OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Microelectronics The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws. OBJECTIVE: Develop vertical photoconductive semiconductor switches (PCSS), which are triggered by a suitable optical source(s). The vertical PCSS should be capable of sub-nanosecond switching and hold off the voltage in excess of 100 kV with the current at least 10 kA. The current conducts via simultaneous multiple current paths (filaments) formed through the bulk of the semiconducting material. Moreover, the jitter associated with multi-formation of current paths (filamentation) should not exceed 20 ps for providing simultaneous switching operations. Also, the objective assembly must have a suitable optical source driver for initiating the PCSS triggering process. Finally, efficient delivery and use of a minimum of optical energy for PCSS triggering are of paramount importance. DESCRIPTION: Many conventional pulse-power systems need ultra-fast switching devices that can operate in high-voltage, high current regimes. A current popular lateral PCSS is normally triggered with above band-gap radiation, which is strongly absorbed (less than 1 micron absorption depth) and which can trigger filaments in a linear array parallel and close to the illuminated surface of the device (10-100 microns). On the other hand, when using sub-band-gap radiation with an exceptionally long absorption depth (many millimeters) to trigger a vertical PCSS, filaments can be formed through the thickness or depth of the device in a two-dimensional array. A 1 cm 1 cm lateral PCSS with a linear array of filaments spaced 300 micrometers apart can support about 33 filaments and a total current which increases linearly with the width of the device. Therefore, the lateral PCSS structure limits total current, performance, and scales. However, the same surface area on a vertical PCSS can support over 1,000 filaments, and a total current which increases with the illuminated surface area of the device. A vertical PCSS, in which current is conducted in filaments perpendicular to the illuminated surface of the device, has an advantage over a lateral PCSS of supporting many more filaments and hence much higher total current per device. With vertical PCSS, the highest fields can be confined to the bulk substrate away from the surface, so higher fields may be held-off and an insulating liquid may not be required. In addition, more benefits with vertical structures are expected. An issue that reduces electric field hold-off is field enhancement at sharp boundaries of conductive and dielectric interfaces. In conventional lateral geometry switches, these sharp interfaces also induce current crowding where the filaments enter the contacts from the semiconductor, causing high current density-induced degradation of the contacts. The surface-normal filament geometry in the proposed vertical switch mitigates this issue, which, in addition to the 2-D scalability of the number of current-sharing filaments, further greatly increases the current-handling capability of the switch. PHASE I: Conduct a feasibility study and design of a single vertical PCSS/Optical trigger assembly, which includes a suitable optical source driver. The design will include the choice of the semiconducting material (e.g, GaN, SiC, or GaAs), bulk topology/dimensions (thickness/length/width) and choice of contact materials, which must be CMOS process compatible. The design must assure high voltage (minimum 100kV), high-current (minimum 10kA) and low jitter (maximum 20 ps) operation. Optical source may include a laser diode(s) or a stand-alone laser (potentially with optical micro-lenses to avoid wasting trigger light outside the optical apertures). While the proposed effort calls for a single vertical PCSS/trigger source assembly, the driver design should be scalable for supporting future synchronized multi-PCSS operation. PHASE II: Build, test, and deliver a fully functional vertical PCSS/trigger source prototype based on the design developed in phase I. Demonstrate the capability to achieve a conduction current pulse width of more than 50 ns. Carry out experimentations in air and insulating liquid, such as Flourinert, in order to compare switch capabilities in two distinct media. Prototypes must be able to carry out a lifetime of 300 shots with the switching current in excess of 1kA. PHASE III DUAL USE APPLICATIONS: The successful completion of Phase II effort will significantly enhance the performance of ultra-fast PCSSs enabling them to operate in a high-voltage, high-current pulse-power environment. Military applications include various fast switch-based microwave sources for directed energy systems, UWB (Ultra-Wideband) pulse sources, and ground penetration radar. Phase III will result in fabrication of a new generation of pulse-power directed energy systems in many areas supporting military and civilian tasks including counter UAS operations, remote immobilization of vehicles and boats, IED neutralization, and non-lethal area denial. In addition, the vertical PCSS can be utilized for the medical imaging technologies as well as Q-switches used in lasers, where high voltage, high current are required. REFERENCES: 1. Mar, A., Zutavern, F., Vawter, G., Hjalmarson, H., Gallegos, R., & Bigman, V. (2016). Electrical breakdown physics in photoconductive semiconductor switches. Sandia National Laboratories, SAND2016-0109. 2. Alan Mar, Fred J. Zutavern, Harold P. Hjalmarson, G. Allen Vawter, Richard Gallegos (2015). Advanced High-Longevity GaAs Photoconductive Semiconductor Switches. DIRECTED ENERGY PROFESSIONAL SOCIETY Seventeenth Annual Directed Energy Symposium 2-5 March 2015 Anaheim, California 3. Hirsch, E.A., Mar, A., Zutavern, F.J., Pickrell, G., Delhotal, J., Gallegos, R., Bigman, V., Teague, J.D. and Lehr, J.M., 2018, June. High-gain persistent nonlinear conductivity in high-voltage gallium nitride photoconductive switches. In 2018 IEEE International Power Modulator and High Voltage Conference (IPMHVC) (pp. 45-50). IEEE. KEYWORDS: PCSS, Vertical PCSS, Photoconductive Semiconducting Switch.
