DETAILED DESCRIPTION It is therefore an object of the present invention to provide a thin film transistor which is free from the above-mentioned defects of the prior art.
According to the present invention, the active layer of the thin film transistor is formed of amorphous silicon carbide (a-Si.
sub.
1-x C.
sub.
x) and the carbon content x is selected larger than 0.
1.
An increase in the carbon content in the amorphous silicon carbide increases its optical energy gap; when x >0.
1, the optical energy gap amounts to 2.
0 to 4 eV or so.
On the other hand, the optical energy gaps of conventional amorphous silicon and hydrogenated amorphous silicon containing 30 atomic % H are only 1.
6 and 1.
9 eV, respectively.
Accordingly, the optical energy gap of the amorphous silicon carbide for use in the present invention is comparatively large, and no light shielding layer is needed.
In addition, since amorphous silicon carbide with practically no photoconductivity can also be obtained through control of the electric conductivity of the amorphous silicon carbide, a-Si.
sub.
1-x C.
sub.
x, by doping it with boron (B) or phosphorus (P), no light shielding layer is needed.
This permits simplification of the structure of the thin film transistor and makes it easier to manufacture.
According to another aspect of the present invention, the active layer may preferably have a heterojunction superlattice structure.
That is, the active layer is formed by laminating many well layers of hydrogenated amorphous silicon carbide, a-Si.
sub.
1-x C.
sub.
x :H (where x <0.
5), and many barrier layers of hydrogenated amorphous silicon carbide, a-Si.
sub.
1-x C.
sub.
x :H (where x >0.
5), alternately with each other, or by laminating many well layers of hydrogenated amorphous silicon, a-Si:H, and barrier layers of hydrogenated amorphous silicon carbide, a-Si.
sub.
1-x C.
sub.
x :H, alternately with each other.
Since the active layer has such a heterojunction superlattice structure, the mobility of quasi two-dimensional carriers is increased by the quantum effect, yielding a large current driving force
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