Design, characterization, and modeling of gallium nitride based HFETs for millimeter wave and microwave power amplifier applications
GaN Heterostructure Field Effect Transistors (HFETs) have been the subject of intense research over the last decade, and provide exciting opportunities for high power microwave and millimeter wave power amplifiers. While extremely high power densities and efficiencies have been achieved at relatively low microwave frequencies, there are still material and device challenges which prevent the GaN HFETs from being used commercially at higher frequencies.
The work discussed herein attempts to improve transistor power performance at microwave and millimeter wave frequency range by gaining a physical understanding of anomalous device behavior. The work demonstrates that by comparing nominal device characteristics measured using standard techniques (DC, s-parameters) with to pulsed I-V measurements taken at judiciously chosen quiescent bias points, device performance under large single conditions can be inferred.
Physical simulations of GaN HFETs which exhibit good agreement with measurements are described. The effects of layer structure and geometry on device performance are calculated and measured. It is shown that the anomalous transient phenomena collectively known at "current slump" can be accurately simulated by taking into account nonlinear transport of charge along the surface at the drain edge of the gate.
A novel measurement of FET thermal resistance is presented. Using three dimensional heat flow simulations which incorporate temperature dependent thermal conditivities, the thermal characteristics of various GaN HFET layer structures are compared.
Compact-models of GaN HFETs were developed which phenomena logically include anomalous transient behavior. The models accurately reproduced device performance under large signal conditions.