Research On CMOS Photodiodes And Related Circuits In Digital Imaging System

The most important part of CMOS image sensor are CMOS photodiodes and related readout circuits. They determine the characteristics of the whole chip. In order to improve performance of CMOS image sensors, It is valuable to do research on these two critical component. We introduce structure and development of CMOS image sensor, more over, the factors influencing the characteristics of CMOS image sensors are also analyzed.Based on the above discussion, this paper utilizes the continuity equation and various boundary conditions to build a CMOS photodiode model. The analytical solution was verified with numerical simulations based on generic 0.18μm CMOS process when reverse bias voltage is 2.2V and T=300K. An improved analysis of quantum efficiency including surface recombination velocity, epitaxial doping concentration and depth, P-sub and P-epi high-low junction has been derived. The result is the more surface recombination rate, the higher the QE we can get. We also present a study on two mechanisms of dark current in CMOS photodiode. A new one dimensional expression of diffusion current has been derived. The result was verified with simulations based on generic 0.18μm CMOS process at different reverse bias voltage. P+/N-well has highest dark current, however, N+/P-epi and N-well/P-epi have lowest dark current.Integration nonlinearity of photodiode was simulated using a piecewise approximation method.We also simulate 3T and 4T pixel unit of tranditional CMOS image sensors.Then constant Vgs which has has low on-resistance sample-hold circuits are proposed. At last, we design a high dynamic range opamp which is suitable for readout circuits of CMOS image sensors.The dynamic range is 85dB.Settling time is no more than 5ns.Main amplifier's power consumption is less than 7.2mW.Bias circuit's is less than 2mW.