High voltage thin-film transistor

Claim 1

A high voltage thin film transistor of the insulated gate field effect type wherein the source-drain voltage is in excess of 200 volts, comprising: a semiconductor layer having first and second opposed major surfaces; a source electrode and a drain electrode at opposite ends of the semiconductor layer, the spacing between the source electrode and drain electrode being in excess of 0.01 cm; a composite insulating layer on said first major surface of the semiconductor layer and overlapping the area between the source and drain electrodes, the thickness of said layer being in excess of 8,000 A; a gate electrode on the surface of the composite insulating layer and overlapping the area between the source and drain electrodes and seperated from the semiconductor layer by the thickness of the insulating layer; and a depleting layer on the second surface of the semiconductor layer and overlapping the area between the source and drain electrodes, the thickness of said depleting layer being at least 1,000 A; the depleting layer effecting depletion of current carriers from the second surface of the semiconductor layer; the composite insulating layer comprising a film of depletion material and a film of enhancement material, said depletion material and said enhancement material of said composite layer together regulating the density of current carriers in said first surface of the semiconductor layer; whereby said depleting layer effects a first value of depletion of said second surface, and said composite insulating layer effects a second value of depletion of said first surface algebriacally less than said first value; said second value of depletion of said first surface being the algebraic sum of the depletion effected by said depletion material and of the enhancement effected by said enhancement material, with enhancement taken as a negative value of depletion; thereby controlling the values of the offset voltage and leakage current of said transistor.