D.L. McLaughlin

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. >

8.5 MW GaAs pulse biased switch optically controlled by 2-D laser diode arrays

The development of a compact, all solid-state switch system that has switched up to 8.5 MW into a 38- Omega load is described. The system uses a 2-D laser diode array with a peak power of 850 W to trigger a 1.5-cm-long GaAs photoconductor into a high-gain conduction mode known as lock on. The highest power switch was pulse-charged to 55 kV and delivered 470 A to a 38- Omega load in a 160-ns-long pulse.<<ETX>>

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.

Photoconductive Semiconductor Switches for High Power Radiation

In this paper we present the results of experiments on Si and GaAs Photoconductive Semiconductor Switches (PCSS). Our goal is to improve their performance for high power electromagnetic pulse generation. For Si, we show ways to alter carrier lifetime to achieve higher repetition rates, improvements in switch lifetimes to over 107 pulses at high field, and methods that reduce or eliminate thermal runaway and heating. For GaAs, the effect of focused trigger radiation was studied and a further reduction (by a factor of 100) in the required light energy was observed. The gain in these switches is now about 100, 000 electrons generated per absorbed or trigger photon. It was further demonstrated that light can be piped through fiber optics to trigger multiple current filaments in GaAs. These results show the ability to control the location of the current filaments.

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.<<ETX>>

Rise time and recovery of GaAs photoconductive semiconductor switches

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.

High Current Density Contacts for GaAs Photoconductive Semiconductor Switches

The current densities implied by current filaments in GaAs photoconductive semiconductor switches (PCSS) are in excess of 1 MA/cm{sup 2}. As the lateral switches are tested repeatedly, damage accumulates at the contacts until electrical breakdown occurs across the surface of the insulating region. In order to improve the switch lifetime, the incorporation of n- and p-type ohmic contacts in lateral switches as well as surface geometry modifications have been investigated. By using p-type AuBe ohmic contacts at the anode and n-type AuGe ohmic contacts at the cathode, contact lifetime improvements of 5--10x were observed compared to switches with n-type contacts at both anode and cathode. Failure analysis on samples operated for 1--1,000 shots show that extensive damage still exists for at least one contact on all switches observed and that temperatures approaching 500{degrees}C are can be reached. However, the n-type AuGe cathode is often found to have no damage observable by scanning electron microscopy (SEM). The observed patterns of contact degradation indicate directions for future contact improvements in lateral switches.