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Department of Electrical EngineeringImage sensors especially 3-transistor image sensor and I-ToF image sensor have been suffered from reset noise which is represented as kTC noise. Due to their operational properties, reset noise is always remained in photodiode or FD nodes. The noise from reset operation can affect the quality of image. In 3-transitor image sensor, the output image from reset operation of the sensor has noise on the image itself and that is getting worsen in the low light intensity where the light signal is not that much larger than the noise from image sensor. Likewise, reset noise affects the depth accuracy of I-ToF sensor. Because the signal from reflected light is not accurate due to the reset noise. To overcome those kind of problems, active reset technique is proposed. The active reset technique is circuit technique for reset noise suppression. Active reset requires additional components such as an amplifier and loop connection switches. And the noise contribution from those additional components can be neglect. First, conventional hard reset is performed. Since the conventional hard reset can make the same initial state within few nano seconds, there is no residual electrons generated by the previous frame. Then the active reset is activated. The negative feedback helps to suppress the sampled hard reset noise. No matter how much noise is sampled on the photodiode or FD node in I-ToF sensor Thanks to the active reset technique, the estimated reset noise can be inversely proportional to the square root of the gain of amplifier. It means that the reset noise can be suppressed up to 96 %. And the actual measurement results show that the reset noise is suppressed as 92% compared with the reset noise from conventional reset operation.ope

Noise Characterization of a CMOS X-Ray Image Sensor

The objective of this thesis is to validate the noise performance of a high resolution CMOS X-ray imager. We carry out a detailed noise analysis on a four-quadrant CMOS imager and the external hardware. Careful analysis reveals several design issues on the printed circuit board (PCB). We propose solutions to improve the PCB design. Experimental results show the modified system outperforms the original one with a sizable margi

DESIGN OF A BURST MODE ULTRA HIGH-SPEED LOW-NOISE CMOS IMAGE SENSOR

Ultra-high-speed (UHS) image sensors are of interest for studying fast scientific phenomena and may also be useful in medicine. Several published studies have recently achieved frame rates of up to millions of frames per second (Mfps) using advanced processes and/or customized processes. This thesis presents a burst-mode (108 frames) UHS low-noise CMOS image sensor (CIS) based on charge-sweep transfer gates in an unmodified, standard 180 nm front-side-illuminated CIS process. By optimizing the photodiode geometry, the 52.8 μm pitch pixels with 20x20 μm^2 of active area, achieve a charge-transfer time of less than 10 ns. A proof-of-concept CIS was designed and fabricated. Through characterization, it is shown that the designed CIS has the potential to achieve 20 Mfps with an input-referred noise of 5.1 e− rms

Analysis of Subthreshold Current Reset Noise in Image Sensors

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 m k T / 2 C P D when t → ∞ , in full agreement with previously published results. In this equation, C P D 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 log t . 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, C P , 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 m k T ( C P D + C P 1 ) 2 2 q 2 A ( C P D + ( 1 + A ) C P ) 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

Analysis of Subthreshold Current Reset Noise in Image Sensors

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 m k T / 2 C P D when t → ∞ , in full agreement with previously published results. In this equation, C P D 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 log t . 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, C P , 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 m k T ( C P D + C P 1 ) 2 2 q 2 A ( C P D + ( 1 + A ) C P ) 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