Identification of an RF degradation mechanism in GaN based HEMTs triggered by midgap traps

Abstract

Quantitative defect spectroscopy was performed on low gate leakage operational S-band GaN HEMTs before and after RF accelerated life testing (ALT) to investigate and quantify potential connections between the evolution of observed traps and RF output power loss in these HEMTs after stressing. Constant drain current deep level transient spectroscopy and deep level optical spectroscopy (CID-DLTS and CID-DLOS, respectively) were used to interrogate thermally-emitting traps (CID-DLTS) and deeper optically-stimulated traps (CID-DLOS) so that the entire bandgap can be probed systematically before and after ALT. Using drain-controlled CID-DLTS/DLOS, with which traps in the drain access region are resolved, it is found that an increase in the concentration of a broad range of deep states between EC–1.6 to 3.0eV, detected by CID-DLOS, causes a persistent increase in on-resistance of ~0.22Ω-mm, which is a likely source for the 1.2dB reduction in RF output power that was observed after stressing. In contrast, the combined effect of the upper bandgap states at EC–0.57 and EC–0.72eV, observed by CID-DLTS, is responsible for only ~10% of the on-resistance increase. These results demonstrate the importance of discriminating between traps throughout the entire bandgap with regard to the relative roles of individual traps on degradation of GaN HEMTs after ALT.