1. Field of the Disclosure
The present invention relates generally to semiconductor devices. More specifically, examples of the present invention are related to image sensor pixel cells having global shutters.
2. Background
For high-speed image sensors, a global shutter can be used to capture fast-moving objects. A global shutter typically enables all pixel cells in the image sensor to simultaneously capture the image. For slower moving objects, the more common rolling shutter is used. A rolling shutter normally captures the image in a sequence. For example, each row within a two-dimensional (“2D”) pixel cell array may be enabled sequentially, such that each pixel cell within a single row captures the image at the same time, but each row is enabled in a rolling sequence. As such, each row of pixel cells captures the image during a different image acquisition window. For slow moving objects, the time differential between each row can generate image distortion. For fast-moving objects, a rolling shutter can cause a perceptible elongation distortion along the object's axis of movement.
To implement a global shutter, storage capacitors or storage transistors can be used to temporarily store the image charge acquired by each pixel cell in the array while it awaits readout from the pixel cell array. When a global shutter is used, a transfer transistor is typically used to transfer image charge from the photodiode to the storage transistor, and then an output transistor is used to transfer the stored image charge from the storage transistor to a readout node of the pixel cell. Factors that affect performance in an image sensor pixel cell having a global shutter include global shutter efficiency, dark current, white pixels, and image lag. In general, global shutter pixel performance improves as global shutter efficiency improves. Global shutter efficiency is a measure of how well signal charge can be stored in the storage node without being contaminated by parasitic light and/or electrical crosstalk.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.