THERMAL-STABILITY ANALYSIS OF ALGAAS GAAS HETEROJUNCTION BIPOLAR-TRANSISTORS WITH MULTIPLE EMITTER FINGERS/

A numerical electro-thermal model was developed for AlGaAs/GaAs hetero junction bipolar transistors (HBT's) to describe the base current, cur rent gain and output power dependence on junction temperature. The mod el is applied to microwave HBT devices with multi-emitter fingers. The calculated results of the common-emitter, current-voltage characteris tics in the linear active region show a ''current crush'' effect due t o inherent nonuniform junction temperature, current density and curren t gain distribution in the device. The formation of highly localized h igh temperature regions, i.e., hot spots, occur when the device is ope rating beyond the current-crush point. This thermally induced current instability imposes an upper limit on the power capability of HBT's. T he dependence of this effect on various factors is discussed. These fa ctors include the intrinsic parameters such as the base current ideali ty factor, the ''apparent'' valence band discontinuity, and the temper ature coefficient of the emitter-base turn-on voltage, as well as the extrinsic factors such as the emitter contact specific resistance, the substrate thermal conductivity and the heat source layout.