SEFET Image Sensor Pixel Architecture for Sub-Micron Sensitivity
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Solution Overview
Problem
Conventional image sensors face challenges in reducing pixel size below 1 um due to narrow dynamic range, small full well capacity, and a Signal to Noise Ratio (SNR) drop, which limits their performance and conversion gain.
Innovation Solution
The development of a high sensitivity image sensor utilizing a single electron field effect transistor (SEFET) with a substrate, a first well, source and drain regions, and a gate region, coupled with a source follower transistor and a selective transistor, enabling the detection of single electrons and improved conversion gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If pixel size is reduced below 1 um, then device size is decreased, but conversion gain decreases due to increased relative capacitance
Solution Approach 1:
The pixel is segmented into distinct functional regions: a photo detecting area for charge generation, a charge storing area for electron accumulation, and a readout circuit area for signal processing. This segmentation allows each region to be optimized independently, enabling small pixel size while maintaining sufficient charge storage capacity and conversion gain through the dedicated charge storing area.
Solution Approach 2:
The patent transitions from planar charge storage to three-dimensional charge storage by forming a charge storing area that extends vertically into the substrate with controlled depth and lateral dimensions. This dimensional change increases the charge storage capacity without proportionally increasing the pixel footprint, thereby maintaining conversion gain in reduced-size pixels.
2Length of moving object
If pixel size is reduced below 1 um, then device size is decreased, but dynamic range becomes narrower
Solution Approach 1:
By separating the photo detecting area from the charge storing area, the patent enables independent optimization of charge generation efficiency and charge storage capacity. The charge storing area can be sized to provide adequate full well capacity even when the overall pixel dimension is reduced below 1 um, thereby maintaining dynamic range.
Solution Approach 2:
The patent adjusts key parameters including the depth and lateral dimensions of the charge storing area, the doping concentration in the substrate, and the spacing between source and drain regions. These parameter changes enable the pixel to maintain adequate charge storage capacity and dynamic range while achieving sub-1 um pixel size.
3Length of moving object
If pixel size is reduced below 1 um, then device size is decreased, but Signal to Noise Ratio drops to 10-20:1
Solution Approach 1:
The patent extracts the charge storage function from the photo detecting area and places it in a separate charge storing area. This extraction reduces the capacitance of the photo detecting area, thereby increasing conversion gain and improving signal strength. The separated charge storing area can be optimized for minimal noise while maintaining storage capacity, leading to improved signal-to-noise ratio in small pixels.
Solution Approach 2:
Different regions of the pixel are given different properties: the photo detecting area is optimized for charge generation with minimal capacitance, the charge storing area is optimized for noise-free charge accumulation with appropriate depth and doping, and the readout circuit area is optimized for signal processing. This local optimization of properties enables high signal-to-noise ratio despite reduced pixel size.
4Quantity of substance
If photo detecting area capacitance is increased, then charge storage capacity is improved, but conversion gain decreases
Solution Approach 1:
The patent divides the pixel into a photo detecting area with low capacitance for high conversion gain and a separate charge storing area with sufficient capacity for charge accumulation. The low-capacitance photo detecting area converts photons to voltage efficiently, while the charge storing area provides the necessary charge storage capacity, thereby resolving the contradiction between capacitance and conversion gain.
Solution Approach 2:
The charge storing area acts as an intermediary between the photo detecting area and the readout circuit. It receives charges from the photo detecting area, stores them with minimal noise, and transfers them to the readout circuit. This intermediary structure allows the photo detecting area to maintain low capacitance for high conversion gain while the system as a whole achieves adequate charge storage capacity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration allows for high sensitivity image sensing with increased conversion gain and the ability to detect light with a smaller pixel size, overcoming the limitations of conventional sensors by enhancing the efficiency of charge-to-voltage conversion.
Implementation Method 1
a pixel including a single electron field effect transistor (SEFET) which detects light and outputs a quantity of the detected light
Data Source
AI summary
A high sensitivity image sensor including a pixel, the pixel including a single electron field effect transistor (SEFET), the SEFET including a first conductive type well in a second conductive type substrate, second conductive type source and drain regions in the well and a first conductive type gate region in the well between the source and the drain regions.