P+ Doped Polysilicon Transfer Gate for Image Sensor Leakage Control
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Solution Overview
Problem
Short gate lengths in transfer transistors of image sensors lead to leakage current, which is mitigated by increasing enhancement implant, but this degrades image sensor performance by increasing image lag.
Innovation Solution
Implementing a P+ doped polysilicon transfer gate with a higher threshold voltage by adjusting the photoresist mask pattern to allow P+ dopant implantation, reducing leakage current without additional processing complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the transfer gate is scaled to have a shorter gate length, then integration and pixel fill factor are improved, but leakage current from the photosensitive element to the floating node increases
Solution Approach 1:
The patent applies different doping types to different regions: the transfer gate is doped with P+ dopant while the barrier/well region is doped with N+ dopant. This local differentiation creates opposite polarity doping regions that generate compensating effects, reducing the leakage current through the transfer gate while maintaining the short gate length for high integration.
Solution Approach 2:
The patent changes the doping parameter by introducing P+ doped polysilicon for the transfer gate, which has a higher threshold voltage characteristic. This parameter change effectively suppresses the leakage current without requiring increase in gate length, thus resolving the contradiction between short gate length and leakage current reduction.
2Object-generated harmful factors
If the enhancement implant under the transfer gate is increased to reduce leakage current, then leakage current is reduced, but image sensor performance degrades due to increased image lag
Solution Approach 1:
The patent applies opposite polarity doping (P+ in transfer gate, N+ in barrier/well) to create localized regions with compensating electrical effects. This reduces leakage current through the transfer gate without requiring increased enhancement implant, thereby avoiding image lag and maintaining image sensor performance.
Solution Approach 2:
The patent converts the potentially harmful leakage current into a beneficial effect by using opposite polarity doping to create compensating effects. The P+ doped transfer gate and N+ doped barrier/well region work together to reduce leakage current while maintaining proper electrical characteristics, eliminating the need for excessive enhancement implant that would cause image lag.
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
Enables scaling of transfer gates to shorter lengths while minimizing leakage current and maintaining image sensor performance by reducing the enhancement implant, thus improving the barrier/well potential at the photodiode interface.
Implementation Method 1
The transfer gate is formed from P+ doped polysilicon
Data Source
AI summary
An active pixel using a transfer gate that has a polysilicon gate doped with P+ is disclosed. The pixel includes a photosensitive element formed in a semiconductor substrate and an n-type floating node formed in the semiconductor substrate. An n-channel transfer transistor having a transfer gate is formed between the floating node and the photosensitive element. The transfer gate is doped with a p-type dopant.