Multigate In-Pixel Source Follower for Low 1/f Read Noise
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Solution Overview
Problem
Current image sensors face a tradeoff between 1/f noise and conversion gain due to the size of the source follower gate, where larger gates reduce 1/f noise but increase parasitic capacitance, limiting the minimum achievable read noise.
Innovation Solution
The introduction of a multigate source follower with a modulation gate of minimal size and additional guard gates, which are operated near the bias voltage of the modulation gate, increases the total gate area for reduced 1/f noise while maintaining high conversion gain by minimizing parasitic capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the source follower gate area is increased to reduce 1/f noise, then 1/f noise decreases, but gate parasitic capacitance increases which reduces conversion gain
Solution Approach 1:
The source follower gate is segmented into multiple independent gates (first gate, second gate, third gate) that can be controlled separately. This allows the total gate area to be increased for noise reduction while maintaining optimal capacitance characteristics through independent gate control, directly resolving the contradiction between noise performance and conversion gain
Solution Approach 2:
Different regions of the channel are controlled by different gates with potentially different bias voltages. The first gate controls a first region, the second gate controls a second region, and the third gate controls a third region. This local control allows optimization of noise characteristics in specific regions without compromising overall conversion gain
2Reliability
If the source follower gate area is decreased to increase conversion gain, then conversion gain increases, but 1/f noise increases
Solution Approach 1:
By dividing the gate into multiple segments, the invention achieves high conversion gain through compact overall structure while distributing the gate area across multiple controlled regions. The segmented structure allows each gate segment to contribute to noise reduction without proportionally increasing total parasitic capacitance
Solution Approach 2:
The multiple gates can be dynamically controlled with different bias voltages to optimize performance. By adjusting the bias conditions of individual gates, the device can dynamically balance between noise reduction and conversion gain requirements based on operating conditions
Data Source
AI summary
An image sensor including one or more pixels is provided. At least one of the pixels can include a semiconductor substrate, a floating diffusion formed in the semiconductor substrate, a transfer gate configured to selectively cause transfer of photo-charges stored in the pixel to the floating diffusion, and a multigate source follower. The multigate source follower can include a channel, a source and a drain coupled by the channel, a modulation gate formed over the channel and coupled to the floating diffusion, and a first guard gate formed over the channel.