Impact of Epi-Layer Quality on Reliability of GaN/AlGaN/GaN Heterostructure Field-Effect Transistors on Si Substrate

This paper reports the impact of epi-layer quality on the short-term reliability of GaN/AlGaN/GaN heterostructure field-effect transistors fabricated on Si substrates. At an early stage of 50 V high-temperature operating life tests, they exhibited a significant lowering of the forward breakdown voltage. The mean time to failure (MTTF) determined by this degradation mode was exponentially increased by decreasing full-width at half-maximum (FWHM) values of X-ray rocking curves. In small FWHM samples, extrapolated MTTF values exceeded 1×10⁷ h at T_ch=150°C. Alternatively, another degradation mode such as drain current degradation will dominate the long-term reliability at this temperature range. High-resolution transmission electron microscope and energy dispersive X-ray spectroscopy analyses revealed thinning or partial vanishment of GaN cap layer in a degraded sample. Gate current-voltage characteristics calculation based on the self-consistent Schrödinger-Poisson analysis suggested lowering of the effective barrier height by decreasing cap layer thickness is responsible for the Schottky degradation. When considering these experimental and theoretical findings, degradation mechanisms assuming diffusion of interfacial materials along dislocations are discussed.