DELAY OF KIRK EFFECT DUE TO COLLECTOR CURRENT SPREADING IN HETEROJUNCTION BIPOLAR-TRANSISTORS

In this work, we present experimental evidence and develop a simple th eory for the delay of base push out (Kirk effect) due to collector cur rent spreading in heterojunction bipolar transistors (HBT's). This eff ect is very pronounced in small area devices even with short collector s. A correction factor relating the observed emitter current density a t which peak cut-off is observed to the classical Kirk effect current limit is derived. This theory has very good agreement with measured da ta for several different a epitaxial structures and has important impl ications for the design of both digital and microwave transistors and circuits.