# Photoconductive Semiconductor Switches: Materials, Physics, and Applications
## 7 (Al)GaN PCSS Devices  
GaN is a semiconductor now entering technological maturity that is well-suited to realize the promise of ultra-low jitter PCSS devices due to its high saturation drift velocity, large and direct bandgap, radiation hardness, high-temperature operability, and high field strength [78]. Triggering in linear mode has been demonstrated with intrinsic and extrinsic light sources [79], and nonlinear operation has been recently observed [6]. While early reports of GaN PCSS devices, especially vertical designs, suffered from low breakdown fields, reports utilizing recently available free-standing material achieve near-ideal breakdown values up to 1.6 MV/cm [80; 81]. The related binary and ternary UWB compounds AlN and Al\({}_{x}\)Ga\({}_{1-x}\)N offer similar promise to GaN as a PCSS medium. Photoconductive switching has not been demonstrated to date in either material, in large part because bulk (freestanding), low-dislocation density material has only recently become available. PVT or hydride vapor-phase epitaxy (HVPE) is used to develop freestanding, high-quality AlN; however, the same technique cannot readily be used to grow freestanding AlGaN due to the added complexity of achieving a uniform composition of the ternary [82; 83]. SiC is generally the best-understood and most widely studied of the PCSS media. It has high thermal conductivity and its bandgap makes it well-suited to intrinsic trigger  
Figure 9: Camera images of filamentary conduction through the bulk of GaAs switches taken during lock-on events (reproduced from Ref. [73]). ing by conventional lasers, and doping with V can enable extrinsic triggering at visible wavelengths [24]. SiC, however, typically has higher leakage current than GaAs and lower saturation drift velocity than GaN [69]. No regime of negative differential resistance has been identified in SiC, suggesting a regime of nonlinear conduction may also not exist. If this is the case, triggering requirements must scale in power linearly with switch current-handling requirements. The direct, narrow bandgap and high dielectric strength of GaAs make it efficiently triggerable with very high slew rate [69] in linear and nonlinear regimes [5]. Its shot life, however, is typically limited to hundreds of shots and its relatively low optical damage threshold limits trigger intensity [71; 84; 85]. Recent collaborative efforts between Sandia National Laboratories and the University of New Mexico have advanced the state-of-the-art in PCSS implementation for both material development and circuit integration through the ARPA-E BREAKERS program [86; 87; 88], among others. A medium-voltage (6 kV) DC circuit breaker was chosen as a PCSS demonstration vehicle composed of a normally-on leg assembled from cascaded SiC junction field-effect transistors (JFETs) and a normally-off leg based on a GaN PCSS [88]. This implementation has the advantage of providing (1) a grid-integrable power handling capability utilizing the GaN PCSS, and (2) an example of clear complementarity between SiC and GaN. PCSS fabrication  
Two considerations primarily motivate the selection of dopant: the need for high resistivity, and the desirable characteristic of efficient sub-bandgap optical absorption. The former is necessary for suppressing leakage current in the off-state, and the latter for enabling triggering with compact visible or IR light sources. Mn and Fe are the two most technologically mature GaN doping species known to have very deep ionization states near the mid-gap [20; 89], making them favorable candidates for extrinsically triggered PCSS fabrication. The dopants reported to have been used to produce SI-GaN for PCSSs (Mn and Fe) are shown in the context of other potential, but untested, dopants in Figure 10. The deep traps introduced by V (VB + 1.39/2.07 eV), Cr (VB + 2.44/3.07 eV), Mn (VB + 1.46/1.61 eV), Fe (VB + 0.71/2.80 eV), and Co (VB + 0.88/1.54 eV) have the potential to facilitate the trap-assisted avalanche multiplication believed to be required for lock-on in GaN.