The electric fields of bulk (100) and surface (111) GaAs high-voltage photoconductive switches were imaged utilizing the electro-optic effect of the semi-insulating GaAs substrate. Experimental methodology for obtaining the images is described along with a self-calibrating data reduction algorithm. Use of the technique for observing fabrication defects and time dependent field nonuniformities is shown.

In this paper we report on the experimentation conducted on vertical optically activated switches fabricated from GaAs grown by Liquid Encapsulated Czochralski (LEC) and Vertical Gradient Freeze (VGF) techniques. Heavily doped contact regions have been grown on the bulk GaAs to form n⁺-SI GaAs-n⁺ and p⁺-SI GaAs-n⁺ structures. Dark dc I-V characterization has been used to assess the voltage withstand characteristics of the devices demonstrating 3.5 kV hold-off for reverse biased LEC and VGF p⁺-SI GaAs-n⁺ devices. Optical activation has achieved a 1 ns switch closure time with VGF p⁺-SI GaAs-n⁺ devices reverse biased at 7 kV and 2 ns switch closure times for VGF n⁺-SI GaAs-n⁺ devices biased at 4.5 kV. The voltage drop across the optically activated switches was characterized in terms of two components; a constant and a resistive voltage drop.

The time evolution of the current filaments in an optically triggered, high gain GaAs switch was studied by recording the infrared photoluminescence from the filaments. When the switch is triggered with two laser diode arrays (through a fiber optic) that are activated within 1 ns of each other, two current filaments are observed, each one emanating from the point of illumination. By delaying one laser with respect to the other, the evolution of the filament was recorded in a time resolved fashion. The first filament that is triggered crosses the switch, the voltage drops and the other filament ceases to grow. By varying the delay between the trigger lasers, the tip velocity is measured to be up to 5.9 +/- 1 X 10⁹ cm/s. This speed is 600 times larger than the peak drift velocity of carriers in GaAs. This observation supports switching models that rely on carrier generation at the tip of the filament. The filaments speed up as they cross the switch: for one voltage range initial speeds were 0.7 +/- 1 X 10⁹ cm/s and final speeds (the last 100 ps of motion) exceed 5.5 +/- 1 X 10⁹ cm/s. This experiment also shows a relationship between the rise time of the voltage across the switch and the required trigger energy and switch jitter.

A simple transmission line model, which seeks to explain fast conductivity phenomena in semiconductors, such as photoconductivity or avalanching (induced by either light or displacement current waves), is proposed. The model relies on breaking up the semiconductor drift space into small cells, each of which contains an imaginary transmission line element so as to allow an electromagnetic wave to propagate away from the generated plasma. The same transmission line may be used to convey light energy produced in the semiconductor. The transmission line also serves as the energy storage element. Time varying nodal resistors, located at the transmission line junctions, control the conductivity. The nodal resistors embody the physics of the semiconductor, whereas the transmission line matrix accounts for energy spread. Slower semiconductor mechanisms, such as carrier drift, may be easily incorporated into the formalism, if necessary. The model points out the importance of triggering either an avalanche or displacement current wave in regions where the static field is high. Under certain conditions the model predicts a growing electromagnetic wave with sufficient amplitude to sustain avalanching.

We report on the intensity dependent supralinear photoconductivity in GaAs:Si:Cu material. The results of our measurements show that the effective carrier lifetime can change over two orders of magnitude with variations in the intensity of the optical excitation. Numerical simulations have also been carried out to analyze the effect. The intensity dependent lifetimes obtained from the simulations match the experiments very well. Such a nonlinear intensity dependence could have possible low-energy phototransistor applications.

The prefiring electric field distribution within a bulk avalanche semiconductor switch has a strong bearing upon the propagation of current filaments that form during the conduction state, the voltage breakdown limits for the off-state, and thus the overall peak power scalability of the switch. In this paper, we apply a detailed numerical model for semi-insulating GaAs which includes the full electronic structure of the deep levels to calculate the internal field distribution under prefiring bias voltages. We demonstrate that the electric field is far from homogeneous, with a potential barrier being formed at the cathode end, and the maximum field occurring at the anode end. The more specific details of this electric field distribution are found to depend greatly upon the type of semi-insulating compensation mechanism present in the GaAs substrate. This electric field distribution agrees qualitatively with that determined by a novel optical imaging technique based upon the Franz-Keldysh effect.

This paper presents simulation results for the dc and transient characteristics of the n(superscript +-i-n(superscript + photoconductive diode at room and cryogenic temperatures fabricated in silicon and in silicon carbide. The simulation is carried out using a two-dimensional device simulator called MEDICI which can solve numerically the Poisson equation, electron and hole current equations, electron and hole continuity equations, as well as heat transfer equation. Relevant physical mechanisms, such as lattice heating, Fermi-Dirac statistics, high-field and doping-dependent free-carrier mobility, and various generation and recombination mechanisms are accounted for in the simulation.

We present simulations of time-dependent filament propagation in laser- triggered GaAs photoswitches. Unlike previous modeling, our calculations are self-consistent in 2D axisymmetric (r-z) geometry. Realistic electron and hole mobilities as well as field dependent impact ionization are included. We observe filament propagation with speeds Uf approximately equals (formula available in paper), much larger than the saturated carrier drift velocity, u^satapproximately equals 10⁷ cm/s. The self-consistently determined filament radius and carrier number density are typically Rfapproximately equals 20-60 micrometers and nfAPEQ10(formula available in paper) respectively. Results are presented for filament propagation in systems with both uniform and nonuniform profiles of background carrier density and electric field.

An electrical pulse-generation system using two optically activated Si photoconductive switches can generate shaped electrical pulses with multigigahertz bandwidth. The Si switches are activated by an optical pulse whose leading edge is steepened by stimulated Brillouin scattering (SBS) in CCl₄. With the bandwidth generated by the SBS process, a laser having a 1- to 3-ns pulse width is used to generate electrical pulses with approximately 80-ps rise times (approximately 4-GHz bandwidth). Variable impedance microstrip lines are used to generate complex electrical waveforms that can be transferred to a matched load with minimal loss of bandwidth.

A disadvantage of photoconductors, based on polycrystalline layers, is that when high electric fields are applied complex processes occur which are believed to be associated with thermal quenching of photoconductivity and inconsistencies in the physical properties of the layer, which lead to the formation of high electric field domains and subsequent 'hot spotting'. These effects have previously severely limited the ability of the material to cope with high electric fields and current densities. The authors have developed a process based on thick film technology, to produce sintered polycrystalline layers of solid solutions of CdSe, CdS and Cd.Te, which suppresses the formation of these high electric field domains, thus allowing the materials to be operated close to their practical electric field strengths. The photoconductivity, practical electric field strengths, time and temperature response of this materials is described. This material can be scaled to produce films ranging from a few square millimeters to several square meters in area. Thus making possible the industrial production of low cost bulk photoconductive switches, based on this form of the material, which will be capable of switching kA from kV sources.

A multigigahertz microwave signal was generated and up-converted in a GaAs substrate coplanar strip line. Two 630-nm-wavelength laser pulses-one with a normal wavefront, another with a titled wavefront- were respectively used to generate and frequency up-convert the signal. The relativistic plasma front, induced by the tilted optical wavefront, frequency up-shifts the counter-propagating electromagnetic wave via the Doppler effect. When the speed of the plasma front was about 0.4 times the speed of electromagnetic wave in the coplanar strip lines, the experiments showed that the fall time of a step signal decreased more than 30% after the reflection. Given the bandwidth limitations of the data acquisition system, it is possible that a factor of 2 increase was achieved. A transmission line model was employed to simulate this process. The simulation results were consistent with experimental observations. Using coplanar strip lines on a GaAs substrate for microwave signal compression device has the advantage of a high reflection coefficient, frequency tunability, small laser trigger energy, and all-solid-state construction, making this technique suitable for impulse radar applications.

We will discuss here our efforts to fabricate and test new SiC opto-electronic high voltage switches. We report the ultrafast switching of novel silicon carbide devices using an optical trigger. The switching properties of both commercial SiC pn diodes and in-house fabricated SiC thyristors were investigated. Subnanosecond risetime was observed with both devices. A comparison of SiC pn diode and thyristor switching shows that the thyristor has the highest switching speed and efficiency, and triggers with the least optical energy. We report the first optical triggering of a silicon carbide thyristor into the latched-on state. This switching is characteristic of electrically triggered thyristors, however the optical triggering produced a significantly faster risetime, 370 picosecond, than is possible with electronic triggering. The thyristor switched 100 volts bias with 96% efficiency, corresponding to a device on-state of 4 volts at an average current density of 750 A/cm². The singular advantages of optical triggering, isolation from the trigger source, synchronized triggering of stacked devices, and switching speed, are highly desirable for high voltage, high power operation of conventional power devices. These results provide enabling technology for high-repetition rate, high-voltage impulse generators and expedite the development of high-power electrically triggered silicon carbide devices.

We show the feasibility of an optically controlled silicon Bipolar Mode Field Effect Transistor (BMFET) to switch high electrical powers up to 1 kW by exploiting a 25 mW laser diode. We have compared the performance of the BMFET with that of a commercial high power bipolar junction transistor (BJT). Under the same illumination conditions, the switching power of the BMFET has been always higher than that of the BJT by a factor greater than one order of magnitude.

Screening currents in a superconducting thin film will exclude a perpendicular magnetic field, produced either by a coil or a magnet. When the film is driven into the normal state by a fast optical pulse, the screening currents decay, allowing flux to enter. The process of flux entry can be observed by measuring the induced voltage across a coil closely coupled to the film. This is equivalent to the operation of an opening switch. This concept has been experimentally demonstrated in low and high magnetic fields, corresponding to the reversible and irreversible regimes, using 500-nm thick YBa₂Cu₃O_7-x films and a 1.064-micrometers , 150-ps pulsed laser. This contactless, inductive switching scheme can be used in energy extraction from superconducting magnetic energy storage (SMES). The switch design and theoretical analysis of current redistribution in optically irradiated YBa₂Cu₃O_7-x thin films are discussed in this paper.

Recent high-power, subnanosecond-switching results of the Bistable Optically controlled Semiconductor Switch (BOSS) are presented. The processes of persistent photoconductivity followed by photo-quenching have been demonstrated at megawatt power levels in copper-compensated, silicon-doped, semi-insulating gallium arsenide. These processes allow a switch to be developed that can be closed by the application of one laser pulse ((lambda) equals 1.06 micrometers ) and opened by the application of a second laser pulse with a wavelength equal to twice that of the first laser ((lambda) equals 2.13 micrometers ). Switch closure is primarily achieved by elevating electrons from a deep copper center which has been diffused into the material. The opening phase is a two-step process which relies initially on the absorption of the 2-micrometers laser causing electrons to be elevated from the valance band back into the copper center, and finally on the recombination of electrons in the conduction band with boles in the valance band. The second step requires a sufficient concentration of recombination center (RC) in the material for opening to occur in the subnanosecond regime. These RC's are generated in the bulk GaAs material by fast-neutron irradiation (approximately equals 1-MeV) at a fluence of about 3 X 10¹⁵ cm^-2. Neutron-irradiated BOSS devices have been opened against a rising average electric field of about 36 kV/cm (18 kV) in a time less than one nanosecond while operating at a repetition rate, within a two-pulse burst, of about 1 Ghz.

This paper presents results from three areas of GaAs PCSS research and development: device lifetime, high current switching, and PCSS-driven laser diode arrays (LDA). We have performed device lifetime tests on both lateral and vertical switches as a function of current amplitude, pulse width, and charging time. At present, our longest-lived switch reached 4 X 10⁶ pulses. Scanning electron microscope (SEM) images show damage near the contacts even after only 5 pulses. We are presently searching for the threshold at which no damage is evident after a single shot. In high current tests, we have reached 5.2 kA at 4.2 kV. This was achieved using twenty fiber-optic coupled lasers to distribute current filaments over a 5 mm wide PCSS. Current waveforms and images of the current filaments as a function of current amplitude will be presented. The lasers used to trigger the high current PCSS were driven with a miniature PCSS. Low inductance, high speed GaAs PCSS are very effective as short pulse laser diode array drivers. Some types of arrays gain switch, producing a compressed optical pulse which is only 57 ps wide. Results from tests with a variety of laser diode arrays will be presented.

Spontaneous breakdown is generally considered as one of the most prominent failure modes of optically activated power switches, and may very well obstruct further improvement of these devices. In recent years, considerable progress has been made in describing the phenomenological features of this process; in particular there seems to be consensus that it is intimately connected to the onset of current filamentation. A full understanding of its underlying mechanisms, however, has not yet been established. In this manuscript, we focus on the role that geometrical effects play in the dynamics of spontaneous breakdown. In contrast to previous simulations (which tried to implement a realistic description of the switch physics but had to impose artificial symmetries), we represent the device by a rather simple model but allow for a fully three-dimensional evolution. Our results indicate that geometry effects are indeed important for an understanding of spontaneous breakdown: Both qualitatively and quantitatively, the behavior of the three- dimensional switch model differs from that of similar but lower-dimensional descriptions.

Different types of damages induced in photoconductive silicon at high fields are discussed on the basis of the corresponding physical processes developed in the semiconductor - ambient dielectric system. Catastrophic damages produced at high fields by surface flashover or total bulk breakdown are compared with surface filaments developed in certain conditions in the prebreakdown stage. A physical mechanism of nonohmic conduction inducing surface filaments is proposed. A new high field limitation, well before total breakdown, is defined in the dark condition by the onset field of the nonohmic regime. Below this field the device operation is safe and stable, while beyond this limit, a surface filament, distinctly different from the surface flashover tracks, may appear on the surface of the device, permanently degrading the device quality. Along with the surface filaments, contact damages may also appear in the device in this nonohmic regime. The semiconductor damages are observed using SEM and optical micrographs. The mechanisms leading to these damages are discussed using electrical and optical characteristics at high fields, as well as the localization of light emissions. The practical importance of the new nondamaging high field limitation is discussed.

A means for electrically gating a microchannel-plate (MCP) detector using a fast rise-time, x-ray-driven Auston switch is presented. The Ga:As Auston switches which are used in the pulser were tested to stand off 7 kV DC bias, and can therefore be used without ramped biasing electronics. Calculations for the x-ray sensitivity of the Auston switches in the pulser are presented. The pulsers were tested with visible light and x rays and were shown to exhibit 90 ps rise-time between zero and 1 kV when triggered by a 1.2 mJ/cm² flux of subkilovolt x rays as measured by silicon p-i-n diodes. The pulser was designed to trigger MCP detectors which were used to spectrally characterize an x-ray heated target. A major attribute of this pulser is that it can be preset to gate a detector with less than a few picoseconds timing jitter with no foreknowledge of the time of arrival of the triggering x-ray flux.

A high peak power impulse pulser that is controlled with high gain, optically triggered GaAs Photoconductive Semiconductor Switches (PCSS) has been constructed and tested. The system has a short 50 (Omega) line that is charged to 100 kV and discharged through the switch when the switch is triggered with as little as 90 nJ of laser energy. We have demonstrated that the GaAs switches can be used to produce either a monocycle or a monopulse with a period or total duration of about 3 ns. For the monopulse, the voltage switched was above 100 kV, producing a peak power of about 48 MW to the 30 (Omega) load at a burst repetition rate of 1 kHz. The laser that is used is a small laser diode array whose output is delivered through a fiber to the switch. The current in the system has rise times of 430 ps and a pulse width of 1.4 ns when two laser diode arrays are used to trigger the switch. The small trigger energy and switch jitter are due to a high gain switching mechanism in GaAs.

We describe a circuit for generating a variety of pulsed waveforms. The circuit consists of a fast photoconductive switch (the bulk avalanche semiconductor switch or BASS) and three transmission line sections consisting of a charge line, a tuning stub and an output line. A model was established for predicting the expected waveforms and experimental results are compared to the model predictions.

This paper describes the design and operation of a multichannel, high voltage pulse generator. The pulse generator produces six pulses with rise times of 1- 2 ns and peak voltages over 20kV. Each of the output pulses is separately controlled to provide a variable pulse delay of up to 15ns. Jitter between pulses is <50 ps. The circuit is based on a primary pulse generator and six ferrite loaded transmission lines which simultaneously reduce the rise time of the primary pulse and delay application of the six output pulses to their loads. The pulses in each of the ferrite transmission lines are independently controlled by simple DC current sources.

This paper considers some of the pertinent aspects involved in the efficient design of a high voltage, low jitter trigatron switch. The performance of a high voltage triggered switch was initially evaluated. This switch was tested using a 10 stage 1MV Marx generator and a single stage trigger source. The Marx generator and trigger source were synchronized to enable jitter measurements to be carried out. The information gained led to the design of a trigatron switch with enhanced performance. This switch performs with subnanosecond jitter when operated at either a very high voltage or by using a corona pre-ionizer source at voltages up to 500kV. The corona source produces negative ions which have been shown to reduce the jitter in SF₆ gaps by minimizing the statistical time lag. The measured jitter varied inversely with breakdown voltage for a given gap spacing and pressure. Extensive electrostatic modelling of the trigatron was carried out to ensure optimal triggering in the final design.

This paper discusses the generation of subnanosecond pulses in the kilovolt range using TRAPATT diodes as overvolted, subnanosecond switches in a Marx circuit using the traveling wave method of erection. The design, operation, and simulation of the solid state Marx generator is discussed including the design of the signal probes and inherent signal distortion. A transient circuit simulation code was used to assist in determining the true operation of the circuit and to assist in designing additional circuits.

Optically activated GaAs-switches have some significant advantages for the generation of steep voltage pulses, duration of some 10 ns, in extended stripline-systems: the parallel operation of a number of switching elements makes a proper impedance matching between stripline and load possible. In addition the high voltage stress can be controlled. The activation in the linear photoconducting mode allows a fast switch closure with low jitter. In order to keep the light energy small it is possible to transfer the GaAs-switch into the nonlinear photoconducting mode. This paper reports on investigations using CCD- camera systems to study the development and the formation-process of current filaments characterizing the nonlinear photoconducting mode. Furthermore results of experiments are given obtained from an extended Blumlein-system with an impedance of 2.5 (Omega) respectively 0.1 (Omega) operated at voltages up to 30 kV. The switch design, the conditions how the parallel operation of switching elements is influenced by the voltage applied across the switch, the wavelength and the intensity of the light are discussed.