A semiconductor device has a MOS gate side surface structure, including a gate electrode filling a trench formed in a semiconductor substrate with an insulator film between the trench and the gate electrode, a gate insulator film covering the surface of the gate electrode, a buffer region of one conductivity type in contact with the semiconductor substrate, a base region of the other conductivity type adjacent to the buffer region on the gate insulator film, and an emitter region of the one conductivity type adjacent to the base region on the side opposite to the buffer region. The semiconductor device and the method of manufacturing thereof can further improve the tradeoff between the on-voltage and the turn-off loss by increasing the amount of electrons injected from a cathode on the surface to increase an amount of carriers on the cathode side in a stable turned-on state of the device.