CMOS Image Sensor Switch Circuit With Dummy Transistors for Charge Injection
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Solution Overview
Problem
Conventional switch circuits in CMOS image sensors suffer from charge injection noise, which is exacerbated by advancements in semiconductor technology leading to smaller and faster devices, and existing solutions like boosted control signals or dummy transistors can cause device failure or inefficiencies.
Innovation Solution
The implementation of a switch circuit with two dummy transistors connected to different terminals of a first transistor, operating on lower voltage supplies, and a power supply circuit that allows floating supply voltages to reduce charge injection and accommodate variations in circuitry due to IC fabrication, thereby minimizing readout mismatch.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a dummy transistor is connected between NMOS transistor and capacitor to reduce charge injection, then charge injection noise is reduced, but device complexity increases
Solution Approach 1:
The single dummy transistor approach is segmented into two dummy transistors positioned at different terminals of the main transistor. This segmentation allows each dummy transistor to compensate for charge injection at its respective terminal, providing more effective noise reduction while maintaining manageable circuit complexity through modular configuration
Solution Approach 2:
Dummy transistors are placed at specific locations (different terminals) of the main transistor based on local charge injection characteristics. Each terminal receives targeted compensation from its adjacent dummy transistor, creating locally optimized charge balance that effectively reduces noise without requiring complex global circuit modifications
2Object-affected harmful factors
If boosted control voltage is applied to the gate of NMOS transistor to address charge injection, then charge injection is reduced, but device reliability deteriorates due to accelerated failure
Solution Approach 1:
Dummy transistors serve as intermediary elements that mediate the charge injection problem without requiring boosted voltages. These intermediary transistors provide passive charge compensation through their physical presence and standard voltage operation, eliminating the need for aggressive voltage boosting that compromises device reliability
Solution Approach 2:
The approach changes from modifying voltage parameters (boosted control signals) to modifying structural parameters (adding dummy transistors). This parameter transformation allows charge injection mitigation through structural configuration rather than electrical stress, preserving device reliability while achieving the desired noise reduction
3Manufacturing precision
If smaller and faster semiconductor devices are used to improve integration, then manufacturing precision is improved, but sensitivity to charge injection noise increases
Solution Approach 1:
Dummy transistors act as counterweight elements that balance the charge injection effects in smaller, faster devices. By positioning dummy transistors at different terminals, the configuration creates opposing charge effects that cancel out the increased sensitivity inherent in scaled-down devices, allowing high-performance transistors to operate without being overwhelmed by charge injection noise
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 significantly reduces charge injection noise and readout mismatch, enhancing the performance and reliability of CMOS image sensors by allowing operation within a narrower voltage range and balancing charge injection across terminals.
Implementation Method 1
charge injection that takes place as a result of CTRL 130 being de-asserted. Charge injection redistributes charge in the channel of NMOS transistor 110
Data Source
AI summary
A switch circuit including structures to reduce the effects of charge injection. In an embodiment, a first transistor of the switch circuit is to receive a first signal and first and second dummy transistors of the switch circuit are each to receive a second signal, wherein the first transistor is connected between the first and second dummy transistors. The second signal is complementary to the first signal. In another embodiment, the first transistor, the first dummy transistor and the second dummy transistors are each connected via respective body connections to a first low supply voltage.