Collector‐emitter offset voltage in AlGaAs/GaAs heterojunction bipolar transistors

Factors affecting the collector‐emitter offset voltage in AlGaAs/GaAs heterojunction bipolar transistors have been examined both experimentally and theoretically. In the offset region the collector current, instead of being zero as reported in the literature, is negative and is equal to or less than the base current. The majority of our double heterojunction transistors have negligible offset voltage (50 mV). The prime reason of unusually large offset voltages as observed in some of our devices is found to be the poor quality of the base‐collector (b‐c) junction which is affected by the growth and fabrication processes. The effect of the conduction‐band spike on the offset voltage depends on the compositional grading and in properly graded devices is negligible. For zero offset voltage, high quality of the b‐c junction is important such that its turn‐on voltage is either equal to or higher than that of the emitter‐base junction.