Temperature Dependence and Dynamic Behavior of Full Well Capacity in Pinned Photodiode CMOS Image Sensors

Abstract

This paper presents an analytical model of the full well capacity (FWC) in pinned photodiode (PPD) CMOS image sensors. By introducing the temperature dependence of the PPD pinning voltage, the existing model is extended (with respect to previous works) to consider the effect of temperature on the FWC. It is shown, with the support of experimental data, that whereas in dark conditions the FWC increases with temperature, a decrease is observed if FWC measurements are performed under illumination. This paper also shows that after a light pulse, the charge stored in the PPD drops as the PPD tends toward equilibrium. On the basis of these observations, an analytical model of the dynamic behavior of the FWC in noncontinuous illumination conditions is proposed. The model is able to reproduce experimental data over six orders of magnitude of time. Both the static and dynamic models can be useful tools to correctly interpret FWC changes following design variations and to accurately define the operating conditions during device characterizations.Index Terms-Active pixel sensor (APS), analytical modeling, capacitance, CMOS image sensor (CIS), dynamic behavior, full well capacity (FWC), modeling, pinned photodiode (PPD), pinning voltage, temperature.

Dark Current Blooming in Pinned Photodiode CMOS Image Sensors

“…5 shows the average integrated charge of all the hot pixels of the 4.5-µm pixel pitch CIS as a function of integration time in the dark and at different temperatures. The EFWC (final saturation level) increases with temperature, which is expected due to the increase of the pinning voltage [12]. On the contrary, the intermediate saturation level reduces with temperature, which means that the PPD potential at this level increases with temperature (i.e., higher reverse bias).…”

“…For most of them, the higher dark current is due to the presence of one or several Shockley-Read-Hall recombinationgeneration (SRH R-G) centers [17] in the depletion region of the photodiode, which thermally generate electron-hole pairs when pn − n 2 i < 0 (where p and n are the hole and electron concentrations and n i is the intrinsic free carrier concentration. This condition is verified only when the p-n junction of the PPD is reverse biased, i.e., when the charge stored in the PPD is lower than the equilibrium FWC (EFWC), which is charge stored in the PPD at equilibrium (i.e., for zero bias) [12]. The highest reverse bias of the PPD is called the pinning voltage [12] and is reached when the PPD is completely empty.…”

“…This condition is verified only when the p-n junction of the PPD is reverse biased, i.e., when the charge stored in the PPD is lower than the equilibrium FWC (EFWC), which is charge stored in the PPD at equilibrium (i.e., for zero bias) [12]. The highest reverse bias of the PPD is called the pinning voltage [12] and is reached when the PPD is completely empty.…”

“…On the other hand, blooming effects in CIS are usually expected to appear only in forward bias, i.e., when the charge stored in the PPD is higher than the EFWC [12]. Indeed, it is only in forward bias that the net current through a p-n junction is positive, i.e., that more electrons flow from the n side to the p side than from the p side to the n side [18].…”

Characterization and video chain development of the CMOS detector applied in the multi-angle spectro-polarimeter SPEXone

“…Interestingly and counter-intuitively, the full-well capacity of the DEM is not a constant, but increases logarithmically with light intensity. 13,14 Two extreme cases for very different light intensities are shown in Figure 5. This also means that, whereas the signal at a fixed intensity reaches a constant value at large exposure times, the signal at a fixed exposure time keeps increasing at large intensities (see Figure 6).…”