Performance of Photo-Transistors with thin (50 nm.) a-Si:H Layers

ABSTRACTPhoto-transistor based a-Si:H image sensors allow the integration of photo-elements and pixel circuitry using standard TFT fabrication processes. While pixel circuitry use thin a-Si:H films (≈ 0.05μm) to minimize the contact resistance, photo-transistors require thick a-Si:H films (≈0.5μm) to maximize photo-sensitivity. We fabricated a new, high sensitivity photo-transistor structure using 0.05μ a-Si:H films. High optical sensitivity is achieved by separating the conduction paths of photo-generated electrons and holes using a secondary gate. Further, the optical path within the photo-transistor is doubled through manipulation of device layout. The photo-transistor show an Ilight/Idarkratio in excess of 103.We compared the transient behavior of conventional and high-sensitivity photo-transistors. We found that both devices display an increase in current with time when biased in either the dark or illuminated conditions. The current increases by six orders of magnitudes then saturates within 200 seconds of bias application. Experiments indicate that this transient behavior is due to a rise in the conductivity of the gap region with time; measurements on 0.05μm thick photo-resistor structures showed that their current increases with time. The shape and temporal range of this behavior rules out the presence of parasitic capacitive effects.