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

Abstract

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 (/spl tau//sub b/) and the collector depletion layer transit time (/spl tau//sub c/), small-signal emitter resistance (r/sub e/), small-signal base resistance (r/sub b/) and collector-base capacitance (C/sub 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/sub t/) under high-current conditions, and there is little effect of r/sub b/ and C/sub BC/ in the fall-off of the maximum oscillation frequency (f/sub 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.