Pixel Circuit Threshold Voltage Tracking Booster

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Solution Overview

Problem

Conventional circuits for providing control signals to NMOS transistors in pixel circuits face challenges in accurately determining the threshold voltage, leading to unnecessarily high control voltages that can cause hot carrier degradation and reduce the lifetime of image sensors, especially when the supply voltage exceeds 2.8V.

Innovation Solution

The development of a threshold voltage tracking booster circuit that accounts for the body effect and specific doping implants of individual transistors, using a modified pixel layout to mimic the body effect of each transistor and generate accurate gate control voltages by averaging and filtering the gate-to-source voltage across multiple transistors.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If control voltages are set to supply voltage plus a fixed threshold voltage to ensure transistors are turned on, then transistor switching reliability is improved, but unnecessary high control voltages cause hot carrier degradation and reduce device lifetime

Engineering Contradiction:

Improvetransistor switching reliabilityVSAvoiddevice lifetime

Core Design Contradiction:

ReliabilityVSDuration of action of stationary object

Solution Approach 1:

The patent implements dynamic threshold voltage tracking where the control voltage is continuously adjusted based on the actual threshold voltage of each transistor. Instead of using a fixed conservative threshold, the system dynamically determines the minimum required voltage by measuring the gate-to-source voltage during transistor operation and using this information to generate precise control signals, thereby avoiding excessive voltages while ensuring reliable switching.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent employs feedback mechanisms where the actual threshold voltage of transistors is measured and fed back to a control circuit. This feedback loop allows the system to adapt the control voltage to match the actual transistor characteristics, preventing the application of unnecessarily high voltages that would cause hot carrier degradation while maintaining sufficient voltage for reliable transistor operation.

Inventive Principle:

Principle #23Feedback

2Reliability

If a conservative threshold voltage is used to guarantee sufficient gate control voltage for all transistors, then transistor operation is ensured, but the control voltage exceeds the minimum required and causes hot carrier effects

Engineering Contradiction:

Improvetransistor operation guaranteeVSAvoidhot carrier degradation

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The patent replaces the mechanical approach of using a fixed conservative voltage margin with an electronic measurement and calculation system. By measuring the actual gate-to-source voltage during transistor operation and using this information to calculate the precise control voltage needed, the system eliminates the need for excessive voltage margins while ensuring reliable transistor operation.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent changes the parameter approach from using a fixed conservative threshold voltage to dynamically adjusting the control voltage based on actual transistor characteristics. The system measures the actual threshold voltage and adjusts the control voltage accordingly, changing the operating parameters to match the specific transistor properties and avoiding unnecessary high voltages.

Inventive Principle:

Principle #35Parameter changes

3Device complexity

If the threshold voltage is determined without accounting for body effect and individual transistor characteristics, then the control circuit is simpler, but the threshold voltage determination is inaccurate leading to suboptimal control voltages

Engineering Contradiction:

Improvecontrol circuit complexityVSAvoidthreshold voltage determination accuracy

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent implements self-service by having the transistor circuit itself provide the measurement information needed for threshold voltage determination. By measuring the gate-to-source voltage during normal transistor operation and using this self-provided data to calculate the threshold voltage, the system eliminates the need for separate complex measurement circuits while achieving accurate threshold voltage determination.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The patent makes the transistor circuit multi-functional by using the same circuit elements for both their primary function (signal processing) and the secondary function of providing measurement data for threshold voltage determination. The gate-to-source voltage measurement during normal operation serves dual purposes: ensuring proper transistor operation and providing data for accurate threshold voltage calculation.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Data Source

PatentUS8026968B2Method and apparatus providing dynamic boosted control signal for a pixel

Publication Date: 2011.09.27 APTINA IMAGING CORP

AI summary

A method and apparatus that generates boosted control signals for a transistor in a target pixel circuit. At least one modified pixel circuit is provided with a transistor layout that approximates a layout of the target pixel circuit. In the modified pixel circuit, one transistor that corresponds to a transistor in the target pixel that is to be controlled provides a voltage used to generate a control signal for the corresponding transistor in the target pixel.