Combined Linear-Logarithmic CMOS Image Sensor with FPN Calibration

Abstract

This paper presents a charge compensated phototransistor for a high dynamic range linear-logarithmic CMOS image sensor. The pixel is based on the four-transistor active pixel structure which can automatically switch between the linear mode and logarithmic mode according to the incident light intensity. This paper also proposes a fixed pattern noise (FPN) correction technique for the proposed pixel sensor. The FPN caused by the threshold voltage variation of the transfer gate is corrected by the calibration method of two-step charge transfer. The prototype sensor consisting of a $160 \times 200$ pixel array with the pixel pitch of $8 \times 8 \mu m^{2}$ is fabricated with a 0.18 $\mu m$ 1P6M standard CMOS process. It is found that the dynamic range is achieved 169 dB and the FPN is reduced by 80%.