Amorphous silicon phototransistor as nonlinear optical device for high dynamic range imagers

An amorphous silicon bulk barrier phototransistor is studied as a basic element for the pixel of high dynamic photosensor arrays. The device is an n-i-δp-i-n structure whose optical gain shows a nonlinear dependence on the illumination intensity. For each applied bias voltage, a quasi-hyperbolic decrease of the optical gain as a function of the incident power is found. This behavior can be explained taking into account both the material characteristics (defect distribution, dopant concentration), and the structure properties. Our measurements lead to a minimum detectable signal of about 0.7 nW/cm² independently on the applied voltage, making the device suitable for low illumination conditions. On the other hand, by increasing the input power up to 35 mW/cm², we did not find saturation of output photocurrent leading to a dynamic range of at least of 120 dB. This value can be further increased by using as basic cell of the pixel the phototransistor and a resistor connected between the voltage supply and the collector electrode