Abstract: A three-transistor active pixel sensor and its double sampling readout circuit implemented by a switch capacitor amplifier are designed.The circuit is embedded in a 64×64 pixel array CMOS image sensor and successfully taped out with a Chartered 0.35μm process.The pixel pitch is 8μm×8μm with a fill factor of 57%,the photo-sensitivity is 0.8V/(lux·s),and the dynamic range is 50dB.Theoretical analysis and test results indicate that as the process is scaled down,a smaller pixel pitch reduces the sensitivity.A deep junction n-well/p-substrate photodiode with a reasonable fill factor and high sensitivity are more appropriate for submicron processes.

Key words: CMOS image sensor, active pixel sensor, fill factor, photo-response sensitivity