Theoretical and experimental investigation of the collector-emitter offsetvoltage of AlGaAs/GaAs heterojunction bipolar transistors

The dependence of the collector-emitter offset voltage (V-ceoff) of AlGaAs/ GaAs heterojunction bipolar transistors (HBT's) on the base current, substr ate temperature, and device geometry has been investigated. We found that V -ceoff decreases at moderate base current (I-b) and begins to increase at v ery high I-b. Moreover, V-ceoff increases linearly with the temperature and logarithmically with the ratio of the base-collector junction perimeter to the base-emitter junction area, rather than with the ratio of the base-col lector to the base-emitter junction areas, as previously reported, Further, more, the measured data do not agree with the classical expression of V-ceo ff derived from the Ebers-Moll equations of bipolar junction transistor (BJ T). Therefore, from the literature, an alternative expression is used, whic h provides more insight into the physics of HBT and is demonstrated to agre e very well with the experimental data.