Semiconductor materials with wide bandgaps, including GaN and Al_xGa_1-xN, offer many performance advantages for power electronic devices compared to conventional Si-based devices. These include larger critical electric fields enabling higher reverse breakdown voltages, fast turn-off and turn-on transitions allowing higher-frequency operation, and high thermal conductivity making higher power density device operation possible. Various device architectures have been demonstrated in these materials, with laterally-oriented Al_xGa_1-xN/GaN high electron mobility transistors (HEMTs) maturing first for radio-frequency applications. More recently, lateral architectures are also starting to be seen in power switching applications. Vertical GaN PN diodes have been demonstrated with excellent forward and reverse electrical behavior including high current capacity, low forward specific on-resistance, high reverse breakdown voltage, and fast reverse recovery time. Selective-area doping control in GaN would enable traditional vertical power devices including merged PN-Schottky (MPS) diodes, junction field effect transistors (JFETs), and more advanced device architectures common in Si and SiC, thereby greatly extending the operating functionality and capability of GaN power devices. This talk will report on our work for two advanced GaN device technologies, including fundamental studies of selective-area doping control and the demonstration of a photoconductive semiconductor switch (PCSS). Both areas of work share the common goal of increasing the power density of switching power converters.

Selective-area doping control using epitaxial regrowth processes by metal-organic chemical-vapor deposition (MOCVD) will be presented. Literature reports of GaN PN diodes where the p-layer is formed by epitaxial regrowth have, to date, indicated lower breakdown voltages and/or higher reverse leakage currents compared to continuously-grown PN junctions. The measurement of a “Si spike” at the regrown interface is commonly observed for GaN using characterization methods such as secondary ion mass spectroscopy (SIMS). Our initial studies to examine possible Si sources and the effects of interfacial Si on electrical performance have indicated that the MOCVD reactor is not a major source of Si. Additionally, intentional Si doping at the interface, with similar levels as measured by SIMS in regrown structures ([Si] = 6E16 cm^-3 and [Si] = 5E17 cm^-3), shows no noticeable degradation in forward- or reverse-bias electrical performance in planar c-plane devices fabricated on GaN substrates (Figure 1). However, regrown GaN diodes formed on m-plane GaN substrates have shown worse electrical performance than c-plane diodes. The increased chemical reactivity of the m-plane and the resulting increase in impurity uptake is likely a contributing factor, and studies are underway to identify the specific impurities responsible. Mitigation strategies to manage the challenges of the m-plane are also being investigated. Further, fabrication-induced damage due to dry-etching processes was shown to increase reverse leakage currents by at least an order of magnitude in planar, c-plane diodes. We will report on these results and the study of various surface treatment methods to reduce these leakage currents in planar, c-plane diodes.

The details of a laterally-oriented, GaN PCSS switch will also be presented. PCSS technology is of interest for applications including high-voltage pulsed-power, electrical grid protection, and increased electrification through new build-outs of high-efficiency/performance power systems. PCSS devices have been demonstrated in several semiconductor materials, but GaN promises an improvement in switch capability due to its advantageous material properties. The device is operated by optically triggering photocarriers in a high-field area generated between two electrodes. Using commercially-available, semi-insulating GaN wafers, surface electrodes were fabricated with a 600 µm gap, and the device was optically triggered using a sub-bandgap 532 nm laser source. Low-voltage (low field) operation of the device demonstrated a linear photoconductive response. Increasing the field over a threshold value of 10-15 kV/cm demonstrated a “high-gain” or “lock-on” operating mode where current continues to flow after the laser source is turned off, if a sufficient field exists (Figure 2). This operating mode has been observed for other semiconductors, including GaAs, and is desirable for high-efficiency switch operation because of the low optical energy needed to turn on the switch. High-gain switching has been demonstrated with PCSS powers of 150 kW (1250 V, 120 A) triggered by optical energies as low as 35 µJ. Further increases in switch power are expected with device improvements.

The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the IDEAS and PNDIODES programs directed by Dr. Isik Kizilyalli. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.