Main Mechanism Responsible for pGaN Gate Breakdown and Lifetime Projection in GaN HEMTs

This work reports that impact ionization is the main mechanism responsible for p-type gallium nitride (pGaN) gate breakdown in enhancement-mode GaN high-electron-mobility transistors (HEMTs) under forward gate bias ( $+ {V}_{\text {GS}}$ ) stress. This conclusion is supported by the following original results: 1) no failure was measured under -12 V reverse gate bias, but all devices failed before ${V}_{\text {GS}}$ could even reach 12 V; 2) light emission was detected on the gate lines across the entire device by emission microscopy (EMMI) under $+ {V}_{\text {GS}}$ , but no emission can be seen under $- {V}_{\text {GS}}$ ; and 3) physical failure analysis revealed that gate breakdown initiated at the gate metal/pGaN interface, leading to dielectric breakdown due to hole trapping exceeding the critical field of the dielectric layer. Two Weibull slopes identified at lower $+ {V}_{\text {GS}}$ stresses can be explained by the two physical processes originated from the impact ionization mechanism. By understanding the fundamental gate breakdown mechanism, a physics-based gate lifetime model is developed from first principles, which provides a good fit to the measured data points. The model projects low failure rate with 25 years of lifetime, if the gate is operated at the recommended operating condition of 5 V. The projected result demonstrates the excellent gate robustness of pGaN HEMTs.