In this article, an ultrahigh repetition frequency sub-nanosecond pulse generator based on vanadium-compensated semi-insulating (VCSI) 4H-SiC photoconductive semiconductor switch (PCSS) under high bias electric field is presented. In this work, the 0.8 mm-thick 4H-SiC PCSS was found to work with electric fields up to 200 kV/cm and power capacity of 176 kW. The VCSI 4H-SiC PCSS response test was performed with a 1 MHz, 1030 nm laser cluster driver with configurable optical pulsewidth and current viewing resistor (CVR) high frequency response circuit. In the experiment, the laser pulsewidth was varied between 300 and 500 ps, and 1.5 ns. The system generated a 1 MHz repetition frequency pulse with a minimum half-width of 365 ps and less than 100 ps of jitter. The results revealed higher response performance for single/continuous pulse triggered light of various pulsewidths. Due to the low absorption efficiency of 4H-SiC for non-intrinsic light, an axial optical reflection trap structure was utilized to boost the light absorption efficiency of PCSS. The proposed structure significantly improved the light absorption performance of PCSS devices. The photocurrent increase ratio of roughly 115% was obtained. This study presents the first results of MHz re-frequency high-power switching in bulk semi-insulating 4H-SiC.

A frequency-agile microwave generating system is examined in this letter. Vanadium-compensated semi-insulation (VCSI) 4H-SiC photoconductive semi- conductor switch (PCSS) has been introduced to achieve wide-range frequency tuning. The output microwave frequency is tuned by adjusting the pulse width of the laser. Both the simulation and the experimental results show a tuning frequency range of 0.5 GHz to 10.0 GHz. The maximum operating output power of the system at 10 GHz is 1.13 W, equivalent to a charging voltage of 1.5 kV and a modulated light wavelength of 1064 nm. Further experiment revealed that with 355 nm laser and charging voltage of 5 kV, the output power could be increased to 86.5 kW.

Velocity saturation characteristics of a V-doped 6H silicon-carbide (SiC) photoconductive switch under high electric field are presented. A vertical-geometry switch device based on vanadium-compensated silicon carbide is triggered by a 532-nm laser pulse with light peak-power of several hundred kW and operational voltages from 2 to 20 kV. The maximum electrical peak-power achieved by the device is up to 1 MW (~50 A, 20 kV, 1.1 ns pulse width). When the working voltage is increased to increase output power, the carrier-velocity saturation under high electric field unavoidably impedes the current growth and directly limits the peak power. The parameters of parallel high-field-dependent mobility are obtained with different fields and laser energies: low-field mobility μ = 227 cm2/V s and fitting parameter β = 1.7. With working voltage increasing steadily, a crack-forming process results in device degradation as a result of large current and high electric field. The IV characteristic curves of the simulations and tests show current saturation and possible damage under high electric field.

Vertical extrinsic photoconductive semiconductor switches (PCSSs) are presented with initial characteristics comparison between V-doped 4H-SiC and Fe-doped GaN PCSS under axial triggering such as dark resistance, photoconductivity, power output, and breakdown behavior. Experiments are carried out under the 532-nm-wavelength laser with mJ-level energy and a pulse width of 30 ns. Photoconductive experiments show that the photoelectric conversion efficiency of GaN PCSS is 2.27 times higher than 4H-SiC PCSS with the same electric field strength under different laser energies from 1 mJ to 5 mJ. 4H-SiC PCSS with a dark-state resistance of 1012 cm can withstand a bias voltage of 8 kV (16 kV/mm) and laser energy of 8 mJ and the maximum output power is up to 428.7 kW, while that of GaN can only stand a bias voltage of 1 kV (2.9 kV/mm) because of low dark resistance and defect. Obvious cracks of 4H-SiC PCSS can be observed from the breakdown image after breakdown occurs, while the dark-state resistance of GaN PCSS drops from 106 cm to 104 cm under high DC voltage.

The competitive relationship between the surface flashover of the coplanar electrodes and the body current channel was investigated. Breakdown behavior of GaAs photo-conductive semiconductor switch (PCSS) with a backside-light-receiving coplanar electrode structure was studied in this paper. GaAs PCSS was triggered by the laser pulse with an extrinsic absorption wavelength of 1064 nm. Special insulating construction was designed for GaAs PCSS, while the surface of the electrodes was encapsulated with transparent insulating adhesive. Our first set of experiments was at a bias voltage of 8 kV, and the surface flashover breakdown of GaAs PCSS was observed with 10 Hz triggering laser pulse. In the second experiment, at a bias voltage of 6 kV, the body current channel breakdown appeared on the backside of the GaAs PCSS. Compared with these results, the existence of a competitive relationship between the surface flashover breakdown and the body current channel breakdown of the GaAs PCSS was confirmed. When the bias voltage is set within a certain range (just reaching avalanche mode), GaAs PCSS with a backside-light-receiving coplanar electrode structure will undergo the body current channel breakdown. This finding is also consistent with the simulation results.

The properties of switching transient in the gallium arsenide photoconductive semiconductor switch (GaAs PCSS) are significant for improving the device lifetime. In this article, the properties of switching transient in the opposed-contact semi-insulating (SI) GaAs PCSS, including the physical process of the ultrafast switching, the characteristics of delayed breakdown, and the current density distribution, are studied, based on an effective 2-D, time-dependent numerical model. The numerical results demonstrate that there exists a positive feedback loop between the increase of the carrier density and the formation and evolution of the avalanche domains, which achieves the ultrafast switching of the opposed-contact SI-GaAs PCSS. The higher bias voltage and triggering optical intensity can promote the earlier formation of the avalanche domains and accelerate the evolution of the domains, leading to a shorter delay time and switching time. The triggering positions determine the distribution of the avalanche domains in the opposed structure, affecting the delay time, among which the cathode triggering leads to the shortest delay time. Current crowding in the opposed structure mainly occurs in the vicinity of the contacts, which first damages the edges of the contacts and makes the areas near the contacts present different failure characteristics. Our analysis indicates that anode triggering is effective for suppressing the current crowding to improve the lifetime of the opposed-contact PCSS.