25 March 1996 Increasing the dynamic range of an array of light sensors using a new thin-film circuit
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Previously realized circuits for solid state light sensitive arrays fabricated in thin-film technology show a comparatively small dynamic range. A wide dynamic range as well as a large sensitivity of the output signal to the change of the input signal (irradiance) is required. A circuit which satisfies these requirements is described. The circuit consists of two thin-film transistors (TFTs), a photoconductor, and a fixed resistor. The photoconductor is used to detect the incoming light whereas the first TFT generates the logarithm of the photocurrent in form of a voltage. The second TFT and the fixed resistor invert the signal of the first stage and determine the slope of the output signal. The sensor's characteristic can be shifted over a wide dynamic range by using an appropriate control voltage. The circuit is simulated by SPICE, employing a special model for the TFTs. The sensor's characteristics show a large dynamic range of 4 decades when shifted by means of the control voltage. At the same time it exhibits a high sensitivity of 1.8 V/W/m2 due to the steep slope of its characteristics. This circuit was then built in a-Si:H thin-film technology. The measured characteristics match very well the simulations.
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Dietmar Horst, Dietmar Horst, Holger Scherr, Holger Scherr, Ernst H. Lueder, Ernst H. Lueder, } "Increasing the dynamic range of an array of light sensors using a new thin-film circuit", Proc. SPIE 2654, Solid State Sensor Arrays and CCD Cameras, (25 March 1996); doi: 10.1117/12.236089; https://doi.org/10.1117/12.236089

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