CMOS image sensor structure with high filling factor

Abstract

The present invention is CMOS image sensor structure with high filling factor. The sensor comprising one photodiode and through N-type transistors is improved to result in increased filling factor. One of the examples is that two of the N-type transistors are replaced by two P-type ones made directly on the N-type well of the photodiode. The other of the examples in that the other N-type reset transistor is replaced by one reset diode made directly on the N-type well of the photodiode, too. The third of the examples is that the reset diode and the source flower transistor are shared. In the said three examples, the output transistor may be omitted to further raise the filling factor.

Description

technical field [0001] The present invention relates to a structure of a complementary metal oxide semiconductor image sensor, and in particular to a structure of a complementary metal oxide semiconductor image sensor capable of increasing the fill factor. Background technique [0002] Today's very popular active photodiode image sensors include at least a photodiode, a reset transistor, a source follower transistor, and an output select transistor. The photodiode is formed by using an N-type doped region and a P-type substrate. When the reset transistor is turned on, the N-type doped region will be charged to the reset level, and when the reset transistor is turned off, the photodiode Will be in a floating state (Floating), its N + The / P junction will generate a depletion region (Depletion). When illuminated, the electron-hole pairs generated in the depletion region will be pulled apart by the electric field, and the electrons will move to the N-type doped region, causing...

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Problems solved by technology

The photodiode is formed by using an N-type doped region and a P-type substrate. When the reset transistor is turned on, the N-type doped region will be charged to the reset level, and when the reset transistor is turned off, the photodiode Will be in a floating state (Floating), its N + The / P junction will generate a depletion region (Depletion). When illuminated, the electron-hole pairs generated in the depletion region will be pulled apart by the electric field, and the electrons will move to the N-type doped region, causing the potential of the N-type doped region to begin to drop. , the magnitude of the falling potential can reflect the intensity of the light, but only the charges generated in the depletion region will be pulled apart by the electric field, so if the depletion region is larger, a larger potential reduction can be reflected under the same light intensity, To obtain higher sensitivity, however, due to the high concentration of N-type doping, its depletion region is small, so the photoelectric conversion efficiency of its photodiode is poor

[0003] This situation can be improved by photodiodes made of N-type wells and P-type substrates. Because N-type wells are doped at a low concentration, their depletion regions are larger, so their photoelectric conversion efficiency is better, which increases the sensitivity of the image sensor. However, due to the limitation of the process design rules, the minimum size of the N-type well is very large, and the distance between the other three N-type transistors (N-channel MOS-FET) and the N-type well in the component is also very large, resulting in this method The resulting image sensor element size becomes larger, its fill factor (Fill Factor) is reduced, and the remaining characteristics of the image sensor are then deteriorated.

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