D.J. Brown

Photoconductive semiconductor switches

Optically activated GaAs switches operated in their high-gain mode are being used or tested for pulsed power applications as diverse as low-impedance, high-current firing sets in munitions; high impedance, low-current Pockels cell or Q-switch drivers for lasers; high-voltage drivers for laser diode arrays; high-voltage, high-current, compact accelerators; and pulsers for ground penetrating radar. This paper will describe the properties of high-gain photoconductive semiconductor switches (PCSS), and how they are used in a variety of pulsed power applications. For firing sets, we have switched up to 7 kA in a very compact package. For driving Q switches, the load is the small (30 pF) capacitance of the Q switch which is charged to 6 kV. We have demonstrated that we can modulate a laser beam with a subnanosecond rise time. Using PCSS, we have demonstrated gain switching a series-connected laser diode array, obtaining an optical output with a peak power of 50 kW and a pulse duration of 100 ps. For accelerators, we are using PCSS to switch a 260 kV, 60 kA Blumlein. A pulser suitable for use in ground-penetrating radar has been demonstrated at 100 kV, 1.3 kA. This paper will describe the specific project requirements and switch parameters in all of these applications, and emphasize the switch research and development that is being pursued to address the important issues.

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.

Fireset applications of improved longevity optically activated GaAs photoconductive semiconductor switches

The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) for fireset applications operating at 1 kV/1 kA levels and higher has been greatly improved by multiple filament triggering and improved contacts. While longevity operating at /spl sim/100A or below has been greatly increased by doping the semi-insulating GaAs underneath the contact metal to improve the ohmic contacts, this technique has not yet improved PCSS longevity for firesets. This paper will compare various approaches to optical triggering of the switches and methods of establishing electrical connection to the devices with regard to switch longevity at kA current levels. Data on device performance and lifetime will also be presented for different structures. The device characterization also includes examination of the switch behavior due to neutron irradiation. This irradiation provides an enhancement of DC voltage holdoff, improvement of radiation hardness, and modification of switching behavior. To improve lifetime at 1 kV-1 kA and above, we employ multi-filament operation and InPb solder/Au ribbon wirebonding. These studies have resulted in the demonstration of fireset switches that have >400 shot lifetime at nominally 1 kA operating current.

Longevity of optically activated, high gain GaAs photoconductive semiconductor switches

The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) for pulsed power applications has been extended to well over 10 million pulses by reducing the density of carriers at the semiconductor to metal interface. This was achieved by reducing the density in the vertical and lateral directions. The first was achieved by varying the spatial distribution of the trigger light, thereby widening the current filaments that are characteristic of the high gain switches. The authors reduced the carrier density in the vertical direction by using ion implantation. These results were obtained for currents of about 10 A, current duration of 3.5 ns, and switched voltage of /spl sim/2 kV. At currents of /spl sim/70 A, the switches last for 0.6 million pulses. In order to improve the performance at high currents, new processes such as deep diffusion and epitaxial growth of contacts are being pursued. To guide this effort, the authors measured a carrier density of 6/spl times/10/sup 18/ electrons (or holes)/cm/sup 3/ in filaments that carry a current of 5 A.

Longevity improvement of optically activated, high gain GaAs photoconductive semiconductor switches

The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) has been extended to over 100 million pulses. This was achieved by improving the ohmic contacts through the incorporation of a doped layer that is very effective in the suppression of filament formation, alleviating current crowding. Damage-free operation is now possible at much higher current levels than before. The inherent damage-free current capacity of the bulk GaAs itself depends on the thickness of the doped layers and is at least 100 A for a dopant diffusion depth of 4 /spl mu/m. This current could be increased by connecting and triggering parallel switches. The contact metal has a different damage mechanism and the threshold for damage (/spl sim/40 A) is not further improved beyond a dopant diffusion depth of about 2 /spl mu/m. In a diffusion-doped contact switch, the switching performance is not degraded at the onset of contact metal erosion, unlike a switch with conventional contacts. For fireset applications operating at a 1 kV/1 kA levels and higher, doped contacts have not yet resulted in improved longevity. The authors employ multi-filament operation and InPb solder/Au ribbon wirebonding to demonstrate >100 shot lifetime at 1 kV/1 kA.

Optically-activated GaAs switches for ground penetrating radar and firing set applications

Optically activated, high gain GaAs switches are being tested for many different applications. Two such applications are ground penetrating radar (GPR) and firing set switches. The ability of high gain GaAs photoconductive semiconductor switches (PCSS) to deliver fast risetime pulses makes them suitable for their use in radars that rely on fast impulses. This type of direct time domain radar is uniquely suited for the detection of buried items because it can operate at low frequency, high average power, and close to the ground, greatly increasing power on target. We have demonstrated that a PCSS based system can be used to produce a bipolar waveform with a total duration of about 6 ns and with minimal ringing. Such a pulse is radiated and returns from a 55 gallon drum are presented. For firing sets, the switch requirements include small size, high current, DC charging, radiation hardness and modest longevity. We have switched 1 kA at 1 kV and 2.8 kA at 3 kV DC charge.

Final report of LDRD project: Electromagnetic impulse radar for detection of underground structures

This report provides a summary of the LDRD project titled: Electromagnetic impulse radar for the detection of underground structures. The project met all its milestones even with a tight two year schedule and total funding of

High gain GaAs photoconductive semiconductor switches: switch longevity

400 k. The goal of the LDRD was to develop and demonstrate a ground penetrating radar (GPR) that is based on high peak power, high repetition rate, and low center frequency impulses. The idea of this LDRD is that a high peak power, high average power radar based on the transmission of short impulses can be utilized effect can be utilized for ground penetrating radar. This direct time-domain system the authors are building seeks to increase penetration depth over conventional systems by using: (1) high peak power, high repetition rate operation that gives high average power, (2) low center frequencies that better penetrate the ground, and (3) short duration impulses that allow for the use of downward looking, low flying platforms that increase the power on target relative to a high flying platform. Specifically, chirped pulses that are a microsecond in duration require (because it is difficult to receive during transmit) platforms above 150 m (and typically 1 km) while this system, theoretically could be at 10 m above the ground. The power on target decays with distance squared so the ability to use low flying platforms is crucial to high penetration. Clutter is minimized by time gating the surface clutter return. Short impulses also allow gating (out) the coupling of the transmit and receive antennas.

Doped contacts for high-longevity optically activated, high gain GaAs photoconductive semiconductor switches

Optically activated, high gain GaAs switches are being tested for many different pulsed power applications that require long lifetime (longevity). The switches have p and n contact metallization (with intentional or unintentional dopants) configured in such a way as to produce p-i-n or n-i-n switches. The longevity of the switches is determined by circuit parameters and by the ability of the contacts to resist erosion. This paper will describe how the switches performed in test-beds designed to measure switch longevity. The best longevity was achieved with switches made with diffused contacts, achieving over 50 million pulses at 10 A and over 2 million pulses at 80 A.