Electrical property modelling of photodiode type CMOS active pixel sensor (APS)

In this research, few electrical characteristics of photodiode (PD) type CMOS active pixel sensor (APS) pixel were modeled. BSIM3v3 threshold voltage equation was simplified for hand calculation with better than 2% peak-to-peak error for full back-gate bias and supply voltages for CMOS process technologies that has minimum feature sizes between 0.18μm and 2.0μm. Two fitting function coefficients (FFC) were included in the BSIM threshold model equations for simplification of the equation. FFCs were extracted by using circuit simulation for given process. Using the simplified threshold equation, electrical characteristics of 3T CMOS PD-APS pixel were modeled. Models include; photodiode reset level, pixel amplifier signal range, and pixel reset level boosting factor. Models were evaluated by using wide variety of available CMOS process technologies and compared with the simulation results. Pixel reset level and signal range model equations produce better than 6% and 12% peak-to-peak accuracy with simple hand calculation, respectively. Models were also confirmed with a designed photodiode-type CMOS APS pixel. A CMOS photodiode type APS imager fabricated in a 0.5μm, 2P3M, 5Volt CMOS process with 15μm square pixel size was used for comparison.