Impact ionization effects on the microwave performance of InAs channel HFETs: the role of channel quantization

Millimeter-wave heterostructure field-effect transistors (HFETs) incorporate (Ga,In)As channels with increasingly higher indium mole fractions to maximize operating frequencies and minimize noise figures. The shrinking channel energy gaps make the devices more susceptible to impact ionization. Until recently, the consequences of impact ionization on the microwave performance of narrow bandgap channel HFETs have largely been ignored. In the present work, we studied the effects of impact ionization and quantum confinement on the microwave characteristics of narrow bandgap InAs/AlSb millimeter-wave HFETs. In the course of this work, we systematically varied the level of impact ionization in HFETs through channel quantization and bias conditions, and compared the microwave behavior of InAs/AlSb HFETs (with different quantum well widths) to that of conventional GaAs MESFETs also fabricated in our laboratory to provide a low ionization benchmark. The observed trends in the S-parameter data enable us to identify the physical effects of impact ionization on the microwave behavior of narrow-gap channel HFETs. We found that the conventional FET equivalent circuit model is grossly inadequate when our InAs channel HFETs operate under high impact ionization conditions. Attempts to model InAs/AlSb HFETs with a recently developed ionization-augmented FET model fail when the devices are operated under high impact ionization conditions.