Delay of Kirk effect due to collector current spreading in heterojunction bipolar transistors

In this work, we present experimental evidence and develop a simple theory for the delay of base push out (Kirk effect) due to collector current spreading in heterojunction bipolar transistors (HBT's). This effect is very pronounced in small area devices even with short collectors. A correction factor relating the observed emitter current density at which peak cut-off is observed to the classical Kirk effect current limit is derived. This theory has very good agreement with measured data for several different a epitaxial structures and has important implications for the design of both digital and microwave transistors and circuits.