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 when , in full agreement with previously published results. In this equation, is the photodiode (PD) capacitance and m is a constant. The noise has an asymptotic time dependence of , even though the asymptotic time dependence of the average (deterministic) PD voltage is as slow as . 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, , 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 , becomes 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.
Highlights
Four-transistor (4-Tr) complementary metal-oxide-semiconductor (CMOS) image sensors [1] are widely used in various applications, such as mobile phone cameras, digital still cameras, security, industrial, medical equipment, etc
Hard reset is originally applied in the reset of the floating diffusion of CCD (Charge coupled device), and is the original reset method of 3-Tr CMOS image sensor
Its timing diagram is shown in device), and is the original reset method of 3-Tr CMOS image sensor
Summary
Four-transistor (4-Tr) complementary metal-oxide-semiconductor (CMOS) image sensors [1] are widely used in various applications, such as mobile phone cameras, digital still cameras, security, industrial, medical equipment, etc. While aim of to them reduce reset noise itself, detection capacitance, increasing voltage. Other approaches to thethus problem have signal been attempted; them is atocharge reducesensitive effectiveamplifier detection or capacitive transimpedance amplifier is introduced [27,28].a charge. Another approach is to introduce incapacitance, increasing signal voltage. A fundamental time-domain analysis of various reset methods is presented, and the reset noise is studied in detail. Feedback reset with reverse taper control methods will be analyzed. One capable of providing an intuitive and simple analytical solution without numerical or Monte Carlo
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