# Photoconductive Semiconductor Switches: Materials, Physics, and Applications
## 1 Introduction  
Photoconductive semiconductor switches (PCSSs) are optically-gated switches that rely on photogenerated carriers to conduct current through a semiconducting medium between two contacts. While any semiconductor can operate as a PCSS, research has disproportionately focused on GaAs, SiC, and GaN due to their electrical properties (enumerated in Table 1) and technological maturity. PCSSs offer several advantages over conventional high-power switches like spark gaps or insulated-gate bipolar transistors (IGBTs): they can be >10\(\times\) more compact than a comparable conventional switch, they can be triggered remotely (making them potentially immune to electromagnetic interference), exhibit ps-scale jitter, and avoid the high-pressure gas requirements of spark gaps. Fabrication typically involves a few steps on a substrate doped to be semi-insulating: a single metalization step to form an _m-i-m_ structure, and optionally, a dielectric deposition step to suppress surface flashover. Devices can be fabricated in vertical or lateral geometries, with the latter being typically either linear or radial. An overview of switch design and fabrication considerations is available elsewhere and is not the primary focus of this work [1]. The commercial application of devices has been limited due to very short shot lifetimes, being typically not greater than 10\({}^{7}\) for Si, 10\({}^{4}\) for GaAs, and substantially less than this for other materials [2, 3]. Future commercialization requires fundamental efforts to understand the impacts of doping and electrode geometry on efficiency and electrical damage. PCSSs operate in one of two modes. The most common and conceptually straightforward is the linear mode, wherein switch conductivity persists only as long as the opticalgate signal is applied; more exotic and potentially more technologically useful is the nonlinear high-gain mode, to-date observed only in direct bandgap materials [4; 5]. PCSS devices fabricated from Si, SiC, Ga\({}_{2}\)O\({}_{3}\), and diamond are ubiquitously operated only in linear mode. PCSS devices fabricated with direct-bandgap materials including GaAs, InP, and GaN, in addition to linear mode operation, exhibit a high-gain mode, known as lock-on, wherein a switch closed above a threshold field remains closed after the end of an optical gate signal so long as a critical field (corresponding to the \"lock-on field\") persists across the switch leads [5; 6]. Under appropriate doping and triggering conditions, whether the voltage applied to switches fabricated from the latter group of materials results in a field above (below) a material-specific threshold value determines whether the switch operates in lock-on (linear) mode. Though high gain operation is commonly referred to as lock-on in different material systems, the underlying mechanisms differ by material, as will be discussed in the next section. In this work, we use the term lock-on with the significant caveat that it may require differentiation in the future as an understanding of the fundamental physics of the mode in each material improves. At the circuit-level, high gain operation is material-agnostic; however, the optically-closed switch conductivity remains constant, permitting constant-impedance discharging of the primary until the field dips below the critical value and the switch opens. The origins of this phenomenon have variously been attributed to negative differential conductivity (NDC), avalanche multiplication, and dismissed altogether as surface flashover. It is likely that the bandstructure of each material, provided its bandgap is direct, furnish a material-dependent set of physical mechanisms whereby lock-on occurs; trap-to-band ionization, negative differential mobility (NDM), avalanche injection, and as-yet unidentified surface effects appear most plausible [6; 7; 8].