M.W. O'Malley

Photoconductive semiconductor switch experiments for pulsed power applications

Experiments have been performed to develop photoconductive semiconductor switches (PCSSs) for pulsed power applications that cannot be implemented with traditional high-power switching technology. A scalable lateral PCSS configuration has been tested and has demonstrated a potential for faster risetimes, less jitter, lower inductance, faster recovery, and optical triggering for new pulse power projects. These switches have been used as both closing and toggling switches at repetition rates up to 40 MHz. A high-field, gain mechanism (lock-on) was explored and tested which may eliminate the major disadvantage of this type of switch, its requirement for large optical trigger energies. >

High Gain Photoconductive Semiconductor Switching

Switching properties are reported for high gain photoconductive semiconductor switches (PCSS). A 200 ps pulse width laser was used in tests to examine the relations between etectric field, rise time, delay, and minimum optical trigger energy for switches which reached 80 kV in a 50 /spl Omega/ transmission line with rise times as short as 600 ps. Infrared photoluminescence was imaged during high gain switching providing direct evidence for current filamentation. Implications of these measurements for the theoretical understanding and practical development of these switches are discussed.

Triggering GaAs lock-on switches with laser diode arrays

Progress toward the triggering of high-power photoconductive semiconductor switches (PCSSs) with laser diode arrays, is reported. An 850-W optical pulse from a laser diode array was used to trigger a 1.5-cm-long switch that delivered 8.5 MW to a 38.3- Omega load. Using 166-W arrays, it was possible to trigger a 2.5-mm-long switch delivering 1.2 MW with 600-ps rise-times at pulse repetition frequencies of 1 kHz. These 2.5-mm-long switches survived 10/sup 5/ pulses at 1.0 MW levels. In single-pulse operation, up to 600 A was switched with laser diode arrays. The goal is to switch up to 5 kA in a single-shot mode and up to 100 MW repetitively at up to 10 kHz. At electric fields below 3 kV/cm GaAs switches are activated by creation of one electron-hole pair per photon. This linear mode demands high laser power and, after the light pulse, the carriers recombine in nanoseconds. At higher electric fields GaAs acts as a light-activated Zener diode. The laser light generates carriers as before, but the field induces gain such that the amount of light required to trigger the switch is reduced by a factor of up to 500. The gain continues until the field across the sample drops to a material-dependent lock-on field. The gain in the switch allows for the use of laser diodes. >

Recovery of high-field GaAs photoconductive semiconductor switches

The authors discuss the recovery of GaAs PCSS (photoconductive semiconductor switches) after they are triggered into a high gain switching mode called lock-on. Fast recovery of GaAs switches after high field switching is of particular interest for high repetition rate applications where it is difficult to provide the large optical trigger energy required for switches operating at low fields. Three categories of circuits for inducing fast recovery after lock-on by temporarily reducing the field across the switch are examined. Measurements of recovery times from 35-80 ns, multiple monopolar and bipolar bursts at 5-40 MHz, and hold-off fields ranging from 5-44 kV/cm (corresponding to 15-66 kV across individual switches) are presented. >

Properties of high gain GaAs switches for pulsed power applications

High gain GaAs photoconductive semiconductor switches (PCSS) are being used in a variety of electrical and optical short pulse applications. The highest power application, which we are developing, is a compact, repetitive, short pulse linear induction accelerator. The array of PCSS, which drive the accelerator, will switch 75 kA and 250 kV in 30 ns long pulses at 50 Hz. The accelerator will produce a 700 kV, 7kA electron beam for industrial and military applications. In the low power regime, these switches are being used to switch 400 A and 5 kV to drive laser diode arrays which produce 100 ps optical pulses. These short optical pulses are for military and commercial applications in optical and electrical range sensing, 3D laser radar, and high speed imaging. Both types of these applications demand a better understanding of the switch properties to increase switch lifetime, reduce jitter, optimize optical triggering, and improve overall switch performance. These applications and experiments on the fundamental behavior of high gain GaAs switches is discussed. Open shutter, infra-red images and time-resolved Schlieren images of the current filaments, which form during high gain switching, are presented. Results from optical triggering experiments to produce multiple, diffuse filaments for high current repetitive switching are described.

Sub-nanosecond avalanche transistor drivers for low impedance pulsed power applications

Ultra compact, short pulse, high voltage, high current pulsers are needed for a variety of non-linear electrical and optical applications. With a fast risetime and short pulse width, these drivers are capable of producing sub-nanosecond electrical and thus optical pulses by gain switching semiconductor laser diodes. Gain-switching of laser diodes requires a sub-nanosecond pulser capable of driving a low output impedance (5 /spl Omega/ or less). Optical pulses obtained had risetimes as fast as 20 ps. The designed pulsers also could be used for triggering photo-conductive semiconductor switches (PCSS), gating high speed optical imaging systems, and providing electrical and optical sources for fast transient sensor applications. Building on concepts from Lawrence Livermore National Laboratory, the development of pulsers based on solid state avalanche transistors was adapted to drive low impedances. As each successive stage is avalanched in the circuit, the amount of overvoltage increases, increasing the switching speed and improving the turn on time of the output pulse at the final stage. The output of the pulser is coupled into the load using a Blumlein configuration.

CHARACTERISTICS OF CURRENT FILAMENTATION IN HIGH GAIN PHOTOCONDUCTIVE SEMICONDUCTOR SWITCHING

Characteristics of current filamentation are reported for high gain photoconductive semiconductor switches (PCSS). Infrared photoluminescence is used to monitor carrier recombination radiation during fast initiation of high gain switching in large (1.5 cm gap) lateral GaAs PCSS. Spatial modulation of the optical trigger, a 200--300 ps pulse width laser, is examined. Effects on the location and number of current filaments, rise time, and delay to high gain switching, minimum trigger energy, and degradation of switch contacts are presented. Implications of these measurements for the theoretical understanding and practical development of these switches are discussed. Efforts to increase current density and reduce switch size and optical trigger energy requirements are described. Results from contact development and device lifetime testing are presented and the impact of these results on practical device applications is discussed.

High gain GaAs photoconductive semiconductor switches for ground penetrating radar

The ability of high gain GaAs photoconductive semiconductor switches (PCSS) to deliver high peak power, fast risetime pulses when triggered with small laser diode arrays makes them suitable for their use in radars that rely on fast impulses. This type of direct time domain radar is uniquely suited for observation of large structures underground because it can operate at low frequencies and at high average power. This paper summarizes the state-of-the-art in high gain GaAs switches and discusses their use in a radar transmitter. The authors also present a summary of an analysis of the effectiveness of different pulser geometries that result in transmitted pulses with varying frequency content. To this end, they developed a simple model that includes transmit and receive antenna response, attenuation and dispersion of the electromagnetic impulses by the soil and target cross-sections.

High gain GaAs Photoconductive Semiconductor Switches for impulse sources

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.

Physics and Applications of the Lock-on Effect

The lock-on effect is a high gain, high field switching mechanism that has been observed in GaAs and InP. This switching mode is exciting because the amount of light required to trigger it is small when compared to triggering the same switch at low fields. For this reason we can use laser diode arrays to trigger high voltages, currents and power. This paper will describe the lock-on effect, and our recent experiments to understand the effect. We will show that impact ionization from deep levels cannot account for the observed current densities, delays, and rise times unless a second mechanism is invoked. We will also describe our applications for laser diode array triggered lock-on switches, the best results that illustrate our potential for the application, and the studies carried out to improve the lifetime and current carrying capability of the switches.