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

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<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\textit {D}\text {,max}}$ </tex-math></inline-formula>) and a positive threshold voltage (VTH) shift are observed in both air and Venus surface conditions. The <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</small>/<sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</small> current ratio (I<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text {ON} }$ </tex-math></inline-formula>/I<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text {OFF} }$ </tex-math></inline-formula>) and <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</small>-state gate leakage current (I<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\textit {G} {, \text {OFF}}}$ </tex-math></inline-formula>) 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<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\textit {D}\text {,max}}$ </tex-math></inline-formula> reduction and 3% VTH shift. I<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text {ON} }$ </tex-math></inline-formula>/I<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text {OFF} }$ </tex-math></inline-formula> improved by 8%, and I<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\textit {G} {, \text {OFF}}}$ </tex-math></inline-formula> decreased by 38%. In comparison, device degradation was more significant in supercritical CO2, where the increased permeability affected the <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</small>-state current. The C-HEMT operated in situ Venus surface conditions exhibited 11% reduction in I<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\textit {D}\text {,max}}$ </tex-math></inline-formula>, 7% VTH shift, 30% reduction in I<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text {ON} }$ </tex-math></inline-formula>/I<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text {OFF} }$ </tex-math></inline-formula>, and 23% reduction in I<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\textit {G} {, \text {OFF}}}$ </tex-math></inline-formula>. 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.