Temperature Dependence of Current Gain, Ideality Factor, and Offset Voltage of AlGaAs/GaAs and InGaP/GaAs HBTs

This paper elucidates the stabilities of the dc current gain, ideality factor, and offset voltage against variations in temperature and currents for GaAs-based heterojunction bipolar transistors (HBTs). The InGaP/GaAs HBT markedly reduces the offset voltage and improves the temperature stability of the dc current gain relative to those of AlGaAs/GaAs benchmark devices. The relative insensitivity of the offset voltage in the InGaP/GaAs HBT to the temperature is established by comparison with AlGaAs/GaAs counterparts. One of the more intriguing aspects of these two HBTs is the replacement of the AlGaAs emitter with the InGaP emitter to improve the temperature stability of the offset voltage. Specifically, the effect of the base current on the offset voltage is abnormal in these devices. In this paper, the conventional Ebers-Moll model cannot account completely for our observed phenomena. Accordingly, the results herein (improved temperature stability of the offset voltage in the InGaP emitter HBT and the uncommon base current effect in these two devices) are interpreted in terms of a physics-based analytical model. A strong consistency existed between the experimental data and the theory. Additionally, some reasonable predictions (not obtained from the measured data) of the specific physically based model are presented. The large-signal model provides a description of device performance that will be of use to device engineers and designers.