Gallium nitride based high electron mobility transistors (HEMTs) have become attractive for use in high power and high frequency applications due to large energy band-gaps, great electron mobility, high breakdown voltages, and considerable 2-D electron gas densities as compared to its GaAs counterpart . As AlGaN/GaN HEMTs gain traction for use in commercial and military applications, it has become increasingly critical for reliability studies to extend beyond the normal temperature-accelerated life tests. The failure rate at operating temperatures is extrapolated from the failure rate under thermally accelerated stress conditions through the Arrhenius equation. This equation is limited to expressing acceleration of failure mechanisms that are due to physio-chemical reactions. With the bulk of reliability predictions focusing on Arrhenius extrapolations of device lifetime, they do not capture failure mechanisms that govern degradation under dc or RF operation that cannot be accelerated by elevated temperatures.
Employment of GaN HEMTs for high power radar systems will require devices to be subjected to large-signal RF while being driven into saturation, resulting in devices experiencing high electric fields and high current densities. Impressive mean-time-to-failure values of greater than 107 hours have been reported at operating temperatures below 200 °C, with activation energies ranging from 0.18 eV to 2 eV. However, these studies neglect the effect of high electric field and current on device lifetime. Additionally as the demand increases for faster data communication at higher frequencies, scaling of the gate will be driven below 0.2 µm (4). In particular, we have investigated field-driven failure mechanisms that may limit the device lifetime at normal operating temperatures.
As the device design and material processing technology for AlGaN/GaN HEMTs has matured over the years, several failure mechanisms that limited device lifetime have been addressed and improved. However, there are still numerous degradation mechanisms that have been reported to plague AlGaN/GaN HEMTs. These mechanisms can be grouped together into three main categories that affect lifetime: contact degradation, hot electron effect, and inverse piezoelectric effect. Both Schottky and Ohmic contacts have shown excellent stability below 300 °C. There are likely multiple degradation drivers present under most stress conditions, and future tests will be required to isolate the effect of current, voltage and temperature under high power conditions.
S. J. Pearton and F. Ren
Stephen Pearton is a researcher at the University of Florida, USA, and a member of the Materials Today Editorial Board.