An insulated gate bipolar transistor includes a substrate; a first conductivity type base disposed on the substrate and having a first trench; a first conductivity type buffer region disposed in the first conductivity type base; a collector doped region having a second conductivity type and disposed in the first conductivity type base; a second conductivity type base to which the first trench extends downwardly; a gate oxide layer disposed on an inner surface of the first trench; a polysilicon gate disposed inside the gate oxide layer; an emitter doped region having a first conductivity type and disposed in the second conductivity type base and under the first trench; a conductive plug extending downwardly from above the first trench and contacting the second conductivity type base; and an insulating oxide layer filled in the first trench, the insulating oxide layer insulating and isolating the polysilicon gate from the emitter doped region.