Gary J. Denison

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.

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.

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.

Photoconductive, semiconductor switch technology for short pulse electromagnetics and lasers

High gain photoconductive semiconductor switches (PCSS) are being used to produce high power electromagnetic pulses for: (1) compact, repetitive accelerators; (2) ultra-wide band impulse sources; (3) precision gas switch triggers; (4) optically-activated firesets; and (5) high power optical pulse generation and control. High power, sub-nanosecond optical pulses are used for active optical sensors such as compact optical radars and range-gated/ballistic imaging systems. Following a brief introduction to high gain PCSS and its general applications, this paper focuses on PCSS for optical pulse generation and control. PCSS technology can be employed in three distinct approaches to optical pulse generation and control: (1) short pulse carrier injection to induce gain-switching in semiconductor lasers; (2) electro-optical Q-switching; and (3) optically activated Q-switching. The most significant PCSS issues for these applications are switch rise time, jitter and longevity. This paper describes both the requirements of these applications and the most recent results from PCSS technology. Experiments to understand and expand the limitations of high gain PCSS are also described.

Long-lifetime silicon photoconductive semiconductor switches

We present the results of experiments aimed at improving the lifetime (longevity) of Si photoconductive semiconductor switches (PCSS). Because damage at the metal-semiconductor interface is the primary damage mechanism in most PCSS, we have tested different contact metallizations. The test setup utilizes: a Nd:YAG laser that operates at 540 Hz with 50 mJ, 10 ns FWHM pulses; a circuit that charges a 50 (Omega) line in 800 ns and discharges it in 20 ns through a 50 (Omega) load; and a lateral switch geometry and 0.25 cm by 0.25 cm switches. The contacts examined include: Cr(diffused)-Cr-Mo-Au, Al(diffused)-Cr-Mo-Au, 31P(ion implanted)-Ti-Pt, Al(diffused)-Pt-Ti-Pd-Au, and edge contacts. In the case of the Cr contacts we have tried thicker Mo or Au layers. For the Al contacts we have tried 1 micrometers and 0.1 micrometers thick depositions. Most contacts survived 107 pulses when switching 32 kV/cm (8 kV over 0.25 cm). The Al diffused went up to 44 kV/cm (1 X 105 pulses). The implanted P switch was switched 2.2 X 107 times at 44 kV/cm and 0.9 X 106 times at 48 kV/cm.

GaAs PCSS: high-gain switching and device reliability

Characteristics of GaAs photoconductive semiconductor switches (PCSS) during the initiation and sustaining phases of high gain switching are studied in this paper. Infrared electro-photo luminescence data are present which show current filaments during high gain switching mode. Triggering of these devices with multiple fiber optics is demonstrated and the implications for high average current density switching are discussed. Switch jitter with high field lateral PCSS has been tested and its impact on multiswitch systems is explored. Results from high repetition rate device lifetime testing are also reported. Lateral switches with several types of contacts (including: refractory metallizations, diffused and ion-implanted Ohmic layers) have been fabricated and tested. Switch characteristics and lifetime results will be discussed for each of these fabrication schemes.

A testbed for high voltage, high bandwidth characterization of nonlinear dielectrics

The dielectric response of many high permittivity materials is nonlinear with both field and frequency. For example, ferroelectric materials exhibit a hysteretic polarization - electric field (P-E) response similar to the B-H curve of ferromagnetic materials. These P-E hysteresis loops are typically measured at low frequencies; the material behavior at high frequencies is less understood. To address this information gap, a test bed has been created to characterize non-linear material behavior at high frequencies and high voltages. This paper will report testbed goals in addition to design, assembly, analysis, and issues. Preliminary results will also be presented from commercially available nonlinear capacitors and in-house fabricated ferroelectric materials, including 10 nF non-linear BaTiO3-based capacitors and 3 mm thick lead zirconate titanate (PZT)-based materials.