Image sensor and electronic device with active reset circuit, and method of operating the same

Abstract

An image sensor including a pixel circuit and an active reset circuit. The pixel circuit includes a light sensing element, a storage node selectively connected to the light sensing element, an output transistor configured to, during a readout operation, output a signal that is based on a potential of the charge storage node to an output line, and a selection transistor that controls the readout operation. The active reset circuit includes a first current path and a second current path, the first current path extending from a power supply node to the output line via the selection transistor and the output transistor, and the second current path extending from the power supply node to the output line via a first transistor and a second transistor. The active reset circuit is configured to, when the selection transistor and the first transistor are both ON, set a potential of the charge storage node based on a potential of a gate of the second transistor.

Description

BACKGROUND[0001]1. Field of the Invention[0002]The present invention relates generally to image sensors and more particularly to reduction of reset spread in image sensors.[0003]2. Description of the Related Art[0004]Digital image capturing devices use image sensors to convert incident light energy into electrical signals. Image sensor designs include Charged Coupled Devices (CCD), Complementary Metal Oxide Silicon (CMOS) image sensors, and Digital Pixel System (DPS) sensors. An image sensor includes a two-dimensional array of light sensing elements called pixels. Each pixel in the array works with the lens system to respond to incident light within a local area of the scene, and produces an electrical signal describing the characteristics of the scene. The electrical signals that are output from the light sensing elements are converted to digital form, and the resulting digital pixel values form the raw data representing the scene. The raw data can be processed by an image processo...

AI Technical Summary

Benefits of technology

This patent describes a method to improve the dynamic range and maximum useable dynamic range of an image sensor. By reducing the variation in the reset signal, the method achieves higher dynamic range at any gain level. This is achieved by including a pixel circuit and an active reset circuit in the image sensor. The active reset circuit sets the potential of the charge storage node based on the potential of a gate, and this helps to stabilize the image sensor and reduce the variation in the reset level. Overall, this method improves the quality of the captured images and increases the reliability of the image sensor.

Problems solved by technology

In CMOS image sensors, various problems arise as a result of reset noise in pixels.

Reset noise relates to a variation in the reset level among pixels, and can cause two pixels that received identical light to nonetheless output different pixel values.

In particular, the variation in reset level among pixels may make it difficult to know which portion of any given pixel signal is attributable to the reset level of the pixel.

However, the reset level variation can still be problematic in image sensors for other reasons.

For example, when using digital CDS cancellation techniques, the variation of the reset signal can result in decreased dynamic range for the image sensor, especially under the conditions of high analog gain.

Thus, although the reset level is ultimately removed from the signal by the CDS technique, the reset variation still can degrade image quality by decreasing the dynamic range of the image sensor, and this effect is magnified under conditions of high analog gain.

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