# Photoconductive Semiconductor Switches: Materials, Physics, and Applications
## 5 \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) PCSS Devices  
B-Ga\({}_{2}\)O\({}_{3}\) (gallium oxide), like diamond, is an emerging UWB material with great promise in PCSS applications. In addition to a bandgap and critical field of 4.8 eV and critical field strength of \(\sim\)6 MV/cm, it has the additional advantage of its potential for cost-effective, large-scale production through methods such as edge-defined film-fed growth (EFG) [55; 56; 57; 58].  
In \(\beta\)-Ga\({}_{2}\)O\({}_{3}\), the Fe acceptor is positioned about 0.7-0.8 eV below the conduction band (CB). This acceptor can emit electrons to the CB under sub-bandgap illumination, functioning similarly to V in SiC and N in diamond. The primary defects in \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) are Si (shallow donor) and Ir (deep donor), both of which unintentionally dope the material during the growth phase. Fe is commonly used in higher concentrations, often by an order of magnitude greater than Si, in order to achieve semi-insulating characteristics in the material [56; 57; 58; 59].  
The primary optical absorption pathways are illustrated in the band diagram presented in Figure 8. It is assumed that the shallow Si donor is fully compensated by the Fe dopant, resulting in the coexistence of both Fe\({}^{2+}\) and Fe\({}^{3+}\) species within the sample at equilibrium. The lowest energy absorption process involves the release of an electron to the CB, thereby converting Fe\({}^{2+}\) to Fe\({}^{3+}\) (Path I in Figure 8) [58; 60]. Although the Fe\({}^{2+}\) level is positioned approximately 0.7-0.8 eV below the CB, experimental observations have indicated that absorption levels can extend to 1.2-1.5 eV below the CB for this trap. This extension is attributed to the relaxation energy within the lattice, with similar energetic behaviors predicted for Fe residing in both the tetrahedral and octahedral Ga sites. Consequently, the 1.17 eV optical energy associated with 1064 nm light results in only minimal transmission to the CB. Higher-energy optical sources, tested at 2.3 eV and above, have been found capable of activating absorption in the deep Ir\({}^{3+}\) state, as well as other deep donors and acceptors that may be present [61]. Since Ir\({}^{3+}\) functions as a deep donor, its absorption of photons with energies of 2.3 eV or higher leads to two possible pathways: direct electron emission to the CB (Path II in Figure 8) or a charge-transfer transition to Fe\({}^{3+}\) or potentially another defect (Path III in Figure 8) [62; 63].  
Additionally, optical absorption peaks related to Fe in \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) have been observed around 2.2 eV, with the absorption decreasing at higher energy levels. This suggests that Fe\({}^{2+}\) emission is more pronounced in the 2.3-2.7 eV range (corresponding to green and blue light) compared to higher-energy excitation sources [64]. The first demonstration of Fe-doped \(\beta\)-Ga\({}_{2}\)O\({}_{3}\)[61] as a PCSS has confirmed its ultra-high breakdown field of greater than 2 MV/cm and a free carrier recombination lifetime of less than 100 ps; however, its room-temperature gain is 100-1000\(\times\) poorer than state-of-the-art devices fabricated from diamond, GaN, and SiC. With an understanding of the trapping and carrier transport of deep states in \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) still at an early stage [63], the material is not yet competitive with more mature material systems as a switching medium. These characteristics strongly position it as a promising candidate for next-generation high-power and ultra-fast switches, particularly in extreme environments [59; 60; 62; 65].