CMOS Image Sensor with Tunable Conversion Gain for Improved Performance

Abstract

This paper describes the use of shallow trench isolation as a modified trench capacitor in CMOS image sensor. MTC helps to tune the conversion gain at the floating diffusion (FD) node. The use of MTC in pixel improves the sensitivity of the pixel without affecting the dynamic range (DR) of the pixel. The proposed pixel structure uses an existing in-built isolation trench as a capacitor. During low illumination, the MTC is separated from the floating diffusion (FD) node thus makes it easier to detect the low light signals owing to the lower FD node capacitance. However, in the event of high illumination, MTC is connected in parallel to the FD node increasing the capacity to store more charges from the photodiode. This attributed to the improved DR performance of the pixel. Using 2D numerical simulations, we show that an overall 60 dB enhancement in the DR as compared to the conventional pixel. Moreover, the proposed modification does not affect the other pixel characteristics such as dark current and crosstalk.