Extended One-Dimensional Analysis to Effectively Derive Quantum Efficiency of Various CMOS Photodiodes

This paper proposes an extended 1-D analysis to derive quantum efficiency of various commonly used CMOS photodiodes. The theoretical model of the CMOS photodiode with the n-/p-epitaxial/p + substrate (n-/p-epi/p + sub) structure is established from steady-state continuity equations, where most existing boundary conditions are applied. In particular, the minority carrier and current densities are continuous across the interface between two layers with the same dopant type. Models of the other commonly used CMOS photodiodes are also examined. Three CMOS photodiodes with n-/p-substrate (n-/p-sub), p⁺/n-/p-substrate (p⁺/n-/p-sub), and n-/p-epi/p + sub structures are fabricated and characterized to validate the proposed model. Additionally, the surface recombination velocity is adequately determined by fitting the simulated quantum efficiency to the measured value. The simulated quantum efficiency of the proposed model for these three photodiodes is quite consistent with the measured values, revealing the feasibility and effectiveness of the proposed model in characterizing various CMOS photodiodes.