Main Mechanism Responsible for pGaN Gate Breakdown and Lifetime Projection in GaN HEMTs
Han Gao, Angel Espinoza, Ricardo Garcı́a, Siddhesh Gajare, Shengke Zhang
Abstract
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 (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$+ {V}_{\text {GS}}$ </tex-math></inline-formula>) 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {GS}}$ </tex-math></inline-formula> could even reach 12 V; 2) light emission was detected on the gate lines across the entire device by emission microscopy (EMMI) under <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$+ {V}_{\text {GS}}$ </tex-math></inline-formula>, but no emission can be seen under <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- {V}_{\text {GS}}$ </tex-math></inline-formula>; 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$+ {V}_{\text {GS}}$ </tex-math></inline-formula> 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.