Gate turn-off thyristor with independent turn-on/off controlling transistors

Claim 1

A gate turn-off thyristor comprising: (a) a semiconductor substrate of a first conductivity type serving as a first base layer; (B) a first emitter layer and a second base layer both of a seocnd conducitvity type, which are formed on opposing surfaces of said substrate; (c) a second emitter layer of the first conductivity type formed in said second base layer; (d) a first metal oxide semiconductor field effect transistor of a certain channel conductivity type for turning on said thyristor, said first transistor comprising, a first semiconductor layer of the first conductivity type formed in said second base layer to serve as a source of said first transistor and to define a surface portion of said second base layer positioned between itself and said first base layer as a channel region of said first transistor, and a first gate electrode layer insulatively provided above said first base layer to cover said channel region; (e) a second metal oxide semiconductor field effect transistor of the same channel conductivity type as said first transistor, for short-circuiting said first base layer and said second emitter layer with each other thereby to turn off said thyristor, said second transistor being provided in said second base layer to be adjacent to said first transistor, said second transistor comprising, a second semiconductor layer of the first conductivity type formed in said second base layer to serve as a drain of said second transistor, said second semiconductor layer defining a surface portion of said second base layer positioned between itself and said second emitter layer as a channel region of said second transistor, and a second gate electrode layer insulatively provided above said channel region of said second transistor; and (f) a source electrode layer formed on said second base layer, for causing said source and said drain to be electrically conducted together with said second base layer, whereby said source electrode layer serves as an amplifying gate electrode of said first transistor, so that a source current of said first transistor acts as a base current of said thyristor to improve a gate sensitivity thereof; wherein said second base layer comprises, an additional semiconductor layer of the second conductivity type formed in said substrate to overlap with said second base layer, said additional layer having a diffusion depth smaller than that of said second base layer, said first semiconductor layer being formed in said additional layer in such a manner that said channel region of said first transistor is defined in said additional layer to have a smaller diffusion depth than that of said channel region of said second transistor, and said first and second semiconductor layers are formed in said additional layer.