Dark Current and Signal-to-Noise Ratio in BDJ Image Sensors

A Review of the CMOS Buried Double Junction (BDJ) Photodetector and its Applications

“…Almost all these charges contribute to the photocurrent without being affected by electronhole recombination, which is a much slower process. The drift current can thus be calculated by integrating the generation rate over the depletion layer [3, 4, 7, 10, 11, 19, 20, 22, 24, 26, 40, 43]: $I_{dr\theta }\approx q{A}_j{\mathrm{normal\Phi }}_{t}exp\left(-\alpha W_{\theta }\right)$where q is the elementary charge; A j is the active surface area; θ = 1 or 2 depending on the considered current and W 1 and W 2 denote the depletion-layer widths.…”

“…Due to the recombination, only photo-generated carriers within a diffusion length from the depletion border can be collected and they contribute to the photocurrent. To determine the diffusion currents inside the BDJ detector, the concentration of excess minority carriers should first be calculated by solving the continuity equation with appropriate boundary conditions [3, 4, 7, 10, 11, 19, 20, 24, 26, 40, 43]. Diffusion component can then be determined for any kind of layer by: $I_{\mathit{\text{phD}}}=q{A}_j{D}_p{\frac{\partial p_n}{\partial x}|}_x$ $\text{or}{I}_{\mathit{\text{phD}}}=-q{A}_j{D}_n{\frac{\partial n_p}{\partial x}|}_x$where D p and D n represent the diffusion coefficient for holes and electrons respectively; p n and n p represent the concentration of excess minority carriers in the observed quasi-neutral region.…”

“…This current depends on the junction characteristics and is superimposed to the photocurrent across the junction. It places a limit on the integration duration for measuring the light intensity and it generates a dark noise [19, 20, 46-47]. The value of dark current imposes a physical limit on the two critical benchmarks of any photodetector (PD): the optical detection range and the noise floor.…”

“…Furthermore, for image sensors, the dark current may exhibit large variations from pixel to pixel. This dark current non-uniformity increases the Fixed Pattern Noise (FPN) and the Dark Signal Non-Uniformity (DSNU) [19, 20, 48, 49]. …”

“…Note that in the last forms of Equation (12), the area and sidewall contributions are not separated and the capacitance value is estimated with respect to the effective width of the depletion layer [19-20]. …”