Investigation of InAlN/GaN Circular Transistors for Venus and Other High-Temperature Applications

Abstract

This study examines the performance and long-term reliability of depletion-mode In0.18Al0.82/GaN-on-Si circular high electron mobility transistors (C-HEMTs) in high-temperature environments. Transistors were operated at 472 °C in air and 465 °C under simulated Venus conditions (supercritical CO2, 1348 psi) over 5 days. During heating, a reduction in maximum drain current (I $_{\textit {D}\text {,max}}$ ) and a positive threshold voltage (VTH) shift are observed in both air and Venus surface conditions. The ON/OFF current ratio (I $_{\text {ON} }$ /I $_{\text {OFF} }$ ) and OFF-state gate leakage current (I $_{\textit {G} {, \text {OFF}}}$ ) exhibit unique trends during heating depending on the ambient, yet values upon reaching nominal 468 °C are similar. During the 5-day operation at high-temperature in air, the Mo/Au gate contact demonstrated robust performance with 9% I $_{\textit {D}\text {,max}}$ reduction and 3% VTH shift. I $_{\text {ON} }$ /I $_{\text {OFF} }$ improved by 8%, and I $_{\textit {G} {, \text {OFF}}}$ decreased by 38%. In comparison, device degradation was more significant in supercritical CO2, where the increased permeability affected the OFF-state current. The C-HEMT operated in situ Venus surface conditions exhibited 11% reduction in I $_{\textit {D}\text {,max}}$ , 7% VTH shift, 30% reduction in I $_{\text {ON} }$ /I $_{\text {OFF} }$ , and 23% reduction in I $_{\textit {G} {, \text {OFF}}}$ . Correlation between electrical performance shifts and surface morphology changes, as observed through scanning electron microscopy (SEM) and atomic force microscopy (AFM), provides deeper insights into degradation mechanisms. Despite this, the transistors showed remarkable resilience, and their wide range of available bias points ensures versatile operation for circuit-level implementations in extreme conditions. These findings underscore the suitability of InAlN/GaN HEMTs for uncooled, high-temperature applications without hermetic sealing.