W.D. Helgeson

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.

Measurement of the velocity of current filaments in optically triggered, high gain GaAs switches

Pictures are presented of the time evolution of current filaments during optically triggered, high gain switching in GaAs. Two filaments are triggered with two laser diode arrays and the time delay between them is varied. When the filament that is triggered first crosses the switch the voltage drops and the other filament ceases to grow. By varying the delay between the lasers, the tip velocity is measured to be 2±1×109 cm/s, 100 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.

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.

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.

Optically-activated GaAs switches for compact accelerators and short pulse sensors

This paper describes research and development of high gain GaAs photoconductive semiconductor switches (PCSS) for two very different types of applications: compact, repetitive accelerators and short pulse, active optical sensors. The accelerator is being tested with a spark gap driven modulator. It is a short pulse, linear induction accelerator (LIA) with an electron diode. Its design goals are: 700 kV, 7 kA, 30 ns pulses at 50 Hz. After characterizing the accelerator with the spark gap modulator, it will be tested with a GaAs PCSS modulator, which is under construction. Forty-eight, 2 inch diameter PCSS will switch 70 kA in a 250 kV coaxial Blumlein to deliver 220 kV, 35 kA, 30 ns pulses to the LIA. One module (1/8/sup th/) of the PCSS modulator is being tested. Results from these tests and projections for the complete system are discussed. The short pulse sensors are for military and commercial applications in optical and electrical range sensing, 3D laser radar, and high speed photography. The highest optical power produced with PCSS-driven laser diode arrays is presently 50 kW in 75 ps wide pulses or 12 kW in 1 ns wide pulses. A variety of sizes of GaAs PCSS are being tested around voltage and current specifications of several applications. Voltages range from 2 to 100 kV, currents range from 10 to 500 A, and electrical pulse lengths range from 1 to 50 ns. This paper discusses developmental issues of GaAs PCSS, which are common to all: fundamental research in high gain GaAs, device longevity, optical triggering, circuit configuration, and switch performance.

Temporal switching jitter in photoconductive switches

Gallium arsenide photoconductive semiconductor switches (PCSS) are being studied as enabling technologies for a variety of applications. High grain PCSS can be triggered with small laser diodes or laser diode arrays. Some of the applications require low temporal jitter of the switches relative to the trigger laser. The purpose of this study was to compare the temporal switch jitter times for different systems: we varied the type of trigger laser and its risetime, the type of pulse charger and transmission line that was discharged through the PCSS, and the geometry of PCSS used. One of the PCSS was an opposed contact PCSS geometry used by the Air Force Research Laboratory. The other was a coplanar geometry switch made by Sandia National Laboratories. It is found that the optical trigger laser characteristics are dominant in determining the PCSS jitter while the nature of the contact geometry (opposed or coplanar) is not as important.

High-gain GaAs photoconductive semiconductor switches (PCSS): device lifetime, high-current testing, optical pulse generators

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.