GaN HEMTs in High-Frequency Overvoltage Switching: Electrical or Thermal Failure?

Abstract

GaN high-electron-mobility transistors (HEMTs) have no avalanche capability and are known to fail electrically when the transient overvoltage reaches their dynamic breakdown voltage (BVDYN). This work explores the failure mechanisms of GaN HEMTs in continuous overvoltage switching at high frequency up to 1 MHz. By employing a soft-switched converter based testbed, the BVDYN and failure mechanisms of three commercial GaN HEMTs with different device structures and voltage ratings are studied. It is found that two devices under test (DUTs) show the expected electrical failure at a consistent BVDYN under different switching frequencies. A new failure behavior is observed in the third DUT, where the electrical failure dominates at low frequencies and the thermal failure occurs at high frequencies over 200 kHz with an overvoltage boundary lower than BVDYN. This failure mechanism is found to be induced by a drastic, nearly-unrecoverable on-resistance (RON) increase during the high frequency overvoltage switching, which induces the thermal runaway in continuous converter operation. This RON increase is believed to originate from series electron trapping in the device structure. These results reveal a new overvoltage failure mode of GaN HEMTs and a new overvoltage boundary lower than the BVDYN in high-frequency overvoltage switching.