Video signals and switching transients in capacitor-photodiode and capacitor-phototransistor image sensors

A new configuration of the capacitor-photodiode type of image sensor is described which results in appreciable reduction of switching transients at the sensor's output. The modification consists of replacing the photodiode by a phototransistor operating in the common base configuration. A 32 × 32 element array capable of operating in either the photodiode or phototransistor mode was fabricated. The dynamic range, transfer characteristic, and switching transient voltage were calculated for both modes and compared with experiment. For the experimental array an order-of-magnitude improvement in signal-to-noise ratio has been realized in the transistor mode of operation.