Photoconductive switching is a technology that is being increasingly applied to generation of high power microwaves. Two primary semiconductors used for these devices are silicon and gallium arsenide. Diamond is a promising future candidate material. This paper discusses the important material parameters and switching modes critical issues for microwave generation and future directions for this high power photoconductive switching technology.

The continuing development of optically controlled bulk high power semiconductor switches with sub-ns closure times and sub-ns synchronization has made the direct generation of microwave energy feasible. The object of this paper are to 1) describe two transmission line approaches for generation of multiple cycles of high power microwave energy using optically controlled switches 2) describe the optically controlled switch requirements for each approach 3) compare the point design performance of the above types of direct high power microwave generators and 4) draw several conclusions from this system study. I.

We examine the limitations of light activated semiconductor switches from a pulsed power point of view particularly the power and the speed. The factors considered fall into three categories namely electromagnetic principles physical properties and carrier generation. We show how after a full examination of the switch in a real operating environment we are led to the choice of a silicon junction with linear activation as the optimum design for a repetitively switched high power device.

The objective of this paper is to demonstrate the feasibility of the generation of stable repetitive kilovolt amplitude pulses of picoseconds duration. The jitter in the pulses is less than 10 picoseconds so that they can be viewed on an ordinary inexpensive sampling scope rather than on an optical autocorrelator.

We are conducting research on the switching properties of photoconductive materials to explore their potential for generating highpower microwaves (HPM) and for high reprate switching. We have investigated the performance of Gallium Arsenide (GaAs) in linear mode (the conductivity of the device follows the optical pulse) as well as an avalanchelike mode (the optical pulse only controls switch closing) . Operating in the unear mode we have observed switch closing times of less than 200 Ps with a 100 ps duration laser pulse and opening times of less than 400 ps at several kV/cm fields using neutron irradiated GaAs. In avalanche and lockon modes high fields are switched with lower laser pulse energies resulting in higher efficiencies but with measurable switching delay and jitter. We are currently investigating both large area (1 cm2) and small area 1 mm2) switches illuminated by AlGaAs laser diodes at 900 nm and Nd:YAG lasers at 1. 06 tim.

Picoseccond photoconductivity has been achieved for a variety of semiconductor materials by techniques which have now become almost standard1. Enhanced scattering by the excessive amount of deep level defects which provide for picosecond recombination lifetimes significantly reduces the mobility, degrading the responsivity of the photoconductor. This paper will present a concept where improved responsivity is achievable by utilizing a graded bandgap AlxGaixAs active detecting layer grown on a high defect density GaAs layer by molecular beam epitaxy (MBE).

Opticallytriggered thyristor switches often operate in adverse environments such as high temperature and high dose-rate transient radiation which can result in lowered operating voltage and premature triggering. These effects can be reduced by connecting or monolithically integrating a reverse-biased compensating photodiode or phototransistor into the gate of the optically-triggered thyristor. We have demonstrated the effectiveness of this hardening concept in silicon thyristors packaged with photodiodes and in gallium arsenide optically-triggered thyristors monolithically integrated with compensating phototransistors.

The gas avalanche switch a high-voltage picosecond-speed switch has been proposed. The basic switch consists of pulse-charged electrodes immersed in a high-pressure (7800 atm) gas. An avalanche discharge is induced in the gas between the electrodes by ionization from a picosecond-scale laser pulse. The avalanching electrons move toward the anode causing the applied voltage to collapse in picoseconds. This voltage collapse if rapid enough generates electromagnetic waves. A two-dimensional (2D) finite difference computer code solves Maxwell''s equations for transverse magnetic modes for rectilinear electrodes between parallel plate conductors along with electron conservation equations for continuity momentum and energy. Collision frequencies for ionization and momentum and energy transfer to neutral molecules are assumed to scale linearly with neutral pressure. Electrode charging and laser-driven electron deposition are assumed to be instantaneous. Code calculations are done for a pulse generator geometry consisting of an 0. 7 mm wide by 0. 8 mm high beveled rectangular center electrode between grounded parallel plates at 2 mm spacing in air. In one operational mode a uniform distribution of initial electrons is induced in the gap between the center electrode and the lower plate. With the center electrode at 227 kV positive and 15 atm pressure voltage pulses of 24 1 kV 2 Ps rise times 8 ps full widths at half maximum (FWHM) 30 ps durations and 24 GHz 3 dB bandwidths are induced at the ends

We demonstrate optical switching using split contact semiconductor absorptive nonlinear optical amplifiers. All-optical time switching of 500 MBit/s data with a rise time of 100 ps has been demonstrated. Fall times of 85ps can be achieved by reverse biasing the absorber section of the amplifier. The switch had a minimum switching threshold of 1 iW a switch gain of 1O dB and can operate over a 60 nm wavelength range around 1 . 55pm. Such a switch may be suitable for application in packet switched networks with multi-GBit/s header speeds. Wavelength switching using the same nonlinear amplifier is also demonstrated. 400 MBit/s data was converted over 35THz from 1 . 3 1im to wavelengths between 1 . 53 . tm and 1 . 585 pm. The minimum power at 1 . 3 1 jim required was ''60 iW. Such nonlinear optical amplifiers can give added flexibility in both TDM and WDM systems and are therefore likely to play a major role in future all-optical networks.

An ultra-rapid all-optical switch useful for application in the future high-bit-rate systems is presented. Ti:LiNbO3 waveguides are considered owing to the acceptable substrate nonlinear relative dielectric constant (a 41 01 8 m2/V2) and the consolidated waveguide fabrication technology. Logic gates which can perform the XOR AND and NOT functions are investigated. The optical switching is provided via optical control pulses in one or both arms by exploiting either the stable or the unstable region of the waveguided optical power versus the effective refractive index.

Experiments in this laboratory and the Corning Glass Works laboratories have established that highly resolved patterns of refractive index gradients ranging from 0. 001 to 0. 01 can be produced by photolysis of organotin compounds physisorbed onto Corning''s code 7930 porous Vycor glass followed by thermal consolidation of the glass at 1200CC. Photolysis binds the metal compound to the glass and thermal activation removes the unreacted adsorbate and converts the photoproduct to a transparent metal oxide. The photochemical and thermal reactions leading to gradient index formation will be described and examples of the devices produced by this microphotolithographic technique will be displayed.

Laser source requirements for optically activated high power switches are reviewed. Various configurations of two dimensional semiconductor lasers along with their present and projected performance levels are discussed.

The light activated semiconductor switch is potentially the fastest most powerful switch available for pulse power and microwave generation applications. We will examine the requirements of the laser employed in activating this type of switch such as the optical energy required for activation and risetime. We will also present different configurations and compare them in terms of e. g. cavity complexity magnitude of optical prepulse and ability to suppress jitter.

The generation of subnanosecond risetime pulses with 2 MW peak power at 1 kHz PRF has been achieved by triggering a hybrid pulser with a laser diode array. The hybrid pulser combines an optical switch centered in a metallized teflon disk that serves as an energy storage medium as well as a radial transmission line. Nearly 90 ofthebias voltage (10 kV) was delivered to a 50 ohm load impedance as a result of the variation of impedance of the radial line. Avalanche switching yielded 4 nano seconds pulses with a risetime of less than 500 picoseconds. _

Laser diode arrays have been used to trigger GaAs Photoconducting Semiconductor Switches (PCSS) charged to voltages of up to 60 kV and conducting currents of 580 A. The driving forces behind the use of laser diode arrays are compactness elimination of complicated optics and the ability to run at high repetition rates. Laser diode arrays can trigger GaAs at high fields as the result of a new switching mode (lock-on) with very high carrier number gain. We have achieved switching of up to 10 MW in a 60 1 system with a pulse rise time of 500 ps. At 1. 2 MW we have achieved repetition rates of 1 kHz with switch rise time of 500 Ps for i0 shots. The laser diode array used for these experiments delivers a 166 W pulse. In a single shot mode we have switched 4 kA with a flash lamp pumped laser and 600 A with the 166 W array.

Switches comprising semiconductor devices such as GaAs or Si switch devices and semiconductor lasers as activating devices are small efficient and reasonable in cost. Optically controlled Si PIN or bulk devices having long carrier lifetime switch cw high power signals in broad-band RF applications. They utilize pulsed laser diode arrays that require low average power. A cw HF switch was obtained utilizing a twodimensional (2-D) laser array that was pulse biased to 10 kHz. Optically activated GaAs devices with nano-second risetimes which utilize the phenomenon of lock-on are used for mega-watt switching applications such as impulse radar and firesets. A new high impedance 2-D laser array which was switched on in 700 Ps and delivered a peak power of 545W has also been achieved.

Measurements and model calculations on semi-insulating GaAs as material for optically and electron-beam controlled semiconductor switches have shown that the steady state current is a strongly nonlinear function of both the applied voltage and the radiation intensity. The nonlinear shape of these curves can be influenced over a wide range by doping with suitable deep acceptors or donors, a result which opens the possibility of "tailoring" the materials to meet specific demands. As an example, it is discussed how a current-controlled negative differential conductivity due to Cu-doping can be utilized for a fast (sub-nanosecond) e-beam controlled switch which operates at low dark current, high hold-off voltage and a forward resistance which lies considerably below the lock-on resistance.

Abstract not available.

We present high-speed shutter and streak photographs synchronized with sample current measurements which show clearly that in surface flashover of silicon in a vacuum ambient the current flows primanly in the silicon not in the ambient. We present S. E. M. photographs which show that this current is filamentary. Results obtained from samples with diffused p and n contacts show that the contacts exert a strong influence over the flashover characteristics.

A new diagnostic technique for measuring electric fields on the surfaces of semiconductors is described. The diagnostic uses the Pockels effect which mixes the electric field on a semiconductor surface with that of an incident optical pulse in a nonlinear crystal rotating the polarization of the optical pulse. This rotation can be detected and used to extract the surface electric field. Electro-optic sampling as this technique is called allows us to study the physics of semiconductors subjected to high fields with 100-ps time resolution. We have seen field enhancements in GaAs in photoconductive switches which modeling has shown to be due to Gunn domains.

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.

Peak power gain greater than 15 was obtained with a current charged transmission line and an optically activated semiconductor opening switch. The optical pulse used for activating the switch is generated by a Nd:glass laser emitting at 1. 054 pm. It has a slow rise-time (''--''2OO uS) and a fast fall-time (s1O uS). In the experiment a 2 kV output voltage pulse was achieved with a 5 mm cube GaAs p-i-n diode sitch at 500 V charging voltage.

Copper compensated silicon doped semiinsulating gallium arsenide (GaAs:Si:Cu) has been shown to exhibit the characteristics of a high-power optically controlled switch that can be closed and opened on a nanosecond time scale [1]. In such switches it is possible to activate and deactivate photoconductivity on command with two laser pulses of different wavelengths [2]. Infrared quenching measurements at low fields show complete quenching of the persistent photoconductivity. At fields greater than 3. 5 kV/cm the quenching is temporarily effective against " lock-on" currents. In order to better understand the switch behavior and be able to optimize switch performance modeling studies have been performed. Basic deep level data for the modeling have been obtained from photo-induced current transient spectroscopy (PICTS). The method and results of measurements on basic deep level parameters such as activation energy are discussed. Experimental studies on current voltage characteristics at high fields show negative differential conductivity.

Fast rise time applications have encouraged us to look at the rise time dependences of lockon switching. Our tests have shown rise time and delay effects which decrease dramatically with increasing electric field across the switch and/or optical energy used in activating lockon. Interest in high repetition rate photoconductive semiconductor switches (PCSS) which require very little trigger energy (our 1 . 5cm long switches have been triggered with as little as 20 J) has also led us to investigate recovery from lock-on. Several circuits have been used to induce fast recovery the fastest being 30 ns. The most reliable circuit produced a 4-pulse burst of +/- 10-kY pulses at 7 MHz with lOO-jtJ trigger energy per pulse.

Optical control of microwave devices particularly MMIC is a rapidly growing research area. The GaAs MESFET is the prime candidate as the optical detector for MMIC applications. In this paper a theoretical analysis is presented which predicts the photoresponse in the MESFET. The analysis includes both internal and external photovoltaic and photoconductive effects. The paper also describes the operation of an optically activated GaAs MMIC switch using GaAs MESFET as the optical detector.

The excitation of persistent photoconductivity followed by photoquenching has been demonstrated in copper-compensated silicon-doped semi-insulating GaAs (GaAs:Si:Cu). These processes allow a switch to be developed which can be closed by the application of one laser pulse ()1064 nm) and opened by the application of a second laser pulse (A21OO nm). In order to further understand the operation of such a switch the dark current-voltage (I-V) characteristics of a switch based on the GaAs:Si:Cu material are investigated using both dc and pulsed electric fields. Experimental studies are also performed on the closing phase of the switch using a 0-switched Nd:YAG laser. In these studies the transition from persistent photoconductivity to non-ohmic current transport is illustrated.

An optically gated thyristor based on GaAs has been designed fabricated and investigated for pulsed power applications. The device included a 200-pm semi-insulating base layer and was triggered with an 848-nm 1-pJ 100-nsec laser diode. The DC blocking voltage of the thyristor was observed to be V the peak current 300 A and the current rate of rise A/sec. Lock-on effect was also observed and is discussed.

In the nanosecond time regime gallium arsenide photoconductive devices are nearly ideal circuit elements for the generation of wideband high-power waveforms. The ability to activate these devices in an avalanche mode further enhances their utility since semiconductor lasers with nanojoule-range pulsed output can enable turn-on. Furthermore characteristics of gaffium arsenide provide the ability to fabricate switches with significant DC hold off capability. In general microwave matching requirements tend to be in opposition to high-voltage integrity constraints. This creates formidable design challenges. Careful compromises in the packaging design have led to megawatt-level peak power outputs at multi-gigahertz frequencies from surprisingly small devices. Work to date has concentrated on monocycle generation for use with a wideband antenna structure. Resultant radiated outputs have applications in ultrawideband radar electronic warfare and communications.

Recently several groups (see for example Refs. 1-9) have been investigating optically initiated GaAs avalanche switches first introduced by Williamson et al. (10). Although the configurations vary, these switches have the following common properties: a gain on the order of 1000 in current over that expected from the injected photocarriers, switching at fields several times lower than the published GaAs avalanche field, a very fast rise time typically occurring a few nanoseconds after the start of the laser pulse, and the device eventually evolving into a lock-on condition characterized by a fixed voltage drop proportional to the electrode spacing. At this time a complete understanding of the device physics is lacking. Theoretical investigation of field enhancement from the injected carriers has shown that this effect is not large enough to produce the initial avalanche (8). Both Gunn (3-5,7,9) and impurity (2,3,6) effects have been suggested as important aspects of the device operation during the lock-on condition. In the next two sections, we describe an empirical model of the initial switching of the device followed by comparing the model predictions to data obtained on doughnut hole configuration devices. The last section discusses the implications of the model on the device physics.