Linear-Logarithmic CMOS Image Sensor with Reduced FPN Using Photogate and Cascode MOSFET

Abstract

We propose a linear-logarithmic CMOS image sensor with reduced fixed pattern noise (FPN). The proposed linear-logarithmic pixel based on a conventional 3-transistor active pixel sensor (APS) structure has additional circuits in which a photogate and a cascade MOSFET are integrated with the pixel structure in conjunction with the photodiode. To improve FPN, we applied the PMOSFET hard reset method as a reset transistor instead of NMOSFET reset normally used in APS. The proposed pixel has been designed and fabricated using 0.18-μm 1-poly 6-metal standard CMOS process. A 120 × 240 pixel array of test chip was divided into 2 different subsections with 60 × 240 sub-arrays, so that the proposed linear-logarithmic pixel with reduced FPN could be compared with the conventional linear-logarithmic pixel. We confirmed a reduction of pixel response variation which affected image quality.

Keywords

• CMOS image sensor
• hard reset
• photogate
• cascode MOSFET