Analytical Model for Crosstalk in p-n_well Photodiodes

Abstract

The response and crosstalk (CTK) of the p-n well photodiode were studied through device simulations performed with ATLAS and experimental data. As a result, a closed-form and explicit 2-D analytical model for its photoresponse and CTK was developed. The model has very few fitting parameters since it is physically based and describes the CTK dependencies on light conditions and physical, geometrical, and process parameters. This is of great interest for pixel design optimization to fulfill high resolution and small area requirements driven by pixel size reduction. As this model extends a previous one focused on p-n + devices, the behavior of both the structures was also compared.

A Review of CMOS Photodiode Modeling and the Role of the Lateral Photoresponse

“…Considering the photodiode model reported previously [45], and the modeled CTK current component, the final expression for the CTK model can be obtained, CTK (%) = I o + I CTK I aa + I W + I lateral × 100 (27) The expressions developed for the CTK allowed us to reproduce both experimental measurements and simulation data obtained for a copious variety of geometrical parameters and illumination wavelengths in the photodiodes under study for both p-n + and p-n well photodiodes, [52], [68]. A comparison between a p-n well (x well = 3.06 μm, x ph = 1.06 μm) and a p-n + photodiode with a diffusion of the same size of the well (x ph = 3.06 μm), and the same total width, x = 5.06 μm, with uniform illumination, in the visible range and beyond, for a standard 180 nm technology can be seen in Fig.…”

“…In the case of p-n well photodiodes, an identical expression is found with x well instead of x ph , but in this case a second contribution to the lateral photocurrent due to photocarriers in the surrounding volume which does not form p-n junctions with the well must be added. Device simulations showed that this contribution is not negligible and can be modeled as I lateral x s /x well , [52]. In order to state the relative importance of the lateral contribution on the overall photocurrent, we consider the case of a p-n + junction in which the whole device area is uniformly illuminated.…”

“…Analytical models for I total have already been presented for both p-n and p-n well photodiodes in Section IV-C. A parallel process leads to an analytical expression for I n accounting for the neighboring photodiodes, [52], [68]. The current is obtained considering two terms: the saturation current of the reverse-biased diode A, I o , and the CTK lateral current related to generated carriers in the surroundings of neighbor B which is zero-biased and under illumination, I CTK ,…”

Simulation of CMOS APS Operation and Crosstalk in SPICE With Generalized Devices

“…The crosstalk in APS originates when a pixel at dark behaves as if it were impinged by light. While the optical crosstalk could originate if photons reach a dark pixel due to light beam scattering in the optical microlenses, the electrical crosstalk happens when carriers photogenerated in the illuminated pixel diffuse further towards neighbor pn junctions and produce a signal [8]. Such a crosstalk is becoming a critical factor in new technologies as it gets worsened when pixels shrink, seriously damaging the sensor resolution [9].…”

“…Crosstalk modeling, especially electrical crosstalk, is thus mandatory to optimize pixel design. However, developing an analytical model is challenging [8] and only few analytical and semianalytical models can be found in literature [8]- [11]. Instead, in this paper we adopt a different approach using the Generalized Devices and demonstrate that electrical crosstalk can be predicted using SPICE simulations only, allowing fast design optimization including the circuit without the need to rely on TCAD simulations.…”