Characterization of CMOS Photodiodes for Image Sensor Applications

N-well photodiodes and N+-diffusion photodiodes are available from standard CMOS processes. The comparison of quantum efficiency and dark current density for both structures have been described. The N-well photodiodes showed higher Quantum Efficiency (Q.E.) and lower dark current density than the N+-diffusion photodiodes. The photodiode capacitance is another important parameter. It determines the conversion gain of the sensing element. A photodiode with a lower capacitance will convert a certain amount of photo charge with a larger output signal and therefore has a higher conversion gain. The capacitance comparison of the N-well and N+diffusion photodiodes is shown in Figure 13. This comparisons are made through two standard CMOS processes with minimum gate length 2.0 micron and 0.5 micron and the dimension of the photodiode is 500 micron by 500 micron. The N-well photodiode shows a higher conversion gain than the N+-diffusion photodiode. This difference can be explained by a wider depletion width of the lower doped p-n junction in the N-well photodiode.