2, University of Arkansas, Fayetteville, Arkansas, United States
3, Avogy, San Jose, California, United States
4, Quora Technology, San Jose, California, United States
5, ARPA-E, Washington, District of Columbia, United States
Vertical gallium nitride (GaN) power semiconductor devices grown on native GaN substrates are emerging as the next frontier in power electronics, with a unipolar figure of merit (UFOM) surpassing that of SiC. This is due to the high critical electric field (EC) of GaN, which is believed to exceed 4 MV/cm. Vertical GaN PiN diodes exhibit high breakdown voltage, low on-resistance, negligible reverse-recovery loss, and avalanche ruggedness. While these properties hold great promise to improve the efficiency and power density of power conversion circuits, little reliability characterization has been reported to date for such devices. As such, this talk reports on the reliability evaluation of 1200 V vertical GaN PiN diodes. Moreover, since hard-switching circuits represent one of the most demanding environments for power semiconductor devices, this work has focused on electrical stress of vertical GaN PiN diodes in a modified double-pulse test circuit (DPTC).
The DPTC, which consists of a power transistor in series with a diode-inductor loop, is commonly used to characterize device switching performance. Typically, a long “charging” pulse is first applied, which is followed by a short characterization pulse (hence the term “double pulse”). However, the ideal DPTC cannot be used in a continuous switching mode, as required for reliability evaluation, since the current through the circuit increases monotonically with the number of switching cycles. The presence of loss in a real DPTC, however, permits the circuit to be operated in a continuous mode with the appropriate choice of frequency and duty cycle. The talk will present an analysis of the requirements necessary to utilize the DTPC in a continuous switching mode for hard-switching reliability evaluation.
Prior to switching reliability evaluation, temperature-dependent current-voltage (I-V) measurements of the vertical GaN PiN diodes were conducted, to establish the baseline performance of the devices. Consistent with previous reports, the breakdown voltage of the diodes exhibited a positive temperature coefficient (i.e., the breakdown voltage was higher at higher temperatures), characteristic of avalanche-induced breakdown. This is critical for circuits requiring avalanche ruggedness and contrasts with lateral GaN power HEMTs, typically grown on Si substrates, that do not exhibit avalanche breakdown. Following the initial I-V characterization, the diodes were subjected to continuous switching stress in the DPTC, which was interrupted periodically to measure forward and reverse I-V curves. The vertical GaN diodes showed minimal degradation in I-V characteristics under a range of stress levels (e.g. supply voltages) tested, indicating good robustness under hard-switching conditions. These results are encouraging for the application of vertical GaN devices in demanding power switching applications.