Elimination of the emitter/collector offset voltage in heterojunction bipolar transistors

Heterojunction Bipolar Transistors (HBT´s) exhibit a large collector/emitter offset voltage due to the difference in base/emitter and base/ collector turn-on voltages, We will show that careful design of the base emitter heterojunction can eliminate this problem. A heterojunction bipolar transistor (HBT) varies significantly from a homojunction bipolar transistor because the emitter injection efficiency is determined largely by the heterojunction band gap difference rather than the emitter doping. To obtain high current gain (β) from an Np⁺n HBT it is necessary that a significant amount of the emitter/base heterojunction band gap difference falls across the valence band. In many material systems this is not the case and the use of compositional grading has been introduced in order to achieve this (1).