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An amorphous silicon field-effect phototransistor is fabricated using a processing technology compatible with conventional amorphous silicon-silicon nitride thin-film transistors. The phototransistor has an offset structure between the source and gate electrodes, where light is absorbed to produce a photocurrent. In an electron accumulation mode, the photocurrent is greater than the dark current by three orders of magnitude. In addition, the phototransistor is found to have output characteristics showing good saturation. Typical photoconductive gain of this saturation current is 17.

Instability mechanism of amorphous silicon-silicon nitride thin-film transistors (TFTs) is examined. By investigating double-layer insulator TFTs, it is demonstrated that the instability is caused by an electrical charge stored at the interface between amorphous silicon and silicon nitride. The amount of stored charge at the interface (Q) does not depend on either drain voltage or drain current. Study on TFTs with several insulator thicknesses has shown that Q strongly depends on the band bending in the amorphous silicon that is related to the gate electric field (E) through the gate insulator. The Q-E relationship is found to be a more general expression of the dependence of threshold voltage shift on gate voltage, and is incorporated into a formula suitable for examining the interface quality.

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