This paper presents initial experimentation conducted on optically activated Gallium Arsenide (GaAs) switches when operated at high electric fields. A Nd:YAG laser operating at 1.06 micrometers has been used to initiate conduction in three sizes of planar GaAs switches. The optical energy required for initiation and delays observed prior to collapse into the sustained current mode are discussed. In addition, dark dc and pulsed experimental characterization studies of ion implanted GaAs switches are presented. The switches were implanted up to approximately 1018 carriers/cm3 under the cathode contact region.
The spectral and temporal nature of the recombinant radiation from a GaAs photoconductive switch is described. A simultaneous measurement of the electrical avalanche pulse and associated temporal luminescence peak is utilized to extract a propagation velocity for the filamentary tip of approximately 5 X 108 cm/s. The measured spectral content of the filaments in a bulk structure is presented for various bias voltages and compared to a theoretical model for band-to-band recombination considering temperature, carrier density, and self-absorption effects.
The electric field structure in GaAs photoconductive switches was recorded by means of an optical diagnostic technique which is based on the Franz-Keldysh effect. At low voltages a 100 micrometers wide region of high electric field strength was seen at the cathode only. With increasing voltage, but below the lock-on value, strong domain like field structures emerge in the anode region. At voltages where lock-on of the photocurrent occurred, current filaments were recorded which seem to shorten the electric field structures. Damage due to filamentation was observed mainly at the contacts. Increasing the intensity of the activating laser and consequently the photocurrent caused the electric field in the domains near the anode to increase and resulted in a lowering of the threshold value for lock-on.
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.
The reliability of the optically triggered Bulk Avalanche Semiconductor Switch (BASS) has been assessed in a variety of test conditions. For a nominal characterization diagnostic, we have adopted a 50-ohm video transmission line having a pulsewidth of 0.5 ns. At 11 kV input voltage we have found BASS lifetimes in excess of 1 Billion pulse. This level of device lifetime is a record achievement for optically triggered semiconductor switches. The reliability distribution is found to be of Weibull type. We discuss the implications of the reliability distribution and the performance of the device during lifetime evaluations.
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 novel technique for deep level characterization of high-resistivity semiconductor materials, mainly GaAs has been developed. The technique analyzes the current transients generated by photo induced current transient spectroscopy (PICTS) using a digital data acquisition approach. It is observed that by using the curve fitting technique closely spaced traps within the bandgap could be separately identified and characterized. In addition, the multiple decay time constants obtained from curve fitting consists of sum of exponentials throughout the temperature range centered around a peak obtained from the PICTS spectrum. Each decay time constant (mode) around the specified temperature range represents a trap energy level present in the sample. Since the decay time constants are obtained directly from the transients, trap parameters calculated employing the curve fitting method are more accurate and reliable than those obtained from the two-gated rate window method. The method has been used to characterize high-resistivity materials of interest to optically activated switches and a comparative analysis is reported.
We present the results of empirical studies of the breakdown of p+-i-n+, n+-i-n+, and p+-i-p+ silicon structures with approximately 1 cm long intrinsic regions. These results show that the contacts have a strong effect on the breakdown characteristics of these devices. We present results for laminated structures and for a device with a ribbed surface which show that the breakdown characteristics of silicon devices can be improved significantly by such geometry modifications. We also present empirical evidence that double injection effects occur in p+-i-n+ devices, and that these effects will limit the efficacy of such devices when subject to relatively long voltage pulses.
New experimental results are reported for silicon samples at high fields in different ambients: vacuum, N2, air, SF6 and SF6 + N2 mixtures, for different experimental conditions. The strong influence of the semiconductor quality and ambient dielectric on the prebreakdown and breakdown response of the system is presented. The high surface flashover sensitivity of semiconductor-vacuum system, as well as the significant interface activity in air and N2 is explained. Using a new physical model of the breakdown in semiconductor- dielectric systems, a new technique was developed consisting of an in-situ treatment of the semiconductor sample (in vacuum) before performing the high field measurements in SF6. Dramatic improvement of the voltage hold-off was obtained in one atmosphere (absolute) SF6. All standard good quality samples, subjected to the vacuum pre-treatment, supported without surface flashover, high fields in the range of 75 - 85 kV/cm, close to the critical breakdown field for SF6 gas at one atmosphere (approximately 89 kV/cm). The role of different defects in producing premature breakdown by surface flashover in SF6 is discussed.
High Tc superconductor jitter-free-opening switches, made of YBCO films, with switching risetime of 100 ps and switching efficiency of 80% have been successfully demonstrated. The switches were used in an inductive energy storage pulsed power system (IESPPS). Ultrashort electrical pulses with pulse compression and peak power gain of 40 were obtained.
High-Tc superconducting thin films are used to design a new type of opening switch. The superconducting film screens the magnetic flux linkage between the primary and the secondary coils of a transformer. Short laser pulses (150 ps) are used to trigger the transition of the superconductor to its normal state allowing the flux produced by a primary current to couple to the secondary. This results in an induced voltage pulse in the load across the secondary and corresponds to the open state of the switch. Experiments have confirmed the feasibility of this inductively coupled switch, and rise times of 50 ns have been observed for the secondary voltage pulse. The switch has the potential for high current (1 kA) and fast switching times (< 1 ns).
Photoconductivity of copper compensated, silicon doped GaAs (GaAs:Si:Cu) which is used as a bulk optically controlled semiconductor switch (BOSS) material is studied. Copper is diffused into GaAs:Si using a spin-on dopant source (Cu-doped silicon dioxide), and a leaky tube diffusion system for annealing. The switch photoconductivity is compared to that of samples processed using closed tube diffusion systems, where the Cu source for diffusion is a Cu film deposited by thermal evaporation on one surface of the GaAs wafer. High efficiency switch material is obtained by using the leaky tube technique and low power semiconductor laser is used as the excitation source for the switch photoconductivity measurements.
The formation of low resistance ohmic contacts to semiconductor devices in which no melting ('alloying') of the contacting metals occurs is highly desirable. To form these contacts a very thin depletion layer must be created at the metal-semiconductor, MS, interface with tunneling occurring at the Fermi level. We describe two contacting schemes to p-type GaAs that avoid the alloying step and permit the use of robust refractory metals for the contacts. The p-contact will be important in proposed photo conductive switches using a P-i-N structure instead of the more common N-i-N structure. The work reported describes our studies of (a) a simple diffusion apparatus to create degenerate hole concentrations near the semiconductor interface and (b) a very stable highly doped epitaxial layer grown using Liquid Phase Epitaxy (LPE).
We report recent subnanosecond photoconductive measurements on a p-i-n device made from a copper-compensated silicon-doped semi-insulating GaAs (GaAs:Si:Cu) substrate. Photoconductivity in this relatively large volume device (0.05 X 0.5 X 1.0 cm3) is generated by extrinsic absorption of a 1-micrometers laser pulse, and optically quenched with a 2-micrometers laser pulse. It is shown experimentally that these large, high- power (multimegawatt) switches have the potential to switch subnanosecond pulses when a copper-compensated GaAs material with suitable fast electron-hole-pair recombination rate is engineered. Test and evaluation of BOSS devices with subnanosecond switching performance is reported. In one case, observation of an electron-hole pair recombination time constant of 0.25 ns is reported in GaAs:Si:Cu irradiated with a neutron dose of 1.8 X 1015 cm-2, a factor of seven faster than previously reported.
We describe a numerical model of an avalanche switch that incorporates the possibility of filament creation. Detailed results of this method are presented which show time delay between beginning of initiating laser pulse and switching, turn-on time much faster than carrier transit time, and formation of high current filament. Quantitative results from this model are in reasonable agreement with experimentally observed values.
A streamer model for high gain photoconductive switching in GaAs is proposed and described. The electric field associated with the streamer is strongly inhomogeneous, having a peak intensity at the streamer tip several times larger than the average applied field. Such a large field incites impact ionization effectively, allowing the streamer to propagate at speeds well in excess of the saturated drift velocity. This model is consistent with all observations of high gain photoconductive switching, including transient measurements.
Order of magnitude estimates suggest that optically controlled bulk semiconductor switches should be able to withstand voltages up to the product of their thickness and the dielectric strength of their material. In reality, however, the devices fail--i.e., exhibit a behavior that resembles dielectric breakdown--already at voltages which are much lower. This deficiency threatens to limit the prospects of the device concept quite seriously and has so far not completely been understood. In our paper, we discuss several mechanisms which may underlie the observed phenomenon, and focus in particular on the dynamical aspects of it, namely on the sudden transition ('sudden breakdown') which takes the switch within a few ns from the resistive off-state to a highly conductive on-state. We investigate a scenario that relates this transition to a second effect also seen during breakdown, namely to the spontaneous onset of current filamentation, and speculate that the magnetic self-contraction of the current (known as the 'pinch effect') may play an essential role in the process. On the basis of a mathematical device model which incorporates the effects of particle transport and magnetic interaction, we obtain quantitative results for the speed and the threshold of magnetically driven filamentation, and find those numbers to lie in the A and the microsecond(s) region, respectively. We conclude that the magnetic pinch may play a essential role in the dynamics of current filamentation and fast breakdown, but cannot explain the fast observed current rise in the ns-range by itself.
A transmission-line model was used to study the asymmetric illumination of photoconductive switches. We studied the evolution of the electric field within the first 2 ns of optical illumination. The electric fields were significantly compressed in the regions of low photocarrier density. Also, the field collapse was slower and the switching efficiency reduced in the assymetric illumination case. Both the symmetric and the asymmetric cases were investigated experimentally with our electro-optic sampling system. The experimental results confirmed the predictions of the transmission-line model.
A simple transmission line model, which seeks to explain phenomena associated with avalanche/displacement current waves in semiconductors, is discussed. The model relies on breaking up the semiconductor drift space into small cells, each of which contains a transmission line element so as to allow an electromagnetic wave to propagate away from the generated plasma. The transmission line element also serves as the energy storage element. A time varying resistor controls the conductivity, induced by either a light signal or an avalanche. As expected, the model points out the importance of triggering an avalanche/displacement current wave in regions where the static field is high. Under certain conditions the model predicts a growing electromagnetic wave with sufficient amplitude to sustain avalanching. The model offers a possible explanation of the observed fast risetime pulses resulting from either optical or avalanche excitation of a small, spatially limited region of the semiconductor region.
A general microcomputer based engineering model for high voltage photoconductive switch operation in the linear mode (closure by high photon flux) and nonlinear mode (closure by low photon flux) has been devised and implemented. The engineering model has been used to predict nonlinear photoconductive switch closure times with reasonable accuracy. Using an interactive graphics 'shell' the model is being used to search for an improved switch design through the use of selective doping distributions. This paper presents an overview of the model and some simulated data results.
Large increases in conductivity induced in GaAs and other semiconductors by photoionization allow fast switching by laser light with applications to pulse-power technology and microwave generation. Experiments have shown that under high-field conditions (10 to 50 kV/cm), conductivity may occur either in the linear mode where it is proportional to the absorbed light, in the 'lock-on' mode, where it persists after termination of the laser pulse or in the avalanche mode where multiple carriers are generated. We have assembled a self-consistent Monte Carlo code to study these phenomena and in particular to model hot electron effects, which are expected to be important at high field strengths.
The use of linear photoconductive switches rather than nonlinear switches for the generation of Ultra-Wide-Band (UWB) pulses provides advantages such as jitter-free operation, low losses, and a reduction of the electrical and mechanical stresses in the switch. These advantages lead to the operation of many switches in series and/or parallel, higher average powers and longer lifetimes. Energy Compression Research Corporation (ECR) has demonstrated an advanced UWB source based on light activated silicon switches (LASS). The UWB source consists of a single LASS device mounted on a low impedance (< 0.5 (Omega) ) microstrip transmission line and a high fidelity impedance transformer connected to a 50 (Omega) coaxial connection. The voltage was measured at low impedance and 50 (Omega) to verify the efficiency and fidelity of the impedance transformer. After a transformation of 110:1 in impedance, the measurement at the end of the transformer verified that pulse rise-time was less than 100 ps and the overall efficiency was 50%. The system was tested up to 10 kV into 50 (Omega) before connector breakdown limited further increase. Larger powers can be radiated if the transformer is directly connected to the antenna.
We report on the application of new electro-optic modulators which are controlled by photoconductive switches. Various configurations of modulator are presented and are applied to several applications. Experimental results are presented in which a photoconductively controlled modulator is used to Q-switch and cavity dump a Nd:YAG laser, utilizing only a single external trigger. The same modulator is also used to Q-switch, mode-lock and cavity dump the same laser. The use of the modulator results in a single laser being able to generate pulses in three duration ranges, i.e., approximately 20 ns, approximately 2 ns and approximately 70 ps. A similar modulator is used to suppress the pre-pulse generated by a regenerative amplifier and help attain a 105 contrast ratio between signal and pre-pulse. Other modulators and applications are presented, including an approximately 30 ps Pockels cell for use as an optical gate.
The stimulated Brillouin scattering effects in liquids is used in conjunction with a 10 ns Q- switched Nd:YAG laser to produce optical pulses of subnanosecond rise time. Various optical schemes are described using Brillouin oscillators and Brillouin amplifiers. The compressed optical pulse is intended to activate a GaAs photoconductive switch producing ten's of MW of electrical power with subnanosecond rise time.
Opening switch work at the University of Maryland is primarily focused on the materials of GaAs and ZnSe. We discuss recent progress in work with both materials. Recent work in GaAs has revealed that a non-'lock on' state of sustained conductivity could be produced in GaAs photoconductive semiconductor switches (PCSS's). The sustained conductivity is produced when a PCSS is switched into a high impedance (500 (Omega) ) load by a high intensity (90 mJ - 12 ns) laser pulse. Furthermore, we have shown that the conductivity of the GaAs switch could be quenched at a time determined by circuit parameters. Thus, the switch could be used as an opening switch in an inductive energy storage pulsed power system without the need for pulse shaping or a different wavelength laser to open the switch. We have demonstrated such an inductive energy storage system with a power gain of 4.2 and a switching opening time of < 10 ns. We have also undertaken experiments with polycrystalline ZnSe which indicate that it may operate as well as a closing switch material. The ZnSe switches were found to exhibit a nonlinear effect at high applied electric fields. In this paper we report on our investigations of ZnSe opening switch using a simple lumped inductive circuit. At high fields and short wavelength, we observed in ZnSe a behavior similar to 'lock-on' as observed in GaAs. No such behavior was observed for longer wavelengths even at higher bias fields. This suggests the strong local field caused by the nonlinear effect observed in previous experiments is responsible for the onset of the observed 'lock-on' behavior.
The on-resistance of a GaAs coplanar waveguide-photoconductive switch was characterized as a function of laser photon energy, switch temperature, and applied dc electric field. An electric-field-dependent resonance at photon energies near the GaAs energy band-gap edge has been observed. This resonant behavior is believed to be caused by a competition between carrier recombination in the switch bulk and carrier sweep-out effects near the switch surface. This field-induced resonance was verified with 5, 10 and 20 micrometers switch gaps that were fabricated on three separate semi-insulating GaAs wafers. For fixed-wavelength laser sources, it has been shown that one can optimize the optical coupling by varying the switch temperature. The switch resistance decreased by a factor of three as a result of an increase in the switch temperature of 20 degree(s)C at photon energies near the absorption edge. A conductive-mode plasma model has been developed that adequately predicts the nonresonant switch behavior.
Several electron beam activated diamond switches have been constructed and operated. In an initial set of experiments the electron source consisted of a LaB6 photocathode illuminated by approximately 15 nanosecond pulses of 248 nm light from a KrF laser. The photocathode could be biased at voltage of 10 - 80 kV. The type IIa diamond wafer was 12 microns thick with top and bottom electrodes consisting of Ti/Pt/Au sputtered metallizations (unannealed). Limited by surface flashover across a 12 micron broken edge of the diamond wafer, pulses with a peak power at the kilowatt level into 50 ohms were generated. The output pulse duration was set by the electron beam duration or the round trip time in the charged transmission line, whichever was shorter. Measurement of the output pulse rise time was limited by the diagnostic oscilloscope resolution but was less than one nanosecond. It was observed that the output pulse amplitude reached the expected value only when the bombarding electron beam voltage was sufficiently large that carrier pairs were generated throughout the thickness of the diamond sample.
The optoelectronic properties of 6-H silicon carbide (6H-SiC) were investigated using lateral and vertical photoconductive switches. We report the measurement of photovoltaic and photoconductive effects for both geometries and at several wavelengths near the 6H-SiC absorption edge. The carrier lifetime in p-type 6H-SiC is also reported. Although the devices possess dark resistances on the order of 10 (Omega) , the switching efficiency of the vertical switches approached 32 percent, while the resistance of the lateral devices could be reduced by 50 percent with 200 (mu) J of laser radiation at (lambda) equals 337 nm. In addition, we measured photoconductivity in the vertical switches with a device static powers dissipation exceeding 11 Watts. Although the device was glowing from the high level of dc power being dissipated, only the switch mount was damaged. 6H-SiC is indeed a high-temperature optoelectronic material.
Thermal ionization of electrons and holes is proposed as an explanation for the sustained phase of lock-on in semi-insulating GaAs. In this mechanism, thermal ionization leads to a current instability which drives the formation of current filaments. The model predicts bistable states: either a highly conductive on-state or weakly conductive off-state. The model predicts that a transition from the off-state to a filamentary-current on-state can be triggered by illumination in agreement with experiments using photoexcitation.
Optically triggered semiconductor switches are a useful tool to generate steep high power pulses. The properties of these switches allow a fast switch closure with low jitter. Thus a parallel operation of a number of switches in various geometric arrangements is possible. This paper reports on investigations to examine how the two well-known operation modes of such switches, the linear and the non-linear photoconductive mode, can be combined in order to optimize the trigger effort in relation to the switch properties. Furthermore, it is investigated, whether such switches can be applied in a low-impedance pulse generator to generate 30 - 50 kV rectangular voltage pulses, duration 20 ns, at an impedance of 0.2 (Omega) . In addition some experiments are outlined where such switches are used to produce steep high voltage pulses in order to improve the simulation of electromagnetic stresses.