Theoretical and experimental investigation of the collector-emitter offset voltage 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, substrate temperature, and device geometry has been investigated. We found that V-ceoff decreases at moderate base current (I-b) and begins to increase at very 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-collector to the base-emitter junction areas, as previously reported, Further,more, the measured data do not agree with the classical expression of V-ceoff derived from the Ebers-Moll equations of bipolar junction transistor (BJT). Therefore, from the literature, an alternative expression is used, which provides more insight into the physics of HBT and is demonstrated to agree very well with the experimental data.