The source-gated amorphous silicon photo-transistor

We discuss the effect of scaling the thickness of the a-Si:H layer on the performance of photo-transistors. We show that reducing the a-Si:H layer thickness to 0.05 /spl mu/m, in order to increase the speed of pixel-switching thin-film transistors (TFTs), causes a large drop in the photo-transistor's dynamic range. We then propose, fabricate and test the Source-Gated Photo-Transistor, a new a-Si:H photo-transistor which allows the use of thin a-Si:H layers while boosting the dynamic range of the phototransistor. Our experimental results show that the source-gated photo-transistor improves the dynamic range of photo-transistors with thin a-Si:H layers (0.05 /spl mu/m) by more than 30 times. This increased dynamic range is obtained by increasing the illumination current and reducing the dark current. Illumination current is improved by doubling the optical path and increasing the photo-generated carriers lifetime within the a-Si:H layer. Dark current is reduced by pinning the gap-region to a constant potential. Finally, we confirm the principle of photo-generated carriers separation in our source-gated photo-transistor by conducting optical and bias stress measurements on conventional and source-gated photo-transistors.