# Photoconductive Semiconductor Switches: Materials, Physics, and Applications
## 3 SiC PCSS Devices
### Switch geometries  
Various SiC PCSS structures [36] have been proposed to address challenges related to breakdown voltage thresholds, on-state resistance, and conversion efficiency. Typically, these devices are categorized into two structural types: lateral, where both electrodes are positioned on the same plane, and vertical, where the electrodes are located on opposite planes [33]. Table 2 compares the most important characteristics of 4H and 6H SiC PCSS designs. Lateral devices, which often use low-impurity SiC polytypes, generally rely on above-bandgap wavelengths to trigger the switches, as this enables maximum carrier collection through intrinsic absorption near the surface, where a strong electric field is present. Conductivity modulation in this surface region shows promises for achieving a substantial on/off resistance ratio, leading to high output currents [34]. However, when the switch is triggered by a high-power laser pulse, gas ionization may occur at the surface, generating positively and negatively charged ions. These ions are subsequently accelerated by the electric field, initiating an avalanche multiplication effect that forms a discharge channel, eventually resulting in a surface flashover (in SiC, as in GaN, this has been confused with lock-on). To mitigate surface breakdown issues, rear-illuminated devices with radial structures (as shown in Figure 6b) have been developed. These structures have been shown to increase blocking voltages by up to five times by reducing point peak currents at the metal/SiC interfaces, thereby enhancing device reliability [4; 35]. SiC PCSSs with vertical structures have been developed with side illumination (as shown in Figure 6c) that exploit the bulk material's extrinsic absorption of below-bandgap wavelengths. Innovative illumination techniques such as silver back contact mirrors [37] and twisters [38] have been developed to enhance photon absorption uniformity in thick bulk substrates. Transparent conductive oxide (TCO) electrodes, such as aluminum-doped zinc oxide (AZO) [39] and indium tin oxide (ITO), have enabled front and backside exci  
Figure 5: Band diagrams of SiC:V showing the occupancy of the different V states depending on the presence of other co-dopants and the energy levels needed to transit between these states. The provided energy values are for the 4H polytype whereas their equivalences for the 6H polytype are shown in parentheses. Carrier generating transitions are shown in green, noncarrier generating (or parasitic) transitions are shown in yellow, and transitions of carriers from co-dopants to V levels are shown in red. (**a**) is the case of SiC:V without co-dopants where carriers can be generated only from the neutral V\({}_{\text{N}}\) state. (**b**) is the extreme case of N \(\gg\) V, where all V is converted into the acceptor V\({}_{\text{A}}\) state allowing only electron generation from this state into the conduction band. (**c**) is the opposite case, where Al or B \(\gg\) V and all V is converted into the donor state allowing only hole generation from this state into the valence band (re-generated from Ref. [32]). tation (Figure 6d). Vertical structures provide practical solutions for mitigating surface flashover and handling high current densities [40]. However, achieving high-quality transparent electrodes with low resistance and minimal absorption has posed significant challenges due to their complex composition and intricate fabrication processes. Surface roughness and internal defects in TCO materials can negatively impact reflection, absorption, and transmission, ultimately diminishing the device's overall performance. More critically, TCO degradation originating from defects may be a primary cause of PCSS breakdown during high-fluence laser experiments. The pulsed laser used to illuminate the surface interacts with the transparent electrode, resulting in the formation of numerous bubbles at the side of the electrode that directly contact the air under bias voltage.