Photosensor circuit capable of detecting minute light signal in high precision and image sensor utilizing photosensor circuit

Abstract

A photosensor circuit comprises a photodiode PD, which converts a light signal into electricity, an n-channel MOS transistor QDwhich has a logarithmic characteristic in its weakly reversed condition and converts a sensor current generated by the photodiode PD into a detection voltage which has a logarithmic characteristic, and a capacitor which is connected to a detection terminal of this transistor QD. A resetting voltage VG=VS is applied to the gate G of the transistor QDto lower the impedance between the drain and the source of the transistor for resetting the transistor to an initial condition. As the n-channel MOS transistor QDis a depletion type n-channel MOS transistor, when this transistor QDdoes not receive any gate voltage, the transistor stays in a weakly reversed condition and has a logarithmic characteristic. This photosensor circuit has a high SN ratio and is capable of detecting minute light signals in high precision. Furthermore, it shows no after-image phenomenon and requires a relatively small number of power sources.