# Photoconductive Semiconductor Switches: Materials, Physics, and Applications
## 3 SiC PCSS Devices
### Switch geometries
As Joule heating and laser heating accumulate, the transparent electrode absorbs heat up to a critical threshold, leading to gradual melting and vaporization. This process forms localized hot spots at the SiC/TCO-air interface. When the laser directly impacts the substrate, damage within the substrate accumulates over time, eventually creating breakdown channels. As this damage worsens, the device ultimately fails. A vertical structured 6H-SiC:V could accumulate \(\sim\)200 pulses at charge voltages ranging from 15 to 17 kV and 300 to 400 pulses with currents in excess of 1 kA without any discernable damage, or degradation in performance [39; 40]. These approximate field and current density values compare unfavorably with GaAs switches, but favorably with GaN switches in comparable geometries. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Ref.** & **Material** & **Structure** & **Wavelength** & \(P_{laser}\) & \(V_{blocking}\) & \(V_{up}\). & \(I_{out}\) & \(R_{on}\) & \(R_{off}\) \\ \hline
[40] & 6H-SiC:V & Vertical & 532 nm & 530 kW & \(>\)150 kV/cm & 20 kV & 50 A & 244 \(\Omega\) & \\ \hline
[37] & 4H-SiC:V & Vertical & 532 nm & 18.2 MW/cm\({}^{2}\) & 260 kV/cm & 12 kV & 840 A & 7.5 \(\Omega\) & \\ \hline
[41] & 6H-SiC:V & Vertical & 532 nm & 31.2 mJ, 17 ns & 225 kV/cm & 20 kV & 500 A & 5.6 \(\Omega\) & \(10^{11}\)\(\Omega\) \\ \hline
[4] & 4H-SiC:V & Lateral & 355 nm & 1 mJ, 7 ns & 370 kV/cm & 26 kV & 450 A & 2 \(\Omega\) & \\ \hline
[36] & 6H-SiC:V & Vertical & 532 nm & 4 MW & 17 kV & 1500 A & \(<\)1 \(\Omega\) & 2 \(\times\) 10\({}^{10}\)\(\Omega\) \\ \hline
[39] & 4H-SiC:V & Vertical & 532 nm & 100 kW & 200 kV/cm & 20 kV & 100 A & \(10^{12}\)\(\Omega\) \\ \hline
[34] & 4 HSiC:V & Lateral & 355 nm & 0.5 mJ, 7 ns & 231 kV/cm & 12.4 kV & 182 A & 12 \(\Omega\) & 4 \(\times\) 10\({}^{12}\)\(\Omega\) \\ \hline
[34] & 4 HSiC:V & Radial & 355 nm & 0.5 mJ, 7 ns & 178 kV/cm & 32.2 kV & 938 A & 8 \(\Omega\) & 12 \(\times\) 10\({}^{12}\)\(\Omega\) \\ \hline
[24] & 6 HSiC:V & Vertical & 532 nm & 13 mJ & 27.4 MV/m & 11 kV & 150 A & 2 \(\Omega\) & 10\({}^{12}\)\(\Omega\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Survey of state-of-the-art values for SiC:V PCSS device electrical properties observed in the literature. Figure 6: Common designs for PCSS devices used in high-power microwave (HPM) applications (top view (top) and cross-section (bottom)): (**a**) lateral in-line contact, (**b**) lateral radial, (**c**) vertical PCSS design with top–bottom metal contacts with side illumination, and (**d**) vertical structure with top–bottom transparent conductive oxide (TCO) with front-rear excitation. SiC PPSSs have been able to achieve either high-voltage operation or high-current operation but have had difficulty achieving both simultaneously with long device lifetimes. In addition, PPSSs operated at high electric fields (>100 kV/cm) have required pulse charging circuits due to high power dissipation in the off-state. Although intrinsic PPSSs are more efficient than extrinsic PPSSs, both devices when operated in the linear mode currently require large amounts of laser energy to achieve low on-state resistances. This subsequently requires large high-power lasers be used to trigger the PPSSs. Therefore, ultrawide bandgap (UWBG) materials with higher thermal conductivity and critical strength field characteristics continue to be investigated.