High temperature storage test and its effect on the thermal stability and electrical characteristics of AlGaN/GaN high electron mobility transistors

The high temperature storage (HTS) effects on the thermal stability and electrical characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) were investigated. The step-temperature storage tests in the temperature range from 150 °C to 325 °C were performed to evaluate the reliability of device. It was shown that the electrical properties of devices were remarkably changed as temperature increases. The values of the saturation current and the transconductance ranged from 187 mA to 299 mS/mm (fresh device) to 149 mA and 246 mS/mm (after HTS at 325 °C), respectively. In addition, the threshold voltage was shifted in a positive position and the leakage current was significantly decreased. In order to analyze the fundamental mechanisms of device degradation, we investigated the ohmic contact reliability by the transmission line model and analyzed the gate diode characteristics to estimate the diode parameters. It was found that Ti/Al/Ni/Au ohmic contact were stable and showed good thermal performance, but the Schottky contact was more sensitive to the HTS test. Failure analysis using electrical characterization of the gate diode and TEM/EDX cross-section revealed that the gate metal diffusion phenomenon was main degradation mechanism. During HTS test, Ni diffused out from the metal-semiconductor interface and Au formed a Schottky contact on AlGaN layer. To suppress the gate deformation, we adopted the optimum passivation and demonstrated its effect on the device reliability. By comparing the behavior of gate during HTS test, we clarified that the optimum passivation could suppress the diffusion related degradation mechanism and increase the reliability of AlGaN/GaN HEMTs.