GaAs Photoconductive Semiconductor Switches Research Articles

高增益砷化镓光导开关中的特征量分析

高增益砷化镓光导开关中的特征量分析

不同形状的光斑触发砷化镓光导开关

不同形状的光斑触发砷化镓光导开关

A Novel Application of GaAs Photoconductive Semiconductor Switch in Triggering Spark Gap

Based on the characteristics of the GaAs photoconductive semiconductor switch (PCSS) and the spark gap, a new combined switch is developed. The combined switch integrates a traditional GaAs PCSS and a normal spark gap, which could offer two to three times of self-breakdown voltage for the spark gap in nanoseconds. Compared with the traditional single GaAs PCSS, a higher peak current at the output terminal was observed in the experiments when the combined switch was used. Based on the transferred electron effect and the runaway fast electron model, the turn-on process of the GaAs PCSS and the mechanism for the formation of plasma channel in the spark gap are discussed. In addition, due to the interaction between the GaAs PCSS and the spark gap, lock-on effect is avoided in a bulk GaAs PCSS when the bias electric field is larger than the lock-on threshold, which indicates that this combined switch could improve the longevity of the GaAs PCSS under high applied voltage.

A high-voltage and high-current photoconductive semiconductor switch and its breakdown characteristics

Semi-insulating GaAs photoconductive semiconductor switch (PCSS) with withstand voltage of 32 kV and peak current of 3.7 kA has been developed. The breakdown mechanism of the PCSS is analyzed. It is shown that the breakdown of PCSS fabricated from indirect band-gap semiconductors is determined mainly by limited conduction of trap filling, but for PCSS's fabricated from materials that exhibit the transferred-electron effect, such as GaAs, breakdown of the PCSS is caused mainly by the negative-resistance-induced electric field enhancement at the anode boundary. Based on the Gunn effect electronics, the breakdown voltage of the semi-insulating GaAs PCSS is calculated, and the calculated results agree with the experimental results.

Characteristics of photoconductivity oscillation in semi-insulating GaAs photoconductive semiconductor switches

The 4 mm gap and 5 ns pulse width semi-insulating GaAs photoconductive switches were triggered by 532 nm laser pulse with gradual increase of bias voltage from 500 V in steps of 50 V until the emergence of nonlinear electrical pulse. The expermental results showed that the linear and nonlinear electrical pulse waveforms had smaller amplitude and varying degrees of oscillation reduction after going through a main pulse. Then the microscopic state and transport process of carriers （hot-electron） in the switch material were studied in detail using the quantum theory. It was found that in the DC bias electric field, when the relaxation time of the hot-electrons in the electron-electron and electron-phonon interaction process is longer than the carrier life, the photoconductivity oscillation can be caused by the change of mobility in the process of optoelectronic transport, which is the main cause for the output electrical pulse to show oscillations.

Analysis of operation mechanism of semi-insulating GaAs photoconductive semiconductor switches

This paper reports that the quenched-domain mode of luminous charge domain has been observed in semi-insulating (SI) GaAs photoconductive semiconductor switches (PCSSs) and the typical characteristics of lock-on effect have been explained based on the model of luminous charge domain. It is shown that the domain is formed by photogenerated carriers and the quenched domain is due to the interaction of the circuit self-excitation and transferred-electron oscillation in the bulk of switch. During the transit of the domain, the bias electric field (larger than Gunn threshold) across the switch is modulated by the ac electric field, when the instantaneous bias electric field is swinging below the sustaining field (the minimum electric field required to support the domain), and then the quenched-domain mode is obtained. When PCSSs operate in nonlinear mode, the ultrahigh electric field of domain induced by photogenerated carriers leads to strong impact ionization accompanied by electron-hole recombination radiation in the switch. Therefore new avalanche domains can be nucleated uninterruptedly by the carriers generated by absorption of recombination radiation which causes the effective carrier velocities to be larger than the saturation velocity. Lock-on field resulted from the length proportional number of domains and steadfast electric fields inside and outside the domains, and the recovery of lock-on effect is caused by the domain quenching. The calculations agree with the experimental results, and the analysis also indicates that SI-GaAs PCSS is a photoactivated charge domain device.

Multiple charge domains model for the lock-on effect in GaAs power photoconductive switches

This paper reports that the lock-on field of semi-insulating (SI) GaAs photoconductive semiconductor switches (PCSSs) was measured under different bias voltages. Based on the experimental results and the transferred-electron effect, a model for the lock-on effect in GaAs PCSSs is proposed. It is shown that the charge domain with an ultrahigh electric field is due to a high photogenerated carrier density, which gives rise to intensive impact ionization accompanied by electron–hole recombination radiation within the domain. Since new domains can be nucleated uninterruptedly by the carriers generated by absorption of recombination radiation, the forefront domain crosses the switch at a speed alternating between the photon velocity and the carrier saturated drift velocity, which makes the observed velocity of carriers larger than the saturated drift velocity. The lock-on field results from the fixed number of a moving train of avalanching charge domains, the steady-state domains electric fields and the steadfast external electric field of the domains. The recovery of the lock-on effect is caused by domain quenching. The calculations agree with the experimental results. Moreover, the analytical results indicate that SI-GaAs PCSS is essentially a type of photo-activated charge domain device.

30 kV and 3kA semi-insulating GaAs photoconductive semiconductor switch

Current as high as 3.7kA has been generated using a single photoconductive semiconductor switch (PCSS) excited by a laser pulse with the energy of ∼8mJ and under a bias of 28kV. The PCSS with electrode gap of 14mm was fabricated from semi-insulating GaAs. Under different bias voltages the “on” resistances of the PCSS were measured. The longevity of the PCSS reached 350 shots at 20kV and 400A. The breakdown mechanism of the PCSS is analyzed based on the breakdown characteristics. It is shown that the breakdown of GaAs PCSS can be described by the electron-trapping breakdown theory.

A new phenomenon in GaAs photoconductive semiconductor switches triggered by laser diode

AbstractA new phenomenon in GaAs photoconductive semiconductor switches triggered by laser diodes is reported. It occurs when the switch is biased under the threshold and triggered by serial laser pulses. This phenomenon reflects the carrier accumulation effect when the switch operates at the nonlinear mode. The effects of bias voltage and optical pulse energy on the carrier accumulation are investigated. The results indicate that number of carrier can be controlled by adjusting bias voltage and optical pulse energy. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2232–2234, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22678

Monte Carlo simulation of operating modes of semi-insulting GaAs photoconductive switches

An ensemble Monte Carlo simulation method has been employed to study the operating modes of semi-insulated GaAs (SI-GaAs) photoconductive semiconductor switches (PCSS) excited by a femtosecond laser pulse and biased in an electrical field. The results show that, under a bias field lower than the Gunn threshold field (which is 4.2kV/cm for GaAs), or the optical pulse energy is below the optical threshold, the time-resolved current in the switch operates in the linear mode. When the bias field and triggering optical energy are respectively greater than the electrical and optical thresholds, the PCSS operates in the nonlinear mode. The simulation shows that when triggered by laser pulse with higher photon energy, the switch has lower optical threshold in the nonlinear mode. The mechanism of the nonlinear mode for the switch is concluded to be due to the inter-valley scattering of the photo-generated carriers, leading to local high field in the switch.

Study of the dipole characteristic of terahertz wave emitted from photoconductor switches

In this paper，different methods are used to simulate the dipole characteristic of terahertz (THz) wave emitted from low-temperature grown GaAs (LT-GaAs) and Semi-insulting GaAs (SI-GaAs) photoconductive semiconductor switches. The results indicate that the main cause of the dipole characteristic of THz wave emitted from LT-GaAs is the lifetime of optical-generated carriers being shorted than the generation time. For SI-GaAs photoconductive semiconductor switches with lifetime of optical-generated carriers longer than 100ps，the dipole characteristic of THz waveforms is mainly caused by intra-valley scattering and space charge field screening under different experimental conditions (different bias field and different optical pulse energy).

Mechanism analysis of periodicity and weakening surge of GaAs photoconductive semiconductor switches

The authors analyze the periodicity and weakening surge of semi-insulating GaAs photoconductive semiconductor switches. It is shown that the periodicity and weakening surge of the output current wave form is caused by the self-excitation of the circuit. The electric field threshold ET and the sustaining electric field Es (the minimum electric field required to support the domain) are modulated by the ac electric field; the output wave form becomes two main modes of transelectron oscillator, namely, delayed domain mode and quenched domain mode.

Generation of steady and jitter-free ultra-fast electrical pulses with GaAs photoconductive switches

In this paper, experiments of a lateral semi-insulating GaAs photoconductive semiconductor switch triggered by nanosecond, pico-second and femto-second laser pulses were reported. The switches was insulated by solid multi-layer transparent dielectrics. Jitter-free electrical pulse with steady voltage amplitude from the 1mm-gap GaAs switches was observed when biased with a low voltage and triggered by the serial laser pulses. Its change of amplitude was less than 1.2%, the triggered jitter-time was less than 10ps, and pulse width was up to sub-nanosecond. The effect of pulse energy change on the amplitude generated photoconductive semiconductor switch was analyzed. It was indicated that ultra-fast electrical pulse with steady voltage amplitude and pico-second triggered jitter-time can be obtained by controlling switch trigger condition and optimizing the switch design.

Doped contacts for high-longevity 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 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 employing multiple switches connected in parallel. The contact metal has a different damage mechanism, and the threshold for damage (/spl sim/40-80 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 1-kV/1-kA levels and higher, doped contacts have not yet resulted in improved longevity. We employ multifilament operation and InPb solder/Au ribbon wirebonding to demonstrate >100-shot lifetime at 1-kV/1-kA.

Compensation mechanisms and the response of high resistivity GaAs photoconductive switches during high-power applications

Photoconductive semiconductor switches (PCSSs) made from semi-insulating (SI) GaAs are the primary switching component of one class of high-power, ultra-wideband (UWB) microwave sources. The high resistivity of the GaAs can be achieved through different processing techniques. The resultant device characteristics of the PCSS such as breakdown voltage, rise time, and turn-on delay will depend on the actual processing technique that was used for the material. Simulation studies comparing an intrinsic material and a high resistivity SI GaAs PCSS grown through the liquid-encapsulated Czochralski (LEC) process with a deep donor and shallow acceptor compensation mechanism highlight these differences. Simulations also elucidate the role of an n/sup +/-doped layer placed next to the cathode, which increases the breakdown voltage of the device. Extending the n/sup +/ layer length beyond the cathode does not yield further improvement but leads to current confinement along a narrow strip that can initiate local heating or burnout. The doping profile of the n/sup +/ layer also affects hold-off characteristics, a faster gradient ensuring better protection of the cathode against the substrate field, and electron injection. Doping the n/sup +/ region with a higher concentration of carbon impurities does not produce the same effect as doping the n/sup +/-SI interface. These material-related issues are critical to further extending the performance characteristics of PCSSs.

Electron irradiation effects on photoconductive semiconductor switches (PCSSs) used in sub-nanosecond transient generators

Radiation-induced damage occurs in GaAs photoconductive semiconductor switches used in sub-nanosecond transient generators when subjected to 600 keV and 6 MeV electron irradiation. These switches are made from semi-insulating (SI) compensated material through a EL2/carbon compensation mechanism, and the liquid encapsulated Czochralski process. New defect levels are formed as a result of the non-ionizing energy loss (NIEL) process. The formation of new defect levels in the device alters the compensating balance between the existing deep level EL2 trap/donors and carbon acceptors, and changes the material properties. As a result, two important parameters of the device are adversely affected-the hold-off voltage of the switch at the pulse-charging (off) state, and the rise time during the conduction (on) state. The hold-off voltage shifts to a lower value since there are more trap-filled regions available that can fill up and alter the homogenous nature of the device material. Unstable filamentary conduction then occurs at a lower voltage and leads to premature breakdown. As with EL2 trap levels, new defect states induced by electron irradiation will further contribute to the delay in the rise time of the switch. The rise time determines the maximum energy transferred to the load. The electron damage mechanism and its effects on the switch characteristics depend on the material properties. Intrinsic material, or material made through compensation other than through the deep donor and shallow acceptor balancing process are not expected to behave similarly. Simulation results at higher bias show a marked degradation of material properties. The switch current-voltage (I-V) characteristic when the bias increases to the kilovolt range is similar to trap-dominated semiconductors. An initial sublinear current regime at low bias is followed by a super-linear regime of current flow at higher bias, and is in agreement with earlier observations.

Characterization of a semi-insulating GaAs photoconductive semiconductor switch for ultrawide band high power microwave applications

Simulation results depicting physical conditions in a photoconductive semiconductor switch in the pulse charging state, prior to high power switching, are analyzed. Results show that surface conditions and EL2 traps in semi-insulating GaAs influence the conduction process, specifically at high bias. Formation of trap-filled regions renders the device inhomogeneous for stable conduction and premature breakdown occurs, due to a large extent on unstable current filamentation within the device. Unlike insulators, the breakdown of desorbed gas from the surface (surface flashover) does not contribute to premature breakdown.

Longevity of optically activated, high gain GaAs photoconductive semiconductor switches

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/.

Ultra-wideband source using gallium arsenide photoconductive semiconductor switches

An ultrawide-band (UWB) pulse generator based on high-gain (lock-on mode) gallium arsenide (GaAs) photoconductive semiconductor switches (PCSS's) is presented. Revised PCSS contact design shows improved performance in hold-off field, on-state switch potential, and switching jitter, while reducing the switch volume by 75% compared to previous designs. A compact laser diode module operates at 904 nm and triggers the PCSS at pulse repetition rates (PRR) of up to 2 kHz. The 625 W laser diode output power is found to be sufficient to produce switching jitter of 65 ps rms at a switched field of 80 kV/cm. The PCSS switching jitter is found to have a strong dependence upon the switched field when triggered with the laser diode module. The revised PCSS geometry is easily integrated into a compact, parallel-plate source used to drive a TEM horn impulse-radiating antenna. The radiated field has a rise time of 330 ps and an adjustable low-frequency spectrum.

Characteristics of an optically activated pulsed power GaAs(Si:Cu) switch obtained by two-dimensional modeling

Photoconductive semiconductor switches (PCSS) have high-voltage hold-off (many to tens of kilovolts) and fast current rise times (<1 ns). However, lock-on, nonuniformities in the electric field, and filamentary current flow across the device when switching at high fields (∼10 kV/cm) have been reported. These observations raise concerns about the scaling of PCSS to high currents (tens of kiloamperes). To investigate these issues a two-dimensional time dependent computer model of a GaAs PCSS with Si and Cu doping has been developed. The model solves the continuity equations, the bulk energy equation, Poisson’s equation for the electric field, and a circuit equation for external currents. Physical effects in the model include band-to-band impact ionization, trap impact ionization, photoionization, and negative differential resistance. Computed characteristics of GaAs(Si:Cu) switches will be reported. Experimentally observed electric-field distributions are explained.