High voltage MIS transistor and semiconductor device

A high voltage transistor, comprising:

• a semiconductor substrate having a first conduction type;
• a well region of a second conduction type formed by a first step of injecting ions from a surface side of the semiconductor substrate and by a second step of thermal diffusion;
• a first part comprising:
• a first base layer of the first conduction type in a first end portion of the well region, a base contact layer of the first conduction type in the first base layer and to which an emitter potential is applied, an emitter layer of the second conduction type in the first base layer and to which the emitter potential is applied, and a gate electrode on an insulation film and extending from the emitter layer to the well region;
• and
• a second part comprising:
• a second base layer of the second conduction type in a second end portion of the well region, a collector layer of the first conduction type in the second base layer, and a high concentration contact layer of the first conduction type in the collector layer and to which a collector potential is applied;
• and
• a high concentration drain layer of the second conduction type between the first part and the second part and an external circuit connected to the drain layer, the external circuit comprising a switching element for selectively connecting the well region to the collector layer in a short-circuit manner.