A simple and practical modeling of the GexSi1-x/Si heterojunction zero-gapdirectional coupler switch (BOA type-Bifurcation Optique Active) is proposed. Theelectro-optic modulsting mechanism of the zero-gap directional coupler switch is discussed by the plasma dispersion effect. The electricity character of the switch is analysed by theheterojunction ultra high injection principle. On the basis of the analysis and the theory ofthe classical single-mode optical waveguide, structure parsmeters and electrivcity parameters of the Ge0.05Si0.95/Si heterojunction BOA switch with double-mode interference mechanism are designed.