Search

A system is presented that is capable of measuring subnanosecond reverse recovery times of diodes in wide-bandgap materials over a wide range of forward biases (0 – 1 A) and reverse voltages (0 – 10 kV). The system utilizes the step recovery technique and comprises a cable pulser based on a silicon (Si) Photoconductive Semiconductor Switch (PCSS) triggered with an Ultrashort Pulse Laser, a pulse charging circuit, a diode biasing circuit, and resistive and capacitive voltage monitors. The PCSS-based cable pulser transmits a 130 ps rise time pulse down a transmission line to a capacitively coupled diode, which acts as the terminating element of the transmission line. The temporal nature of the pulse reflected by the diode provides the reverse recovery characteristics of the diode, measured with a high bandwidth capacitive probe integrated into the cable pulser. This system was used to measure the reverse recovery times (including the creation and charging of the depletion region) for two Avogy gallium nitride diodes; the initial reverse recovery time was found to be 4 ns and varied minimally over reverse biases of 50–100 V and forward current of 1–100 mA.

Wide-bandgap GaN optically-controlled switches have the potential for driving down the cost and size and improving the efficiency and capabilities of high voltage pulsed-power applications. Key to these applications is the understanding of the high-field photo-conductive properties of this material to determine if it will operate in a high-gain and/or sub-bandgap triggering mode, such as has been observed in GaAs. Photoconductive semiconductor switches (PCSS) were fabricated and tested from GaN wafers from Kyma Technologies and Ammono. With relatively low voltages applied the switch, linear photoconductive currents are measured in response to exposure to laser pulses of sub-bandgap wavelength (532 nm). Above a threshold voltage applied to the PCSS, persistent conductivity is measured that lasts well beyond the duration of the laser pulse and discharges the charged transmission line in the system. The sustaining field for this switching mode is approximately 3kV/cm. Another known distinguishing characteristic of high-gain switching is the formation of filamentary current channels that can be imaged due to the emission of recombination radiation from the plasma within the filaments. These filaments have been also observed in the GaN switches. High-gain switching has been initiated in GaN devices with as little as 2.5 μJ laser energy. This persistent photoconductivity is a distinguishing characteristic of what is known as "lock-on" effect most notably known in GaAs-based photoconductive switches, and is the basis for highly-efficient photoconductive switching requiring relatively little laser energy.

The demand for greater flexibility and increased energy density in pulsed-power systems is moving highly interactive components closer together. The development of compact technologies for less complex and more robust system designs is critical. A key system component that can impact these goals is the trigger generator (TG). Inexpensive, compact, and fiber-optically controlled TGs that deliver trigger pulses with subnanosecond jitter have been created with photoconductive semiconductor switches (PCSSs). However, high-voltage (HV) GaAs PCSSs are typically pulsed charged for less than 100 s so that they can hold off 60-100 kV/cm without self-triggering into high-gain (lock-on) switching or initiating surface flashover. Since many new pulsed-power system designs are based on dc-charged HV switches, pulse charging the trigger system is an additional complication requiring space, HV switching components, and HV cables. A further improvement in PCSS-based TG is to move from pulsed to dc-charged PCSSs. This paper reports results from dc-charged GaAs PCSSs with 0.25-1.0 cm gaps, extending previously reported results on smaller devices at 3 kV to a new regime of 100 kV. To hold off high fields for longer periods and to extend GaAs PCSSs to dc applications, we have utilized neutron-irradiated GaAs (n-GaAs). Neutron irradiation in GaAs increases the defect density, shortens the carrier recombination time, and (for devices with large insulating regions) reduces the dark current, which improves the dc hold-off strength. PCSS contacts in this research were created using rapid thermal annealing (RTA) to produce high adhesion and low contact resistance. However, this can reduce the defect density near the contacts by annealing some of the n-induced defects. Hence, a range of RTA temperatures and neutron doses was studied to understand the tradeoff space for contact adhesion and dc hold-off. This paper presents results from I-V characterization and dc hold-off on irradiated and nonirradiated GaAs PCSSs. These PCSS devices were demonstrated to hold off fields of 39-61 kVDC/cm, respectively. Irradiation doses over a range of 3×1013 - 1×1015 (1 MeV Si equivalent) were explored in search of the optimal performance. Additionally, the impact of the fabrication processes on the benefits of irradiation is explored, and the observation of unusual low-frequency oscillations during GaAs I-V testing is discussed.

The longevity of high gain GaAs photoconductive semiconductor switches (PCSSs) has been extended to well over ten 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 latter was achieved by varying the spatial distribution of the trigger light thereby widening the current filaments that are characteristic of the high gain switches. We 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 we recorded open shutter, infra-red images, and time-resolved Schlieren images of the current filaments, which form during high gain switching. We measured, under varying conditions, a carrier (electrons or holes) density that ranges from 3/spl times/10/sup 17/ cm/sup -3/ to 6/spl times/10/sup 18/ cm/sup -3/.

Summary form only given. Methods of producing multiple current-sharing filaments (MCSF) in GaAs photoconductive semiconductor switches (PCSSs) have produced as many as 30 filaments per switch with a spacing of 330 μm. As the approaches to triggering and isolating MCSF mature, the replacement of high current, conventional pulsed power switches with banks of MCSF PCSSs capable of switching tens of kiloamps become feasible and economical. Multiple banks of MCSF PCSS can eventually produce optically controlled pulsed power systems with faster rise-times, shorter pulses, and higher peak powers using more economical switches with device lifetimes of over one million pulses.This paper will report results from three types of MCSF approaches: (1) line-of-sight optics focused with cylindrical micro-lens arrays into bright narrow lines of light on the surface of the PCSS; (2) high-reflectivity dielectric optical masks which produce shadows of bright narrow lines of light on the surface of the PCSS; and (3) etched-steps in the surface of the PCSS to divide up the illuminating light and isolate the filaments. These approaches are being tested to switch 2-3 kA with three 1cm square GaAs PCSS. Switching parameters, approximate device lifetimes, and current-sharing capability are being measured and evaluated for each of these triggering techniques. Many other approaches have been considered previously, and their potential for other specific applications will be discussed. Increasing PCSS current density through the development of longer-lived PCSS with higher currents per filament is research that has continued in parallel with the multi-filament triggering work. This however is primarily an interface issue where the contact metal meets the GaAs. We will discuss new electrical contact geometries capable of reducing current crowding. Thereby lowering the peak current density to enable higher total filament current. Theoretical bases for improving contacts will also be discussed.

Summary form only given. Ferroelectric materials exhibit a hysteretic response similar to the B-H curve of magnetic materials. With ferroelectric materials the polarization-electric field (P-E) hysteretic loop is typically measured at low frequencies. However, 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 testing commercially available ferroelectric capacitors and materials, including 10 nf non-linear BaTiO-based capacitors and 3 mm thick lead zirconate titanate (PZT)-based materials. The main goal of the test bed is to drive a 1-50 nF ferroelectric sample around its polarization-electric field (P-E) loop with a low inductance circuit (10 nH) and high bandwidth (1 GHz) diagnostics at voltages up to 20 kV and currents up to 10 kA. A secondary goal is to drive the sample through P-E sub-loops to measure the sample response at high frequencies within the P-E loop. Polarization of the sample is obtained by integrating the current passing through the sample and dividing by its cross-sectional area. The electric field is determined by measuring the voltage across the sample and dividing by its thickness. Simulation of the circuit and analysis of the data require modeling of the sample behavior internal to the P-E loop and along its boundaries. A hyperbolic tangent (Fermi function) model that depends on the initial values of P and E and the polarity of the current provides a reasonable model for low bandwidth measurements of most materials. However P-E loops with sharper corners and/or a strong variation with frequency may require a model with a higher order exponential dependence, and a free parameter to characterize the sharpness of the corners independent of the slope of the loop through zero polarization. Simulation and analysis of our preliminary results will be discussed.

As the demand for greater flexibility and increased energy density moves highly interactive components closer together in pulsed power systems, the development of technologies for less complex and more robust system designs is critical. A key system component that impacts these goals is the trigger generator (TG). Inexpensive, compact, fiber-optically controlled TGs that deliver trigger pulses with sub-nanosecond jitter have been created with photoconductive semiconductor switches (PCSSs) [1]. A further simplification in pulsed power design is to move from pulsed to DC charged components. This paper reports results from DC charged GaAs PCSSs with 0.25–1.0 cm gaps extending previously reported results on smaller devices at 3 kV [2] to a new regime of 100 kV.

Three categories of circuits were explored to induce fast recovery, after lock-on by temporarily reducing the field across the switch. Measurements of recovery times from 35 to 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. A comparison is made of the different circuits used to induce recovery from lock-on, and the various factors that influence the recovery are discussed. >

We have presented experimental and simulation results for a SI GaAs and discussed the results in terms of the dominance of the trap levels in determining the I–V characteristics of the devices. Experimental results and simulation analyses indicate that, when subjected to neutron irradiation, there is an improvement in the hold-off characteristics of the devices, which may lead to higher voltage operation and thus an improvement in the rise time of the device. The maximum energy transferred to the load would increase, thereby improving the overall device characteristics. The filamentary conduction processes of the PCSS are more susceptible to failure if a large number of defect sites are formed in the device during a radiation transient. This is mainly because defects are expected to bring about inhomogeneous conduction.

Attempts to use photoconductive semiconductors as high-power (10-100 kV, 0.1-2 kA) toggling switches with recovery times of 5-100 ns have stimulated the exploration of their recovery mechanisms. We have observed that optically triggered GaAs switches exhibit "lock-on," i.e., when triggered, they do not recover as long as they are holding more than 4-8 kV/cm. Experiments are being performed to determine the minimum recovery time of these switches after lock-on. by immediately reducing their fields or currents. As an alternative to GaAs, Si is a semiconductor that does not exhibit lock-on. It has a very long recovery time (> 100 /spl mu/s) that can be shortened to less than 100 ns with highly concentrated gold doping (/spl ges/10/sup 15//cm/sup 3/). Device models that predict the behavior of PCSS and experiments on the recovery of GaAs and Au:Si switches are presented.

Experiments were performed to determine the applicability of photoconductive semiconductor switches (PCSS) for use as output switches in subnanosecond pulse for EMP simulators. Lateral switches made of both Gallium Arsenide and Silicon with 1.5 cm long insulating regions immersed in Fluorinert were tested in a 50 ohm tri-plate transmission line geometry. Mode locked and Q- switched lasers were used to trigger both a gas switched Marx generator which pulse-charged the transmission line in 100- 150 ns and to illuminate the PCSS via an optical delay line. Illuminating beam energies and electric field strengths at switchout were varied to determine minimum risetimes and light energies required for triggering. The GaAs switches were operated in the high gain (lock- on) mode. Risetimes as fast as 600 ps were observed using a mode locked laser and 700 ps using a Q-switched laser. The minimum light energy required to trigger GaAs was 22/spl mu/J and the highest switched fields for both GaAs and Si is about 60 kV/ cm.

Terahertz radiation from optically-induced plasmas on metal, semiconductor, and dielectric surfaces is compared to electron-hole plasma radiation from GaAs and Ge. Electro-optic sampling and electric-field probes measure radiated field waveforms and distributions to 0.350 THz.

Summary form only given. Short pulse lasers can be used to generate two types of short-lived, radiating plasmas. When absorbed by a semiconductor, a sudden optical pulse creates a charge-neutral, electron-hole plasma that radiates as the carriers relax in local crystal, surface, or applied electric fields. When focused onto the surface of an insulator or metal, the electric fields in an intense optical pulse break down the solid surface and surrounding gas. The charged particles in this surface-breakdown plasma radiate as they are created at the solid-gas (air) interface. The sudden creation of either of these types of plasmas with a sub-picosecond optical pulse produces a non-equilibrium distribution of charges that radiates with an extremely wide bandwidth in the THz regime. This paper reports the radiated electric field intensities, spectra, and associated radiation patterns for electron-hole plasmas from gallium arsenide and silicon, and for surface-breakdown plasmas from aluminum, glass (BK-7), gallium arsenide, and silicon. The plasmas are created and diagnosed with a chirped-pulse regenerative amplifier laser that produces a 1 kHz train of 120 fs wide pulses with up to 2 mJ of energy per pulse at a wavelength of 800 nm. To explore the wide bandwidth of these plasma radiation sources, two electric field diagnostics were used. A conventional electric field probe (D-dot) with a 40 GHz bandwidth and a 70 GHz sampling oscilloscope measured the low frequency tail of the radiation spectrum. To measure the high frequency range of the spectrum, electro-optic crystals (zinc telluride) sampled the radiated electric field as it co-propagated with an optical probe beam that was split off the excitation (pump) beam. The bandwidth of the electro-optic sampling is limited by the thickness of the electro-optic crystals, so a range of thicknesses was employed to demonstrate source-limited measurements. The fundamental differences between electron-hole and surface-breakdown plasmas will be discussed. Similarities and contrasts in radiation patterns and efficiencies will be described for all of these plasma radiation sources. Simple models for the picosecond carrier motion in these sources will be presented and compared with measurements. The advantages and challenges of using a kilohertz, millijoule, short-pulse laser for plasma generation and electro-optic sampling will also be mentioned.

Gallium arsenide (GaAs) photoconductive semiconductor switches (PCSS) have been studied as an enabling technology for a variety of applications at both the Air Force Research Laboratory and Sandia National Laboratories. High gain PCSS can be triggered with small laser diodes or laser diode arrays. The requirements of these applications require the switching of high voltage in sub-nanosecond time with low temporal jitter of the switches relative to the trigger laser. There have been several configurations and sizes of these switches studied by the Air Force Research Laboratory over the last several years. The most recent designs are with small structures where the electrical contacts are placed on opposite sides of the bulk material. This configuration allows for different electrical conditions on either side depending on the nature of the semiconductor structure; i.e., p-i-n or n-i-n. In addition to the type of structure used and geometry of the contacts, the performance of these switches (switch time, voltage, and jitter) is dependent on the thickness of the GaAs. Several thicknesses have been studied during the past year. This paper reports on the results of several studies to investigate the ultra-fast switching properties of these structures.

It is shown that Si photoconductive semiconductor switches can be used to switch high voltages (up to 123 kV), high fields (up to 82 kV/cm) and high currents (2.8 kA). The ability of the samples to withstand this type of high-voltage, high-current switching depends on the way in which the current penetrates the semiconductor. The appropriate use of water or contacts greatly improves the switching capability. It is also shown that the wafers can support large currents (4.0 kA for GaAs and 2.8 kA for Si) and large linear current densities (3.2 kA/cm for GaAs and 1.4 kA/cm for Si). For GaAs this linear current density corresponds to about 1 MA/cm/sup 2/ given a penetration depth of about 10/sup -3/ cm. >

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. >

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.

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.

High energy, electrically controlled, compact, short-pulse lasers are useful for active optical sensors. We present a new class of semiconductor laser that can potentially produce much more short pulse energy than conventional (injection-pumped) semiconductor lasers (CSL) because this new laser is not limited in volume or aspect ratio by the depth of a p-n junction. We have tested current filament semiconductor lasers (CFSL) that have produced 75nJ of 890nm radiation in 1.5ns (50W peak), approximately ten times more energy than ISL. These lasers are created from current filaments in semi-insulating GaAs and, in contrast to CSL, are not based on current injection. Instead, low-field avalanche carrier generation produces a high-density, charge-neutral plasma channel with the required carrier density distribution for lasing. This paper will report spectral narrowing, lasing thresholds, beam divergence, temporal narrowing, and energies which imply lasing for several configurations of CFSL. It will also discuss active volume scaling based on recent high current tests.

This paper reports on a recent comparison made between the Air Force Research Laboratory (AFRL) gallium arsenide, optically-triggered switch test configuration and the Sandia National Laboratories (SNL) gallium arsenide, optically-triggered switch test configuration. The purpose of these measurements was to compare the temporal switch jitter times. It is found that the optical trigger laser characteristics are dominant in determining the PCSS jitter.

A new impact ionization theory is proposed for current filaments in optically triggered semi-insulating (SI) GaAs switches. The theory explains the rapid switching and lock-on voltage observed in these switches in terms of hot carriers which become more effective at impact ionization at higher carrier densities. The theory is implemented by hydrodynamic transport equations which include kinetic terms for hot carriers and hot phonons. The solutions of these equations are in good agreement with current versus voltage data for optically triggered GaAs switches.

Infrared images of GaAs photoconductive semiconductor switches (PCSS) show current filamentation during high gain switching. Since high speed switching characteristics for these devices are related to the initiation and control of these filaments, many experiments have been performed to determine how such control can be achieved. The experiments reported here use fiber-optic coupled laser diode arrays (LDA) to initiate switching and to control the location and number of current filaments. This paper describes the nature of these current filaments as the fiber-optic configuration and the switch test parameters are varied. These experiments show that fiber-optic coupled LDAs can be configured to make efficient use of the optical trigger energy, control the current flow, and improve overall switch performance. One unusual property of the current filaments is that their rate of formation is much faster than the saturation carrier velocity in GaAs. This paper will present results from direct measurements of the filament growth velocity which indicate velocities up to 2 {times} 10{sup 9} cm/s. The implications of this result on several models for the initiation of high gain switching will be discussed.

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.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A process was developed for the fabrication of high power GaAs photoconductive switches with ohmic Ni/Ge/Au contact metallization in a lateral NIN switch configuration. The main features of the process were ion implanted silicon in the contact regions and rapid thermal processing for annealing the implants and alloying the metallization. A 50 mill dia. process wafer included 28 photolithographically defined switches of four different types to comparatively investigate the effects of ohmic contacts and switch dimensions on switch lifetime, together with monitors to characterize the ohmic contact process.

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.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.