# Photoconductive Semiconductor Switches: Materials, Physics, and Applications
## 2 Theory of PCSS Operation
Both of these methods enable the interrogation of ab initio physical principles to derive a statistical model of device operation [10]. By simulating particle-level interactions, carrier transport dynamics outside of standard excitation conditions can be self-consistently modelled. These methods each have this advantage over more conventional technology computer aided design (TCAD) software packages. REOS is used to analyze photocurrent and breakdown by solving the continuum equations. This code computes the temporal electrical response to stimuli applied to a material such as a semiconductor, insulator, or a metal [11; 12]. The main external stimulus considered is an electrical current governed by an external circuit. During the simulation session, the sample can be subjected to ionizing radiation, thermal energy, and ion beams. These calculations include defect chemistry, interface effects, tunneling reactions, hot carriers, and lattice heating. These effects are governed by reactive transport equations that have specific terms corresponding to the species flow and the species reactions. The response involves flow and chemical reactions of electrons, holes, and chemical species such as impurities and lattice defects. The species flow is governed by drift-diffusion equations. The reactions are governed by the chemical potentials of the constituents. Additional terms include the effects of the external stimuli. The energy changes are captured in a lattice temperature as well as electron and hole temperatures. The calculations are governed by input information that defines the physical quantities, the initial conditions, the injectedquantities, the reactions, and the electrical circuit. The main computed quantities are the species densities as a function of time and position and the electrical currents as a function of time. The EMC is a particle-based method that can solve the Boltzmann transport equations governing electrons and holes in a semiconductor [13]. This software differs from conventional particle-based calculations because it includes high-density effects. The code self-consistently implements Fermi exclusion and carrier-carrier scattering. The solution of the Boltzmann equation using a particle method seeks the electron distribution function \(f(r,\ k,\ t)\) as a function of position \(r\), wavevector \(k\), and time \(t\). The method obtains the solution by following the evolution of the distribution function as electrons respond to an electric field. In addition, these particles scatter by interactions with phonons and other electrons. The accurate simulation of electron-electron scattering is critical for computing breakdown dynamics. At the high carrier densities involved in breakdown, Fermi exclusion must be implemented. Impact ionization rates are computed using both electron-electron scattering and two-electron impact ionization. The GaAs mechanism  
The initiating optical pulse in the presence of an electric field causes an initial high photoconductivity. These initial carriers, under appropriate conditions, lead to an increased density of charge carriers in continuous regions spanning the contacts [13], termed \"filaments\". The growth mechanism exhibits avalanche-like behavior. The conductive on-state is maintained by a so-called minimum lock-on electric field \(E_{LO}\) = 5 kV/cm. This is quite small compared with the values 100\(-\)250 kV/cm reported for the threshold field that must be exceeded to initiate avalanche breakdown in GaAs. The threshold field \(E_{T}\) = 10-30 kV/cm required to initiate lock-on is also small compared with the fields needed to initiate avalanche breakdown in the absence of an optical trigger. Early studies attributed the self-sustaining current to collective impact ionization [14; 15], which involves conventional impact ionization and models the carrier energy distribution function by considering carrier-carrier interactions. This theory leads to qualitative agreement with the lock-on phenomenon.