Impact ionization and quantum confinement effects on the microwave performance of narrow bandgap channel millimeter-wave HFETs

Millimeter-wave heterostructure field-effect transistors (HFETs) incorporate (Ga,In)As channels with increasingly high indium mole fractions to maximize operating frequencies and minimize noise figures. The ever shrinking channel energy gaps render the devices more susceptible to impact ionization effects. Until recently, the effects 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 study, we systematically varied the level of impact ionization in HFETs through the use of quantum confinement 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. The observed trends in the S-parameter data have enabled 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 inadequate when devices operate under high impact.