A transistor of a semiconductor device includes an isolation region, an active region disposed in the isolation region, a gate extending in a second direction on the active region, and source and drain regions respectively extending in a first direction perpendicular to the second direction in the active region on first and second sides of the gate. The source and drain regions include low-concentration source and drain doping regions including first and second low-concentration source and drain doping regions The source and drain regions further include high-concentration source and drain doping regions respectively disposed in the low-concentration source and drain doping regions and having higher doping concentrations than the low-concentration source and drain doping regions. A first length in the second direction of the first low-concentration source and drain doping regions is greater than a second length in the second direction of the second low-concentration source and drain doping regions.