Abstract
In this paper we present a systematic approach to sort out different types of random telegraph noises (RTN) in CMOS image sensors (CIS) by examining their dependencies on the transfer gate off-voltage, the reset gate off-voltage, the photodiode integration time, and the sense node charge retention time. Besides the well-known source follower RTN, we have identified the RTN caused by varying photodiode dark current, transfer-gate and reset-gate induced sense node leakage. These four types of RTN and the dark signal shot noises dominate the noise distribution tails of CIS and non-CIS chips under test, either with or without X-ray irradiation. The effect of correlated multiple sampling (CMS) on noise reduction is studied and a theoretical model is developed to account for the measurement results.
Highlights
The readout random noise (RN) and the dark current (DC) are two key performance indices for CMOS image sensors (CIS)
One unexpected finding during the course of this study is that the random telegraph noises (RTN) due to the varying reset-gate induced sense node (SN) leakage is identified in non-CIS test chips without any X-ray irradiation
It is known in literature that there are generally two kinds of RTN [24]
Summary
The readout random noise (RN) and the dark current (DC) are two key performance indices for CMOS image sensors (CIS). In literature and product datasheets, usually the average values of RN and DC are quoted They are not sufficient to reveal the true performance of CIS, because the statistical distribution of RN and DC are typically non-Gaussian and highly asymmetrical with significant long tails. As reported in [20,21,22,23], the number of RTN pixels in an 8.3 MP CIS before X-ray irradiation is in the order of 1 percent. One unexpected finding during the course of this study is that the RTN due to the varying reset-gate induced SN leakage is identified in non-CIS test chips without any X-ray irradiation. Part of the results were published recently in [33,34]
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