Semiconductor materials with wide bandgaps, including GaN and AlxGa1-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 AlxGa1-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. Figure 1

The objective of this work was to understand the fundamental physics of extremely high frequency RF effects on electronics. To accomplish this objective, we produced models, conducted simulations, and performed measurements to identify the mechanisms of effects as frequency increases into the millimeter-wave regime. Our purpose was to answer the questions, 'What are the tradeoffs between coupling, transmission losses, and device responses as frequency increases?', and, 'How high in frequency do effects on electronic systems continue to occur?' Using full wave electromagnetics codes and a transmission-line/circuit code, we investigated how extremely high-frequency RF propagates on wires and printed circuit board traces. We investigated both field-to-wire coupling and direct illumination of printed circuit boards to determine the significant mechanisms for inducing currents at device terminals. We measured coupling to wires and attenuation along wires for comparison to the simulations, looking at plane-wave coupling as it launches modes onto single and multiconductor structures. We simulated the response of discrete and integrated circuit semiconductor devices to those high-frequency currents and voltages, using SGFramework, the open-source General-purpose Semiconductor Simulator (gss), and Sandia's Charon semiconductor device physics codes. This report documents our findings.

The development and testing of fiber-optically controlled trigger generators (TGs) based on high gain photoconductive semiconductor switches (PCSSs), constructed from high resistivity GaAs, are described in this paper. The TGs are optimized to trigger the high voltage switches (HVSs) in pulsed power systems, where they control the timing synchronization and amplitude variation of multiple pulse forming lines that combine to produce the total system output. Future pulsed power systems are even more dependent on triggering, as they consist of many more HVS and, in some cases, produce shaped pulses by independent timing of the HVS. The goal of the PCSS TG is to improve timing precision and replace high voltage trigger cables or line-of-sight optics with fiber-optic trigger control. The PCSS trigger has independent EMP-free timing control via 200-mum-diameter optical fibers. This design is simpler than other TG because optical isolation allows PCSS triggers to be remotely located near the HVS at any voltage. PCSS can improve the performance of prime power HVS, diverters, and diagnostics by supplying trigger pulses with subnanosecond jitter and rise time that are more precise and easily adjusted than the conventional TG. For pulse-charged HVS, the PCSS TG can generally derive their trigger energy from the stray fields of the HVS. High gain PCSS capabilities for producing pulsed power TG have been demonstrated previously (not all simultaneously): 220 kV, 8 kA, 350-ps rise time, 100-ns pulsewidth, 50-ps rms jitter, and 10-kHz repetition rate. Furthermore, PCSS has previously triggered a 300-kV trigatron with 100-ps rms jitter.

We are developing advanced optically-activated solid-state switch technology for Firing Sets. Advanced switch development at Sandia has demonstrated multi-kA/kV switching and the path for scalability to even higher current/power, resulting in good prospects for sprytron replacement and other even higher current pulsed power switching applications. Realization of this potential requires development of new optical sources/switches based on key Sandia photonic device technologies: vertical-cavity surface-emitting lasers (VCSELs) and photoconductive semiconductor switch (PCSS) devices. The key to increasing the switching capacity of PCSS devices to 5kV/5kA and higher has been to distribute the current in multiple parallel line filaments triggered by an array of high-brightness line-shaped illuminators [Mar, A., et al., 2001]. This was limited by commercial mechanically-stacked edge-emitting lasers, which are difficult to scale and manufacture with the required uniformity. In VCSEL arrays, adjacent lasers utilize identical semiconductor material and are lithographically patterned to the required aspect ratio. However, we have demonstrated that good optical uniformity in rectangular-aperture (e.g. 5-by-500 mum) VCSELs is difficult to achieve due to the lack of optical confinement in the long dimension. We have demonstrated line filament triggering using 1-D VCSEL arrays to approximate line generation. These arrays of uncoupled circular-aperture VCSELs have fill factors ranging from 2% to 50%. Using these arrays, we are developing a better understanding of the illumination requirements for stable triggering of multiple-filament PCSS devices. In particular, we are examining the dependence of filament formation versus the illumination fill factor and spatial brightness along the length of the filament. Ultimately, we will apply effective index techniques, pioneered at Sandia for leaky-mode VCSELs, to create a lateral photonic lattice that selects a single transverse mode with high brightness and uniformity for even higher fill factors and illumination unformity [Zhou, D., et al., 2000]. These sources will be developed and tested with complementary PCSS designs employing interdigitated multifilament contacts for high-power switching.

Firing systems typically incorporate isolation-based architectures that are established by the safety themes of particular weapon systems. Robust electrical diversion barriers are implemented to isolate energy from detonation-critical components until the event of intended use of the system. An optical trigger assembly is being developed to enhance the safety of new firing systems. It couples a fast trigger signal through an exclusion region barrier without compromising the integrity of the barrier in abnormal environment situations. A laser diode generates an optical pulse that is coupled through a sapphire stub to a photoconductive semiconductor switch (PCSS). The PCSS drives a vacuum switch tube to complete the triggering chain in the firing system. A general discussion and comparison of triggering technology options, and the design characteristics and performance parameters of the specific optical trigger point design are presented in this paper.

This report summarizes an investigation of the use of high-gain Photo-Conductive Semiconductor Switch (PCSS) technology for a deployable impulse source. This includes a discussion of viability, packaging, and antennas. High gain GaAs PCSS-based designs offer potential advantages in terms of compactness, repetition rate, and cost.

Mode locked lasers are a compact, inexpensive source of optical pulses. Currently, some problems with semiconductor sources are low output power, self phase modulation, secondary pulse formation, and timing jitter. These problems are addressed here

The first monolithic photonic integrated circuit for all-optical generation of millimeter (mm)-wave electrical signals is reported. The design integrates a mode-locked semiconductor ring diode laser, an optical amplifier, and a high-speed photodetector into a single optical integrated circuit. Signal generation is demonstrated at frequencies of 30, 60, and 90 GHz.

The first monolithic integrated circuit for all-optical generation of millimeter wave signals is reported. The design integrates a mode-locked semiconductor ring diode laser with an optical amplifier and high-speed photodetector into a single optical integrated circuit. Signal generation is demonstration is demonstrated at 30, 60 and 90 GHz frequencies.

Generation of millimeter-wave electronic signals and power is required for high-frequency communication links, RADAR, remote sensing and other applications. However, in the 30 to 300 GHz mm-wave regime, signal sources are bulky and inefficient. All-optical generation of mm-wave signals promises to improve efficiency to as much as 30 to 50 percent with output power as high as 100 mW. All of this may be achieved while taking advantage of the benefits of monolithic integration to reduce the overall size to that of a single semiconductor chip only a fraction of a square centimeter in size. This report summarizes the development of the first monolithically integrated all-optical mm-wave signal generator ever built. The design integrates a mode-locked semiconductor ring diode laser with an optical amplifier and high-speed photodetector into a single optical integrated circuit. Frequency generation is demonstrated at 30, 60 and 90 Ghz.

We report the threshold characteristics of small oxide-confined vertical-cavity surface emitting lasers. Abrupt threshold transitions 105 times the spontaneous emission background are obtained at injection currents as low as 470 nanoampere.