High purity semi-insulating (HPSI) 4H-silicon carbide (SiC) was used to fabricate lateral and vertical photoconductive semiconductor switches (PCSSs). The lateral PCSSs were illuminated from the frontside (fPCSS) or the backside (bPCSS). The side-illuminated vertical PCSS (vPCSS) was designed to increase the light-matter interaction volume. A 532-nm pulsed laser with adjustable energy was utilized to excite the PCSSs. The turn-on time was found to be highly dependent on the optical illumination energy, and the full-width at half-maximum of the PCSSs output waveforms was related to the peak output voltage. The output electrical pulse from the vPCSS exhibited a shorter turn-on time and a larger pulsewidth than the two types of lateral PCSSs. The vPCSS outperformed the fPCSS and bPCSS in terms of minimum $\mathrm{\scriptscriptstyle ON}$ -state resistance and output pulse amplitude under the same optical illumination energy. The vPCSS, which utilizes a large effective contact area to collect photogenerated carriers, also had higher photon absorption efficiency by arranging the optical path at a right angle to the carrier transport. The vPCSS exhibited a minimum $\mathrm{\scriptscriptstyle ON}$ -state resistance of 0.34 $\Omega $ at optical illumination energy of 8 mJ.

Photoconductive semiconductor switch (PCSS) allowing side illumination was fabricated on high purity semi-insulating (HPSI) 4H-SiC. A 532-nm pulsed laser with variable optical energy was used to trigger the PCSS. The performance of the PCSS was characterized under the two different load conditions, 50- $\Omega $ load and 0.05- $\Omega $ , with a current viewing resistor (CVR). The PCSS exhibited significantly different output characteristics for the two different loads. The equivalent resistance of the PCSS with the 50- $\Omega $ load, which was calculated from the output voltage and current, was inversely proportional to the optical energy, but the one with a 0.05- $\Omega $ load exhibited saturation behavior with the optical energy. While the times at peak output with the 50- $\Omega $ load were similar at various optical energies, the times at peak output with the 0.05- $\Omega $ load were dependent on the optical energy. Output current oscillation was also observed after the PCSS was turned off in the case of 0.05- $\Omega $ load condition. The different output characteristics for the different load resistances were analyzed using the transient response of the equivalent circuits. The PCSS exhibited a minimum on-state resistance of $0.27~\Omega $ with the optical energy of 8 mJ and a maximum output current of 657 A at the bias voltage of 4.8 kV. The operating voltage of the PCSS was limited by surface flashover, which caused an additional output pulse following the first output pulse.

This article presents a high-voltage switched oscillator (SO) with a quarter-wave open-circuited stub (OCS) for generating mesoband high-power microwave (HPM) pulses. Unlike the conventional SO, the proposed SO generates mesoband HPM pulses whose dc offset is greatly reduced because the OCS inserted between a charged transmission line and an antenna blocks the flow of the electric energy from a high-voltage power supply to the antenna. Thus, the proposed SO allows the insulation capability of the antenna to be increased and ground-loop-integrated-wideband antennas to be employed. The transient response of the proposed SO is theoretically analyzed to understand its operation mechanism. The results prove that the proposed SO is capable of producing mesoband HPM pulses with a damped rectangular pulse waveform without a high-voltage dc offset. Also, the amplitude and quality factor are discussed. Compared with the conventional SO, the proposed SO has a larger output amplitude and smaller quality factor. Here, the difference between the quality factors is negligible. In addition to the theoretical analysis, we simulated and fabricated the proposed SO. Consequently, the results of the circuit simulation and experiment demonstrated the validity of the proposed design concept.

The switched oscillator (SWO) is a quarter-wave oscillator capable of generating mesoband high-power microwave (HPM) pulses. This paper presents a novel high-quality factor ( Q) SWO with a capacitive voltage divider (CVD). Unlike conventional SWOs, the transmission line energy of the proposed SWO is gradually coupled with the output load through the CVD. Thus, the proposed SWO can produce a mesoband pulse with a much higher power spectral density within the coupling bandwidth of a target in HPM effect testing. The frequency-domain output-voltage equation of the proposed SWO was obtained through a theoretical analysis of the equivalent circuit of the SWO. Through a numerical analysis, the time-domain output waveform of the proposed SWO was demonstrated to have a high quality factor, and the circuit parameters of the equivalent circuit of the SWO were set to achieve a high quality factor. The circuit parameters selected in the numerical analysis were used in an electromagnetic simulation to determine the design parameters of the parallel-plate transmission line (PPTL)-based SWO with the CVD built on a printed circuit board. The proposed SWO was fabricated and its performance was assessed. For a breakdown voltage of 8.7 kV, the output voltage of the fabricated SWO showed a quality factor of 25, which is more than two times larger than the quality factors of conventional SWOs. Thus, the proposed SWO was demonstrated to produce high- Q mesoband HPM pulses, and it has the potential to be applied in HPM effect testing. Moreover, the proposed SWO improves operator safety by preventing the output load from being charged by a high-voltage power supply.