Abstract: In a bipolar silicon-on-insulator transistor having a substrate having a major surface, an oxide layer on the major surface, a silicon layer of a first conductivity type on the oxide layer, a base region of a second conductivity type extending into the silicon layer, an emitter region of the first conductivity type extending into the base region, and a collector region of the first conductivity type extending into the silicon layer at a lateral distance from the base region, a plug region of the second conductivity type extends into the silicon layer up to the oxide layer on the opposite side of said emitter region relative to the collector region, a portion of the plug region extends laterally along the surface of the oxide layer under at least part of the emitter region towards the collector region at a distance from the base region, and the plug region is electrically connected to the base region.