Impact ionization in InP-based HEMTs

A physical model of impact ionization in InP HEMTs is developed and incorporated in an equivalent circuit model for comparison with experimental observations showing excellent agreement. Impact ionization is modeled by a voltage dependent current source in an RC network at the drain end. The RC branch behaves like a low pass filter that imparts a frequency dependence to the impact ionization current source and enables one to model the inductive nature of S/sub 22/ at low frequencies. Theoretical calculations show a compression of transconductance due to impact ionization with increasing gate bias that is supported by experimental data. Finally, an experimental method is suggested to extract impact ionization induced transconductance using Y/sub 21/.