William C. Nunnally

Surface flashover of silicon

We have previously reported the results of experiments based on high-speed shutter and streak photography which show clearly that in surface flashover of silicon in a vacuum ambient the current flows primarily in the silicon, not in the ambient. Here we present scanning electron microscope (SEM) photographs of the surface damage resulting from flashover which show that this current is filamentary. Photomicrographs obtained from samples with diffused p/sup +/ and n/sup +/ contacts show that the contacts exert a strong influence over the flashover characteristics. Finally, we report the results of experiments in which the sample was illuminated with a weak pulse of visible light either before or just after the application of the voltage pulse to the sample. These experiments show that flashover can be inhibited by such a light pulse, and shed light on the relationship between the flashover mechanism and electric field inhomogeneities.

Investigation of a radial transmission line transformer for high‐gradient particle accelerators

This paper discusses the use of a photoconductively switched radial transmission line transformer as a high‐gradient particle accelerator power system. In particular, a 1.8‐m‐radius radial line with a one‐way transit time of 10 ns was fabricated and the voltage attenuation/amplification of a radially traveling wave measured. These results are compared with numerical solutions to the radial‐wave equations using the method of characteristics. The line was operated as a step‐down transformer by driving the inner radius with a pulse generator and measuring open circuit voltage at its outer radius. The voltage attenuation of the transformer agrees fairly well with an approximate theory for short‐pulse propagation along the line (where pulse lengths are roughly one‐tenth the one‐way transit time of the line), and a more accurate computer solution using the method of characteristics has correctly predicted the voltage response of the line. These results imply that moderate voltage, short duration pulses (approxi...

Investigation of photoconductive silicon as a reconfigurable antenna

A reconfigurable antenna whose size, shape, and polarization can be dynamically modified is a desirable component for todays communication and surveillance systems. To date most proposed reconfigurable antenna concepts have used a matrix of conducting elements connected by electronic/optoelectronic switches with the on-off pattern of the switches defining the size or shape of the antenna. An alternative approach for realizing this concept is to exploit the photoconductive effect in various semiconductor materials. This paper reviews the general theory of photoconductivity in semiconductors emphasizing the parameters relevant to this application. The experimental arrangement and the initial results for our proof- of-concept demonstration are then presented. In this experiment a bow-tie shaped transient carrier distribution was created in a 4 inch diameter, high resistivity (6 - 8 k(Omega) -cm) Si wafer through illumination with a pulsed Nd:Glass laser. The voltage amplitudes measured by the transient Si bow-ties (three different wafers) were 45 - 50% of those received in the same environment by an equivalent copper bow-tie antenna designed to operate from 1.4 - 2.1 GHz. Analysis of this initial data indicates that an optimized design incorporating improved semiconductor processing techniques, an optimized feed/balun, soldered interconnects, and impedance matching would significantly improve performance.

An optical technique for measurement of semiconductor surface electric fields

We present an optical technique for the measurement of semiconductor surface electric fields. The measurement technique uses the Kerr electro‐optic effect in nitrobenzene, a phase sensitive interferometer, and associated data acquisition units to measure the surface electric fields between the contacts of a planar semiconductor device. This technique was used to measure the surface fields on silicon devices used in pulsed power applications, but has the potential for use as an electric field probe for any device having high surface electric fields, both pulsed and dc. The measurement technique showed a temporal resolution of 100 ns, which can be easily reduced to a few nanoseconds using superior data acquisition and detection systems. The spatial resolution was about 50 μm for devices that had a typical contact separation of about 500 μm (power devices). This technique can be applied to measure the surface fields on devices commonly used in microelectronic applications. In this paper we discuss in detail ...

Interferometric determination of the quadratic electro-optic coefficient of nitrobenzene

An interferometric method that was used to measure the electro-optic s(11) coefficient of nitrobenzene is presented. The method uses a Mach-Zehnder interferometer arrangement that produces a finite fringe interferogram. A nitrobenzene Kerr cell is placed in one arm of the interferometer and a pulsed high voltage is applied to the two rectangular electrodes. The voltage application causes the fringe image to shift in time, and this fringe shift is measured by a streak camera that streaks the fringe image across a slit. The streak start time is chosen before the application of the voltage pulse and ends after the voltage pulse has been applied. Thus continuous observation of the event is possible. By accurately measuring the fringe shifts the electro-optic s(11) coefficient can be determined. The experimental arrangement is described, and the results obtained are discussed.

Surface field measurement of photoconductive power switches using the electro-optic Kerr effect

We report our investigations of the surface electric fields present between the contacts of an optically controlled semiconductor switch. The experimental arrangement uses the Kerr electrooptic effect to measure the surface fields when a pulsed voltage is applied across a gap between two electrodes on planar samples fabricated on a silicon wafer. The system arrangement measurement technique and preliminary experimental data is presented for deposited aluminium contacts.

Cryogenic Silicon Photoconductive Switches For High Power Lasers

Silicon photoconductive switches have the potential to replace such plasma discharge switches as sparkgaps and thyratrons that are commonly found in pulsed laser drive circuits. This offers the possibility of developing advanced modulators that are all solid-state, and which enjoy the advantages of improved efficiency, compactness, and life expectancy. Silicon operating at liquid nitrogen temperature is especially attractive as a power switch because at 77K it displays an extremely low coefficient of thermal expansion, a large optical absorption depth for 1.06um light, and a large thermal conductivity. These factors allow low temperature silicon to switch power levels an order of magnitude greater than at 300K, and an experimental cyrogenic silicon switch has been made to switch pulses of 15kV, 1.2kA, 0.5uS duration at 100Hz recurrent frequency. It is shown that silicon switches compare favorably with thyratrons in terms of electrical ratings and energy transfer efficiency, and should be considered in advanced pulser designs for both terrestrial and space applications.

Compact, Ultra-Wideband, Impulse Generators for Electro-Optical Applications

The bandwidth limitations of an “ultra-wideband, short-pulse” circuit is quantified by the temporal response of the electronic circuit. The upper bandwidth limit of an electronic circuit is determined by the maximum rate of current change in the circuit which is further defined by the ratio of the circuit voltage to the circuit inductance. Thus, “ultra-wideband” pulse generation requires a large voltage and a small inductance. Once the operating voltage is maximized and the inductance is minimized for a particular system, the circuit temporal response is then limited by charge carrier transit time and stray capacitance in active circuit devices.

Recent developments in the optical measurement of surface fields on silicon photoconductive power switches

We report further investigations of the surface electric fields present between the contacts ofan optically controlled silicon switch. The Kerr electro-optic effect and a phase sensitive interferometric analyzer is used to measure the surface fields when a pulsed high voltage is applied across the contacts. The experimental arrangement has a temporal resolution of 100 nanoseconds and a spatial resolution of 50 microns. The experimental results show preferential field enhancement near the anode and the surface electric fields are nonuniform in space and time. The temporal non-uniformity is more pronounced at higher electric fields.

Switching performance of a cryogenic silicon photoconductive power switch

The silicon photoconductive power switch has the potential to replace gaseous discharge switches like thyratrons, krytrons, ignitrons, and spark gaps, that are commonly found in high‐voltage and high‐current pulse circuits. This offers the possibility of developing advanced laser and radar drive circuits that are all‐solid state, and which enjoy the advantages of improved efficiency, compactness, and life expectancy. Silicon operating at liquid‐nitrogen temperature is especially attractive as a power switch because at 77 K it displays an extremely low coefficient of thermal expansion, a large optical absorption depth for 1.06‐μm light, and a large thermal conductivity. These factors allow low‐temperature silicon to switch power levels an order of magnitude greater than at 300 K, and an experimental cryogenic silicon photoconductive power switch (PCPS) has been made to switch pulses of 15‐kV, 1.2‐kA, 0.5‐μS duration at 100‐Hz recurrent frequency. Temperature and thermal stress profiles generated in the swi...