# Photoconductive Semiconductor Switches: Materials, Physics, and Applications
## 2 Theory of PCSS Operation
For a given electric field, this mechanism leads to two stable states: a non-conductive off-state and a conductive on-state. These two states correspond to the conductive region of an optically initiated current filament and the non-conductive external region. It fails to predict the magnitude of \(E_{LO}\), however, suggesting a value of \(E_{LO}\) = 50 kV/cm that is much lower than the bulk avalanche field. However, this predicted field is 10 times larger than the observed lock-on field [16]. A similar situation has been found for other materials. This shortcoming of collective impact ionization theory led to a refined version of this theory called two-electron impact ionization [17]. In the mechanism, two electrons in the conduction band share their energy during a collision. A process of this type is responsible for the broadening of the distribution function if the carrier density is sufficiently high. However, in the two-electron mechanism, the more energetic electron undergoes a virtual transition to an excited virtual state. Its final state is a low energy state in the conduction band just as in conventional impact ionization. Instead of impact ionization caused by a single electron, in this theory, the energy of two electrons is combined in an impact ionization event. More precisely, two electrons at nearly the same elevated energies interact with each other. This scattering event causes one to go down in energy and the other one to go up in energy. The state reached by the more energetic electron is a virtual state. The electrons in these virtual states initiate the impact ionization that produces new electron-hole pairs. The end result is impact ionization by two electrons whose shared energy is sufficient to create the electron-hole pair. The REOS and EMC calculations also include carrier-carrier scattering, which arises from the competing effects of two-electron impact ionization and Auger recombination. These results are consistent with the prediction of the collective impact ionization theory that \(E_{LO}<E_{T}\) and predict the formation of the ubiquitously observed current filaments. Despite over 35 years of repeatable observations of lock-on in GaAs, the combination of mechanisms involved is not fully understood. Additional testing and new calculations will be necessary to assess whether the computed sustaining field approaches the lock-on field. The primary mechanism suggested for lock-on in a GaN optically triggered switch involves defects that enhance the field at anode contact. This model relies on acceptor defects that cause both the off-state and the on-state [18]. In the on-state, these defects undergo impact ionization to enable the avalanche injection. They also cause a large field enhancement that causes conventional band-to-band impact ionization. Early theoretical work that explored the effects of defects showed that impact ionization of the acceptors produced a mechanism for optical initiation of lock-on. However, the simulated current flow during self-sustaining conduction was not appreciably larger than the photocurrent that was used to trigger the lock-on state, indicating that this mechanism in isolation is insufficient. Those earlier calculations led to a revised mechanism. In this mechanism, the primary effect of deep defects is to cause a field enhancement of sufficient magnitude to enable conventional band-to-band impact ionization [7; 8; 19]. The mechanism has been validated by using it for avalanche breakdown calculations of a reverse biased GaN diode. This model is then used in calculations applied to an _n-i-n_ GaN structure. These calculations predict a dramatic switching event from a non-conductive off-state to a highly conductive on-state involving lock-on. The calculations discussed here involve a generalized _n-i-n_ structure illustrated in Figure 1. For these calculations, a positive electrical potential is applied to the right-hand contact. This figure shows an _n-i-n_ structure that has donor-doped contacts and also heavily doped acceptor regions denoted by \(p^{+}\).