic and extrinsic small-signal parameters which affect RF performance are directly determined using several steps without numerical optimization in order to properly analyze device parameters. The T equivalent circuit model determined by the method shows excellent agreement with the mean errors of 3.5-6.9% under both low-and high-current conditions. The analysis showed that the intrinsic transit time, which is the sum of the base transit time (τ_b) and the collector depletion layer transit time (τ_c), small-signal emitter resistance (r_e), small-signal base resistance (r_b) and collector-base capacitance (C_BC) all increase under high-current conditions. In addition, we found that the intrinsic transit time is the dominant parameter for the fall-off of the cut-off frequency (f_t) under high-current conditions, and there is little effect of r_b and C_BC in the fall-off of the maximum oscillation frequency (f_max) under high-current conditions. Judging from these results, device parameters are successfully obtained under a wide current range by a new parameter extraction technique and circuit modeling for HBTs under a wide current range can be achieved using the small-signal T-equivalent circuit