A Photoconductive Semiconductor Switch (PCSS) circuit component definition simulates a semiconductor device that conducts electricity when illuminated.

PCSS components should be used when only time-domain results are of interest. This is because PCSS components violate the assumptions of linear system theory, making it incorrect to apply either a Fourier or discrete Fourier transform to the time-domain results. A simulation containing a PCSS produces invalid frequency-domain results—including impedances, S-parameters, and steady-state far-zone fields—even if the time-domain results eventually decay to zero. The time-domain results are unaffected, and therefore are valid.

XF's PCSS component is not illuminated at the beginning of a simulation and is equivalent to a passive resistor, as expressed in the following equation:

\begin{equation} R = R_0 \qquad \text{when} \qquad t \lt T_i \end{equation}At a user-defined illumination time the resistance's rate of change is determined based on the value of $V_s$. When $V_s \gt 0$, the transition follows a smooth curve determined by

\begin{equation} \frac{\dot R}{R} = \frac{1}{T_r}-\frac{\text{tanh}\left(\frac{|V|-V_a}{V_s}\right)+1}{2T_a} \qquad \text{when} \qquad t \geq T_i, V_s\gt 0 \end{equation}If $V_s=0$, then the rate of change is abrupt and follows the piecewise fuctions

\begin{eqnarray} \frac{\dot R}{R} &=& \frac{1}{T_r}-\frac{1}{T_a}&\qquad\text{when}\qquad t \geq T_i, V_s= 0, |V|\gt V_a \\ \frac{\dot R}{R} &=& \frac{1}{T_r}&\qquad\text{when}\qquad t \geq T_i, V_s= 0, |V| \lt V_a \end{eqnarray}where

$R = $ instantaneous cell edge resistance

$\dot R/R = $ time rate of change of $R$

$R_0 = $ initial resistance

$V = $ instantaneous cell edge voltage

$V_a = $ avalanche voltage

$V_s = $ normalized inverse rate of change of $1/T_a$ with voltage

$T_i = $ illumination time

$T_a = $ exponential avalanche carrier generation time

$T_r = $ exponential carrier recombination time

$R$ is capped at $R_0$ in the illuminated state because it would otherwise tend to infinity at low $V$, such as at the end of a simulation.