A theoretical model is given to represent the active part of either an optoelectronic switch, similar to those described by Auston el al., or an optoelectronic gate such as the one we developed. These devices consist mainly of a microstrip line, with a characteristic impedance of 50 Ω, which is deposited on a high-resistivity semiconductor. The centre strip has a break, creating a gap which can be illuminated by an optical laser pulse. The latter generates an electron-hole plasma of high density at the surface of the semiconductor, allowing the transmission of a signal across the gap.