Analysis of Subthreshold Current Reset Noise in Image Sensors

Abstract

To discuss the reset noise generated by slow subthreshold currents in image sensors, intuitive and simple analytical forms are derived, in spite of the subthreshold current nonlinearity. These solutions characterize the time evolution of the reset noise during the reset operation. With soft reset, the reset noise tends to mkT/2CPD when t→∞, in full agreement with previously published results. In this equation, CPD is the photodiode (PD) capacitance and m is a constant. The noise has an asymptotic time dependence of t−1, even though the asymptotic time dependence of the average (deterministic) PD voltage is as slow as logt. The flush reset method is effective because the hard reset part eliminates image lag, and the soft reset part reduces the noise to soft reset level. The feedback reset with reverse taper control method shows both a fast convergence and a good reset noise reduction. When the feedback amplifier gain, A, is larger, even small value of capacitance, CP, between the input and output of the feedback amplifier will drastically decrease the reset noise. If the feedback is sufficiently fast, the reset noise limit when t→∞, becomes mkT(CPD+CP1)22q2A(CPD+(1+A)CP) in terms of the number of electron in the PD. According to this simple model, if CPD = 10 fF, CP/CPD = 0.01, and A = 2700 are assumed, deep sub-electron rms reset noise is possible.

A Review of Optical Sensors in CMOS

“…The parasitic resistances of electrical interconnections to the photodetector are sources of thermal noise [36]. Moreover, the reset switch (when it is switched) on can be considered a resistance and the amplifier connected to the photodetector can also contribute to the total amount of thermal noise [37]. The 1/f noise appears on readout electronics, especially on the in-pixel sourcefollowers normally present on a CMOS imager [38,39].…”

Full Counting Statistics of the Soft-Reset Process

“…The inherent suppression factor is found to be 2 equivalent to the order of the cumulant giving an asymptotic noise value [k B T C/(2q 2 )]. For practical applications, however, further suppression is necessary leading to extra feedback control circuits [6], [23]. Here, the feedback is shown to enhance the emission-transition correlation by introducing the barrier voltage proportional to the emitted charge number as follows [8]:…”

“…T HE soft reset (SR) is a technique to reset a storage node in an image sensor under an emission-limited-or a subthreshold-bias condition of a reset transistor [1], [2], [3], [4]. The technique is effective in suppressing the shot noise of the reset transistor from a Poissonian type to a sub-Poissonian type (less noise than that of the Poissonian-type noise) and asymptotically gives the half amount of the hardreset-noise [5], [6]. A physical model based on a continuoustime Markov chain in which the storage node potential makes a transition by an amount of single charge potential after each carrier emission was shown to numerically reproduce the timedependent SR-noise [5], [7].…”