# Photoconductive Semiconductor Switches: Materials, Physics, and Applications
## 4 Diamond PPSS Devices  
Diamond's exceptional physical properties make it an ideal semiconductor material for PPSSs, particularly for high-voltage and high-power switching applications due to its very high thermal conductivity [42]. As a UWB semiconductor, diamond possesses a bandgap of 5.5 eV. Intrinsic diamond demonstrates a high critical electric field (10-20 MV/cm), high carrier mobility (up to 2100 cm\({}^{2}\)/V\(\cdot\)s for holes and 1060 cm\({}^{2}\)/V\(\cdot\)s for electrons), and excellent thermal conductivity (22-24 W/cm\(\cdot\)K) [35; 36; 37; 38]. Both intrinsic [43] and extrinsic [44] diamond PPSSs have been studied, utilizing excitation wavelengths either above or below the diamond bandgap, respectively. Previous research has observed photoconductivity in intrinsic diamond using UV-band excimer lasers under high electric fields of up to 0.7 MV/cm [45; 46]. In contrast to intrinsic diamond PPSS devices, which rely on photon energies above the bandgap (band-to-band excitation), typically requiring UV wavelengths below 226 nm, extrinsic PPSSs operate with sub-bandgap photon energies, where carriers are excited from dopant or defect levels. This allows diamond devices to function using more accessible visible or infrared wavelengths, while retaining the advantageous properties of diamond. N-doped diamond has been identified as a promising candidate for extrinsic PPSS applications, particularly in vertical structures, due to its high photoresponsivity to sub-bandgap excitation (a direct parallel with SiC\(\cdot\)V and GaN:Mn). The implementation of diamond PPSSs requires precise control over carrier generation, which dictates the wavelength and power of the input optical source, and carrier recombination, which determines the achievable switching frequency. This, in turn, requires the precise control of material quality and doping profiles. The manufacturing of high-quality synthetic diamond with controlled defect concentrations, however, remains a complex process. Furthermore, the available substrates tend to be relatively small and challenging to process. High-purity single-crystal diamond is generally unsuitable for PPSS applications due to its \(\upmu\)s-scale carrier lifetimes [47]. To overcome this limitation, extrinsic photoconductivity, where defects or impurities modulate generation and recombination, is utilized to reduce carrier lifetimes [48; 49]. Polycrystalline diamond PPSSs with picosecond response times have been demonstrated, but the exact nature of deep traps remains unclear, complicating the development of practical devices. Furthermore, switching impedance achieved to-date has been higher than many commercial loads, being generally not less than hundreds of \(\Omega\)[45]. The technological immaturity of diamond as a semiconducting medium, very low gain, and high switching impedance, make it unsuitable at present for integration into commercially relevant loads. While promising based on intrinsic material properties, it requires advances in the knowledge of impurity levels and carrier dynamics to be commercially viable. Impurities enable the use of extrinsic photoconductivity, as demonstrated with SiC\(\cdot\)V. In diamond, N typically acts as a single-substitutional impurity, creating a deep donor level approximately 1.7 eV below the conduction band, which can be accessed through sub-bandgap excitation [50]. This deep donor level, along with a large carrier capture cross section, allows N-doped diamond to exhibit sub-nanosecond carrier lifetimes. Natural diamond PCSS devices have demonstrated lifetimes in the range of 0.5 to 1 ns [43; 51]; however, the N concentrations in these natural diamonds were limited (Figure 7a) [52; 53; 54]. As a result, synthetic diamond, with higher levels of incorporated N, has emerged as an attractive alternative for PCSS applications. The mechanism underlying the photoelectrical detection of \(V_{\rm N}\) centers can be explained by a two-photon ionization process, as depicted in Figure 7b.