# Photoconductive Semiconductor Switches: Materials, Physics, and Applications
## 3 SiC PCSS Devices  
SiC has been widely utilized as a fundamental material for high-power, high-temperature, and high-frequency applications, owing to its outstanding electronic and thermal properties, as well as its commercial availability. When compared to first-generation semiconductor materials such as silicon (Si) and second-generation materials like gallium arsenide (GaAs), SiC demonstrates several advantages. These include a wide bandgap energy (3.0 eV for 6H-SiC and 3.2 eV for 4H-SiC), a high critical breakdown field, and superior thermal conductivity (4.9 W/cm-K for 6H-SiC and 3.7 W/cm-K for 4H-SiC). Additionally, SiC exhibits high electron saturation velocities (1.5-10\({}^{7}\) cm/s for 6H-SiC and 1.9-10\({}^{7}\) cm/s for 4H-SiC), a low dielectric constant, and exceptional thermal stability, with breakdown fields ranging from 3 to 5 MV/cm [4; 22; 23; 24]. SiC is available in numerous polytypes, including 3C, 2H, 4H, and 6H, each defined by the specific stacking order of Si-C bilayers along the c-axis of its crystal lattice. Among these, 3C-SiC is the only polytype that possesses a cubic crystal structure, while 4H-SiC and 6H-SiC both exhibit hexagonal crystal structures [25]. Numerous studies have focused on engineering the material characteristics of SiC to meet the primary requirements for high-power photoconductive semiconductor switches (PCSSs). Various growth techniques, such as undoped Lely [26; 27], physical vapor transport (PVT) [28], and ion-implanted epitaxial material (I\({}^{2}\) epi) [29], have been employed to produce high-quality SiC substrates [30]. However, during the growth process, defects such as ScC precipitates, nanopipes, micropipes, and cavities can arise, particularly when growth conditions are suboptimal. These defects can significantly degrade both the performance and stability of the resulting devices. Additionally, primary impurities, including N and B, are often unintentionally introduced into the material. N acts as a shallow donor, while B serves as a shallow acceptor, enabling N and B doping to create _n_-type or _p_-type semiconductor materials with enhanced conductivity [24; 31]. Doping considerations  
Two primary approaches are available for obtaining high-quality semi-insulating (HPSI) SiC. The first approach involves refining the SiC growth process to minimize the unintentional incorporation of N and B impurities, thereby achieving high purity SiC. Alternatively, a compensating third dopant, such as V or Ti, may be introduced to balance the excess N donors or B acceptors [29; 30]. For example, when SiC is doped with V to form V-compensated semi-insulating (VCSI) SiC, the dopant acts similarly to the \(Z_{1}/Z_{2}\) defect centers found in HPSI SiC. V has three charge states and forms two energy levels within the bandgap as an amphoteric dopant. Neutral V (V\({}_{\text{N}}\)) can capture an electron (or emit a hole) to become an acceptor (V\({}_{\text{A}}^{-}\)), or emit an electron (or capture a hole) to become a donor (V\({}_{\text{D}}^{+}\)). The acceptor level corresponds to the transition between V\({}_{\text{N}}\) and V\({}_{\text{A}}\), often referred to as the shallow donor deep acceptor (SDDA), while the donor level corresponds to the transition between V\({}_{\text{N}}\) and V\({}_{\text{A}}\), known as the deep donor shallow acceptor (DDSA). V effectively compensates _n_- or _p_-type doping and pins the Fermi level near the mid-gap, leading to a high dark resistivity (>10\({}^{9}\)\(\Omega\cdot\)cm). Furthermore, V acts as a recombination center and has been demonstrated to influence the control of carrier lifetime, further enhancing the performance of SiC-based devices [24; 30; 31; 32]. Co-doping interacts with the intrinsic behavior of V defects, making the quantification of optical transitions in SiC particularly challenging. V functions as an amphoteric dopant in SiC, with two energy levels emerging within the bandgap as it transitions between three distinct charge states.