Generation of waveforms utilizing photoconductive switches and striplines has been analyzed for three circuit categories; parallel switching circuits, series switching circuits and passive waveform shaping circuits. The first two circuits utilize multiple switches to synthesize a waveform whereas the latter circuit contains a single switch in conjunction with a passive stripline circuit to generate a waveform. The properties of real switches impose practical limitations on these circuits which, in some cases, can be severe.

A non-linear, tapered radial transmission line which combines the functions of energy storage and voltage gain was investigated. A photoconductively switched impulse generator with a tapered radial transmission line has resulted in a compact design with impedance transformation properties. The radial transmission line device has achieved narrow output pulsewidth as well as voltage gain, which are important properties for the generation of high peak power impulse S.

The ionization coefficient of deep traps in GaAs is determined from a gas breakdown model together with the recent experimental data obtained at LLNL (Lawrence Livermore National Laboratory) and Boeing. Using this coefficient in our nonlinear device transport code, we have investigated theoretically the nonlinear switching phenomena in GaAs devices. The results obtained from our investigations show that if we take into consideration the effect of the field ionization of the deep traps, we can show how the "Lock-On" phenomena could occur in the device.

Ultrafast optics find more and more applications to microwave and millimeter wave technology through picosecond optoelectronics. THOMSON CSF is working on sequential RF waveform generation using a frozen-wave generator and picosecond photoconductive switches. Such a generator produces unique wide-band and high—power microwave pulses. A critical issue consists in radiating that pulse. A solution is a quasi-TEM horn terminating the feeding line. A two-dimensional, boundary-element numerical simulation has been developped to calculate the reflection coefficients of the dominant modes. The standard case of open-ended parallel-plane waveguide enables us to validate the code. Then, the code is applied to exponentially tapered videband horns, showing that a 1:10 bandwidth is theoretically reachable. Work is in progress to calculate the radiation pattern, the fluence of the electromagnetic field inside the horn, and to extend the code to the case of horns partially filled with dielectric.

The "Applied-B-Diode" used for the pulse generator KALIF at the Kernforschungszentrum Karisruhe needs uniform anode plasmas which should be established by a 30-50 kV pulse, duration 20 ns, delivered from a generator with an impedance of the order of 0. 1 (. The pulse risetime and the jitter has to be in the range of 1ns. Investigations have demonstrated that optically triggered semiconductor switches have the best chances to meet these requirements. The paper outlines the design of the pulse generator and the optically triggered semiconductor switch concept. In addition some experimental results are reported.

The general requirements for the two-mode-interferometer considering both step index and parabolic index waveguides are determined. A model is developed to realistically include the effects of diffusion on the refractive index profiles of the guiding structure. The necessity of high-order mode excitation for improved operation is established.

The temporal development of electric field distribution and temperature distribution in photoconductive, gallium arsenide (GaAs:Si:Cu) switches was studied by means of absorption measurements near the bandedge of gallium arsenide. Regions of high absorption, corresponding to enhanced field strength, were recorded close to the cathode contact for low applied fields (E < 20 kV/cm), and at both contacts for higher fields. Breakdown was observed at voltages where the absorption patterns merged. For low intensity laser activation the absorption became temporarily (during laser activation) homogeneous over the switch area, but the pattern emerged again during the tail current phase and became even more pronounced than before. After turning the voltage off, the absorption in certain regions of the switch decayed only slowly, with a time constant of about hundred nanoseconds, indicating local heating of the switch. At high laser intensities the absorption pattern, generated through pulse biasing of the sample, disappeared completely during laser activation. The switch stayed homogeneous, where electric field and temperature are concerned, even during the following lock-on phase, and recovered right after the switch voltage was turned off. The results show that energies in excess of 1 mJ/cm2 are needed to eliminate field inhomogenetics in photoconductive GaAs:Si:Cu switches and to obtain nanosecond switch recovery.

The Bulk Optically Controlled Semiconductor Switch (BOSS) is based on GaAs doped with silicon and compensated with diffused copper. The BOSS device can be turned on with a laser pulse of one wavelength and turned off with a second laser pulse of a different wavelength. The resulting electrical pulse duration may be varied depending on the timing between the two, relatively short, laser pulses. Two factors that limit the lifetime of this photoconductive switch are the choice of contact gcometry/formulation, and the conductivity during the "on-state." Diffused copper forms several deep acceptor levels in GaAs:Si, and the two dominant levels are labeled CUA and CuB which are located 0.14 eV and 0.44 eV from the top of the valence band respectively. The BOSS switching concept relies on the CuB level, and therefore the CUA level is considered to be parasitic. The on-state conductivity of the switch can be enhanced by either maximizing the density of CUB acceptors through processing techniques, or by increasing the total donor/acceptor concentrations. The contact formulation must also be considered due to the high currents through these devices. The lifetimes of similar devices have been directly measured using an automated, high-current, pulse circuit. The following paper describes our recent progress related to the bulk processing of BOSS devices, contact geometry/formulations, and experimental lifetime studies.

Using an electro-optic imaging system, we have measured the spatial and temporal structure of electric fields in photoconductive switches with four different contact configurations. Contacts with heavy doping under the metallization had the best performance. Modeling has verified these results.

Photoconductive semiconductor switches are useful for the generation of high voltage electrical pulses with picosecond rise times. In addition, the availability of high power semiconductor laser arrays allows the elimination of large flash-lamp pumped solid state lasers. In this paper, the role of electro-absorption is investigated. A theoretical model has been formulated which combines measured field and wavelength dependent absorption data with a one dimensional drift and diffusion model. The results of the model indicate the existence of optimum parameters for both pump wavelength and pump intensity before a plateau of diminishing returns is achieved. The results along with a detailed explanation of the theoretical formalism is included in the paper.

A solid state switch is required to pass very high current densities, J, upon demand. Electrical access to the switch is through ohmic contacts which interface the device to the external circuit. Thus, accurate characterization of these contacts is essential in the design of the switch. Most work to date has centered around the electrical characteristics of switches, in particular the resistance R, and a reduction in joule (J2R) heating. In this paper we consider both the electrical characteristics and the thermal stability of the ohmic contacts on the operation of solid state switches. Thermal stability is important due to the inherent heating of the switch during its on time. Further, some applications will place switches in environments requiring operation at high temperature. The paper describes the present status of ohmic contacts to GaAs and InP.

Dark current characterization of GaAs photoconductive switches is examined with an emphasis on low frequency current oscillations caused by travelling charge domains in the semiconductor. The voltage controlled negative differential resistance responsible for this phenomenon is due to field enhanced capture of deep level traps and is utilized for extraction of trap parameters using simple thermionic measurements. The GaAs switch investigated is of the avalanche variety and has been shown to produce current filament.ation in the on state. Since this latter effect is associated with a current controlled negative differential resistance region, we speculate on the nature of the transition between the two states.

Deep level characterization studies have been made for different semiconductor materials (such as chromium doped GaAs, GaAs:Si:Cu, semi-insulating GaAs, polycrystalline ZnSe) of interest for photoconductive pulse power switches. Photo Induced Current Transient Spectroscopy technique using a digital approach for data acquisition has been used for measuring deep level parameters, such as activation energy, trap concentration and capture cross-section for electron and hole capture. Of particular interest to us is information n copper levels and EL2 levels in silicon doped copper compensated GaAs, which has been shown to perform as optically controlled closing and opening switch. The analysis of the current transient is performed by using two different methods a standard rate window method and a curve fitting method. The results obtained by both the methods are compared.

A process was developed for the fabrication of high power GaAs photoconductive switches with ohmic Ni/Ge/Au contact metallization in a lateral NIN switch configuration. The main features of the process were ion implanted silicon in the contact regions and rapid thermal processing for annealing the implants and alloying the metallization. A 50 mill dia. process wafer included 28 photolithographically defined switches of four different types to comparatively investigate the effects of ohmic contacts and switch dimensions on switch lifetime, together with monitors to characterize the ohmic contact process.

A non-alloyed Pd/GeTFi/Pt ohmic contact for an n-GaAs has been investigated. The specific contact resistance was 4.7x10^-7Ω.cm² for an n-GaAs film which was doped to 2x10^{18 -3}.Auger depth profile showed that the interface between the contact and n-GaAs was abrupt. There was almost no change of Auger depth profiles before and after the sample was put in the furnace for 20 hours at 300 °C in an N₂ ambient Scanning electron microscopy showed that the contact surface was very smooth after a 400 °C , 15 second anneal. Auger depth profiles and contact resistance measurements showed that the contacts begin to deteriorate when the rapid thermal annealing time exceeds 35seconds.

This paper presents measurements of electrical and optical properties of GaAs photoconductive semiconductor switches (PCSS) operating in a high gain switching mode previously referred to as "lock-on." High voltage and high current switching properties are given as motivation for understanding this switching mode. X-ray triggering of these switches is demonstrated, and the sensitivity of high field GaAs devices is discussed. Infrared photoluminescence (PL) images of the switches during the initiation of high gain switching reveal dense filaments of high carrier concentration. Spatial modulation of the optical (532 nm) trigger is shown to control the location and influence the number of filaments. The ability to trigger large lateral GaAs PCSS by focusing the light to a point near either contact is described, and the corresponding rise times and delays to high current switching are shown. The sudden rise of current to several orders of magnitude beyond the space charge limited flow which can come from the contacts demonstrates that initiation of this high gain switching mode cannot be explained solely by carrier injection at the contacts and drift across the 1 .5 cm long insulating region.

GaAs avalanche photoconductive switches are promizing devices for high power switching because of their geometric scalability, high voltage hold-off, and very low activating optical energy. The ultimate performances of these devices will depend critically on the semiconductor geometry, the electrodes geometry, the contacts technology and the semiconductor surface preparation. This paper describes comparatives tests of GaAs switches by varying these parameters. In particular, switches passivated by GaAs-glass bonding technology have been tested. Breakdown tests and sub-nanosecond switching tests using a 150 ps Nd:YAG laser are presented. Our goal is to switch 50 to 100 kV and 2 kA in less than 200 ps.

We have studied the current-voltage-temperature response of off-state NIN GaAs Avalanche Photoconductors. These non-destructive tests, evaluated in terms of thermal activation energies (E_a), and Resistance-Voltage (R-V) characteristics, are found to effectively distinguish between two types of devices. The first type possesses E_a < E_gap/2 and resistivity commensurate with bulk SI GaAs; the second demonstrates E_a > E_gap/s and R values greatly surpassing those of bulk Semi-Insulating GaAs. These data are consistent with junction effects at both contact-bulk interfaces, arising from the formation of an uncompensated P-type region near the N⁺ contact layer. We explain the P-type behavior by trapped electron neutralization of the deep compensating donor EL2.

Superconducting YBa₂Cu₃O_7-x films, 8000 angstroms thick, were deposited on MgO substrates and irradiated by 150-ps pulses from a Nd:YAG laser (wavelength equals 1.064 micrometers ). In addition to the bolometric response, a faster nonbolometric component has been observed. A simulation of the bolometric response involving the solution of 1-D heat flow equation in the film and the substrate and electrical modeling of the film was done to confirm the thermal origin of the shower component of the observed photoresponse.

A novel structure comprising a number of identical ZnSe/GaAs double heterostructures arranged in series to form a photoconductive switch for high voltages and for high-power frozenwave generation is described. The GaAs front the separate heterostructures are the only components interconnected to form the series arrangement. The switch has low dark current, low onresistance and high switching efficiency. The output characteristics is controllable by external electronic circuitry.

Zinc Selenide, in polycrystalline and single crystal form, and chemical vapor deposition (CVD) grown diamond films were studied with respect to their application as materials for electron-beam activated switches. The hold-off fields of the three materials were found to exceed that of semi-insulating gallium arsenide by at least an order of magnitude. Highest hold-off fields for pulsed voltage operation were recorded for diamond at 1.8 MV/cm. The electron-beam induced conductance in the 1 mm thick single crystal zinc selenide switches reached values of 0.5 (Ωcm²)^-1 with an electron-beam current density of 20 mA/cm² at electron-energies of 150 keV. This corresponds to an electron-beam induced reduction of switch resistance from 10⁸ Ω to 2 Ω per square centimeter. The dominant carrier loss mechanism in the single crystal zinc selenide switch was found to be direct recombination of electron-hole pairs. In this material, the current, after electron-beam turn-off, decays hyperbolically with 90% to 10% falitimes in the range of hundreds of nanoseconds. The electron-beam induced conductivity in CVD grown diamond films of 1 micrometer thickness is due to the subnanosecond carrier lifetime less than three orders lower than that of single crystal zinc selenide. Both materials, single crystal zinc selenide and diamond, showed a lock-on effect in current. For diamond it could be demonstrated, as before for gaffium arsenide, that this effect can be suppressed by proper choice of contacts.

CdTe material, with its high molecular weight (240), is widely used for radiation detection. The bandgap energy of CdTe is 1.45 eV. When doped with chlorine, which compensates the acceptor level introduced by cadmium vacancies, CdTe is intrinsic and presents a very high resistivity (> 10⁸ (Omega) cm). The contacts were made by electroless metal deposition and further annealing; they were characterized with DC and pulsed voltages. For the first time, this paper presents power switching experiments with CdTe material. We investigated different types of crystals and contact geometries with gap size varying from less than one mm to a few millimeters. The switches were activated by a YAG laser 10 ns FWHM pulses (1.06 micrometers ) or with 160 kV X-rays 30 ns FWHM pulses. The time constant of recovery was found to be more than 10 ns. In some cases, for high voltages corresponding to fields higher than a few kV/cm, large recovery times of more than 100 ns were measured. This apparently long carrier lifetime, combined with the high resistivity, make CdTe an alternative material to Si and GaAs for some switching applications.

Two new circuit structures which exploit the properties of the fast opening GaAs photoconductive semiconductor switch are described. These new circuit structures, series current charged transmission lines, and dual of the Blumlein line, offer increased power output over our previous current charged transmission line circuit structure. In addition, the electrical and optical response of the poly-ZnSe switch under high applied electrical fields has been further investigated. Non-linear behavior is observed in the poly-ZnSe switch under high fields both experimentally and in numerical simulation. This type of non-linear behavior may be useful in high power opening switch applications as predicted by numerical simulation.

The influence of test environments (vacuum and air) on the pre-breakdown and breakdown characteristics of photoconducting p-type silicon samples were investigated. The surface of all the samples were polished (mechanical and chemo-mechanical) to a fine finish (0.06 micrometers ) and cleaned by ultrasonic and RCA methods. The role of thermally grown thin amorphous SiO₂ layer and the insulating epoxy layer on the breakdown properties were also investigated. The breakdown characteristics of the samples were found to be modified by the surface condition while testing in vacuum. Contrarily, the influence of those surface parameters on the breakdown properties of the same samples were found to be insignificant when the test environment was air. In general, the samples exhibited higher breakdown strength in air compared to that in vacuum. Besides, the lock-on behavior of the samples was identified only in vacuum (not in air) environment.

As a material for high power solid state switches, diamond promises to outperform any other semiconductor material because of its high dielectric strength, high electron and hole mobility and its excellent thermal properties. With the conductance controlled by high energy electron- beams, which can be generated using standard vacuum tube technology, thin film diamond switches can be given a very simple and compact design. This manuscript discusses the promises and limitations of this novel switch concept on the basis of steady state and transient device simulation, and investigates in particular, the suitability of electron-beam controlled diamond switches for closing and opening applications.

The role of impact ionization in switching is examined. Rate equations for three different processes are developed and analyzed in a search for two stable solutions that correspond to the on and off states of the switch.

The bulk optically controlled semiconductor switch (BOSS) is a concept based on modulating the bulk conductivity (sigma) of copper-compensated, silicon-doped gallium arsenide (GaAs:Si:Cu). Using laser light at (lambda) equals 1 micrometers , (sigma) can be increased to greater than 1 ((Omega) cm)^-1, and then returned to values comparable to the equilibrium value of (sigma) < 10^-4 ((Omega) cm)^-1 by illuminating the bulk with an infrared laser at (lambda) approximately equals 2 micrometers . This reversible process forms the switching cycle of the BOSS device. Experimental verification of the essential features of the BOSS switching cycle has been reported at low values of current density; however, power scaling experiments have revealed behavior too complex to be explained by a zero-dimensional model based strictly on conductivity. For this reason, a one- dimensional time-dependent computer code has been developed to analyze the effects of current transport and spatial inhomogeneity in a BOSS device. Electron and hole transport are modeled self-consistently with electron and hole continuity equations and the Poisson equation. The computer code includes the effects of deep-level trapping kinetics, and the boundary conditions model those of a forward-biased p-i-n device. The low-current density results of the 1-d model are verified against the 0-d conductivity model; deviations from the 0-d model as the current density is increased are reported and qualitatively compared against available power-scaling data.

The lock-on effect observed in high power, light-activated GaAs bulk switches is very important in determining the GaAs power device performance. An analytical model to explain the physical origin of this effect is presented. In this model, negative resistance associated with transferred-electron effect creates high-field-induced avalanche injection at the anode contact. A regional approximation is used to calculate the field distribution in the device and to derive the device f-V characteristics. Reported experimental results are in good agreement with the model over a wide range of device parameters.

Photoconductive semiconductor switches (PCSS) presently have the greatest potential for dramatic performance enhancements for high power pulsed applications. However, surface flashover severely limits the maximum stand off voltage in the open state. We report the use of a novel technique to PCSS to overcome this limitation. The technique is an extension of the graded ring bushing idea from accelerator technology, but differs by reducing the thickness of the insulator (semiconductor) down to tens of micrometers. Recent results using this technique have yielded electric field values, before flashover, in the range of 70 kY/cm to 114 kVF/cm in silicon and 70 kY/cm to 84 kV/cm in gallium arsenide.

In this paper, high performance single pulse selectors for laser using three kinds of avalanche transistor driver arc described. They have low jilter ( <1 ns ), long life-time (>lO⁷shots ), short delay time ( about 2Ons ), high probability of selecting single pulse (100% ), high signal/noise ratio of selected single pulse ( > 10³ ).The amplitude stability of selected single pulse better than that of' the mode-locked pulse train. Comparison of the performances of avalanche transistor driver with others is given.

Under ideal conditions photoconductive switches utilizing ohmic contacts can be made to conduct high currents that scale directly with input optical trigger power. In practice, however, ohmic contacts can only be approximated by using heavily-doped contact/metallization regions, so that photoswitch structures employing intrinsic substrate layers to support switch voltage can be viewed as n-i-n, p-i-n, or p-i-n, depending on the contact doping. Under bias, these contacts preferentially inject majority carriers (either holes or electrons) into the substrate that can form high local space charge electric fields at elevated current densities. In this paper we show both experimentally and analytically that contact space charge formation in a cryogenic silicon n-i-n photoswitch structure ultimately limits its on-state current capability.