William W. Sullivan

All solid-state high power microwave source with high repetition frequency.

An all solid-state, megawatt-class high power microwave system featuring a silicon carbide (SiC) photoconductive semiconductor switch (PCSS) and a ferrimagnetic-based, coaxial nonlinear transmission line (NLTL) is presented. A 1.62 cm(2), 50 kV 4H-SiC PCSS is hard-switched to produce electrical pulses with 7 ns full width-half max (FWHM) pulse widths at 2 ns risetimes in single shot and burst-mode operation. The PCSS resistance drops to sub-ohm when illuminated with approximately 3 mJ of laser energy at 355 nm (tripled Nd:YAG) in a single pulse. Utilizing a fiber optic based optical delivery system, a laser pulse train of four 7 ns (FWHM) signals was generated at 65 MHz repetition frequency. The resulting electrical pulse train from the PCSS closely follows the optical input and is utilized to feed the NLTL generating microwave pulses with a base microwave-frequency of about 2.1 GHz at 65 MHz pulse repetition frequency (prf). Under typical experimental conditions, the NLTL produces sharpened output risetimes of 120 ps and microwave oscillations at 2-4 GHz that are generated due to damped gyromagnetic precession of the ferrimagnetic materials axially pre-biased magnetic moments. The complete system is discussed in detail with its output matched into 50 Ω, and results covering MHz-prf in burst-mode operation as well as frequency agility in single shot operation are discussed.

A compact 45 kV curve tracer with picoampere current measurement capability.

This paper discusses a compact high voltage curve tracer for high voltage semiconductor device characterization. The system sources up to 3 mA at up to 45 kV in dc conditions. It measures from 328 V to 60 kV with 15 V resolution and from 9.4 pA to 4 mA with 100 fA minimum resolution. Control software for the system is written in Microsoft Visual C# and features real-time measurement control and IV plotting, arc-protection and detection, an electrically isolated universal serial bus interface, and easy data exporting capabilities. The system has survived numerous catastrophic high voltage device-under-test arcing failures with no loss of measurement capability or system damage. Overall sweep times are typically under 2 min, and the curve tracer system was used to characterize the blocking performance of high voltage ceramic capacitors, high voltage silicon carbide photoconductive semiconductor switches, and high voltage coaxial cable.

High Power Lateral Silicon Carbide Photoconductive Semiconductor Switches and Investigation of Degradation Mechanisms

Several generations of high power, lateral, linear mode, intrinsically triggered 4H-SiC photoconductive semiconductor switch designs and their performance are presented. These switches were fabricated from high purity semi-insulating 4H-SiC samples measuring 12.7 mm × 12.7 mm × 0.36 mm and were able to block dc electric fields up to 370 kV/cm with leakage currents less than 10 μA without failure. Switching voltages and current s up to 26 kV and 450 A were achieved with these devices and ON-state resistances of 2 Ω were achieved with 1 mJ of 355 nm laser energy (7 ns FWHM). After fewer than 100 high power switching cycles, these devices exhibited cracks near the metal/SiC interface. Experimental and simulation results investigating the root cause of this failure mechanism are also presented. These results strongly suggest that a transient spike in the magnitude of the electric field at the metal/SiC interface during both switch closing and opening is the dominant cause of the observed cracking.

Performance and optimization of a 50 kV silicon carbide photoconductive semiconductor switch for pulsed power applications

A 50 kV silicon carbide photoconductive semiconductor switch (PCSS) is presented. The SiC PCSS device is fabricated from semi-insulating 4H-SiC in a newly-proposed rear-illuminated, radial switch structure. The improved structure reduces the peak electric field within the switch, extending the blocking voltage to over 50 kVdc. Electrostatic field simulations of the PCSS are presented along with experimental blocking curves. The PCSS demonstrated low on-state resistance, delivering over 27 MW of peak power into a 31 Ω load. Device modeling was performed to further optimize the switch for peak efficiency when illuminated with 355 nm light, a common laser wavelength. The switch structure was modified for peak operation at 355 nm and the experimental and theoretical results are compared.

Shallow Incorporation of Nitrogen in HPSI 4H-SiC through the Laser Enhanced Diffusion Process

This paper investigates n-type doping of point-defect compensated high purity semi-insulating (HPSI) 4H-SiC using a pulsed laser (10 ns FWHM @ 260 nm) for the introduction of nitrogen to shallow depths. A thermal model is presented using COMSOL Multiphysics featuring nonlinear temperature dependent material properties and a volumetric heat source term that takes into account the laser absorption depth for common ultraviolet irradiating wavelengths. The temperature distribution in the material and the amount of time that the surface and near-surface regions are at high temperature determines how many laser pulses are required to dope to the desired depth, and simulation results are presented and fit to measured data. The simulations and measured data show that for shallow doping a short wavelength ultraviolet laser should be used to localize the heat at the surface so the dopant can’t diffuse deep into the material. The laser enhanced diffusion process has been used to incorporate nitrogen into HPSI 4H-SiC with a measured surface concentration greater than 1020 cm-3 and a nonlinear thermal model was built.

Characterization of Annealed HPSI 4H-SiC for Photoconductive Semiconductor Switches

Annealing of high purity semi-insulating (HPSI) 4H-SiC is investigated as a method to improve bulk photoconductive semiconductor switches through recombination lifetime modification. Five samples of HPSI 4H-SiC were annealed at 1810 °C for lengths of time ranging from 3 to 300 minutes. The recombination lifetime of the unannealed and annealed samples was measured using a contactless microwave photoconductivity decay (MPCD) system. The MPCD system consists of a 35 GHz continuous microwave probe and a tripled Nd:YAG pulsed laser. The recombination lifetime was increased from 6 ns, as received, up to 185 ns by annealing for 300 minutes. To experimentally verify switch improvements, identical switches from unannealed and annealed material were fabricated and tested at low voltage. The unannealed device generated a 15 ns pulse with a 2 ns rise-time. The annealed device conducted for upwards of 300 ns with a comparable 2 ns rise-time. The increased recombination lifetime resulted in lower on-state resistance and increased energy transfer.

Performance and characterization of a 20 kV, contact face illuminated, silicon carbide photoconductive semiconductor switch for pulsed power applications

A 20 kV, lateral geometry, contact face illuminated, silicon carbide (SiC) photoconductive semiconductor switch (PCSS) is presented. The SiC PCSS was fabricated from high purity semi-insulating, bulk 4H-SiC (12.7 mm × 12.7 mm × 0.35 mm), in a lateral geometry, with both the anode and cathode contacts located on the same face of the device. The device was illuminated with light from a tripled Nd:YAG laser (355 nm-7 ns FWHM) entering from the contact face. The device demonstrated sub-ohm on-state resistance for laser pulse energies in the mJ range, and micro-ampere leakage currents at 20 kVdc in the off-state. Voltage hold-off and low leakage currents in the off state were achieved through high energy electron beam irradiation of the bulk material. The switchs geometry and packaging are discussed, along with experimental switching and blocking characteristics.

The effects of sub-contact nitrogen doping on silicon carbide photoconductive semiconductor switches

Forming non-rectifying (ohmic) contacts to wide band gap semiconductors such as silicon carbide (SiC) requires a heavily doped subsurface layer to reduce the Schottky barrier height and allow efficient electron injection. Nitrogen, a common n-type dopant in SiC, was incorporated into a SiC sample using a laser enhanced diffusion process in which an impurity is incorporated into the semiconductor to very high surface concentrations (> 1020 cm-3) and very shallow depths (<; 200 nm) with the use of a pulsed 266 nm laser. This paper evaluates the effects of nitrogen introduced through laser enhanced diffusion on the contact formation and the efficiency of silicon carbide photoconductive switches at low and high injection levels under different biasing conditions. Nine lateral switches were fabricated on a high-purity semi-insulating 4H-SiC sample; three with no sub-contact doping, three with sub-contact doping on only one contact, and three with sub-contact doping on both contacts. Results are presented for tests under pulsed laser illumination with sub-contact doping on only the anode, only the cathode, neither, and on both of the contacts.

Shock Wave Simulation of Ferrite-Filled Coaxial Nonlinear Transmission Lines

Ferrite-filled coaxial shock lines have recently been used to significantly decrease the rise time of a high voltage pulse. This decrease can be enhanced by initially axially biasing the ferrite material with an applied external magnetic field, allowing for a faster transition from the unsaturated to the saturated state. The simulation of the ferrite materials operation, including saturation, is discussed as well as the simulation of coaxial nonlinear transmission lines. The project explores the rise time changes with variations of magnetic bias, ferrite geometry, input signal characteristics, and transmission line characteristics. Simulated waveforms are discussed for a nickel-zinc ferrite-filled coaxial line. The pulse steepening effect observed in electromagnetic shock lines occurs primarily because of an increase in phase velocity for points higher on the waveform due to the saturation of the ferrite material. An incident pulse of high enough amplitude will drive the ferrite material into saturation, decreasing the relative permeability to one. This saturation front propagates through the ferrite material in the direction of the incident wave until the entire material is saturated, producing a sub-nanosecond rise time pulse. The shock line is designed for a saturated impedance of 50 Ohms to couple easily into existing systems. Pulsed operation of up to low kilohertz repetition is desired and being explored. Applications of electromagnetic shock lines include laser triggering and ultra-wideband radar generation, as well as others.

Evaluation of a Pulsed Ultraviolet Light-Emitting Diode for Triggering Photoconductive Semiconductor Switches

The power output, forward voltage, conversion efficiency, and spectral characteristics of a 365 nm ultraviolet light-emitting diode (LED) were measured for applications of triggering wide-bandgap photoconductive switches for pulsed power applications. Pulsed currents through the LED ranged from 125 mA up to 2.2 A at widths from 10 μs up to several seconds. Using time-resolved electroluminescence spectroscopy, peak emission was observed to occur at 368.5 nm for short pulses with a red-shift to 371.8 nm for pulses 8 s in duration. A peak light output of 4.1 W was measured for short pulses (<;50 μs) of 2.12 A, corresponding to six times the rated output specification. The LED was used to trigger a high-voltage photoconductive semiconductor switch (PCSS) at voltages up to 6 kV into a high-impedance load. The 365 nm LED is a promising candidate for optical triggering of PCSS devices.