Floating-Gate ChemFET Arrays for Dense pH Sensing
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Solution Overview
Problem
Conventional ISFET arrays for pH measurement have limitations in pixel size, complexity, and linearity due to the body effect, which restricts the number of sensors and increases the size of each pixel, making them unsuitable for large-scale applications like DNA sequencing, and they are prone to performance issues from fabrication and handling processes.
Innovation Solution
The design of a large-scale chemFET array with reduced pixel size and improved signal-to-noise ratio, using CMOS fabrication techniques, where each pixel consists of three transistors and employs a floating gate structure with no electrical connection between the source and body, and incorporates microfluidics for high-speed analyte delivery, along with alternative decoder approaches to facilitate rapid data acquisition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If conventional ISFET arrays are used for pH measurement, then the body effect causes non-linearity and performance issues, but increasing the number of sensors requires larger pixel size and increases device complexity
Solution Approach 1:
The pixel is divided into functional segments: the chemFET sensor element, the floating gate structure for biasing, and separate source/drain regions. This segmentation allows independent optimization of each component, reducing overall pixel complexity while maintaining sensor functionality and enabling higher sensor density.
Solution Approach 2:
The floating gate structure serves multiple functions: it acts as a biasing mechanism for the chemFET, provides a reference potential, and enables the sensor to operate without direct electrical connection between source and body. This multi-functionality reduces the need for additional circuitry, simplifying the pixel design.
2Reliability
If conventional ISFET arrays are used, then fabrication and handling processes cause performance issues, but improving manufacturing precision is difficult
Solution Approach 1:
The floating gate structure converts the potential harm of fabrication variations into a benefit by providing a self-adjusting bias mechanism. The floating gate automatically compensates for manufacturing tolerances in the chemFET, improving performance stability without requiring ultra-precise fabrication.
Solution Approach 2:
The invention changes the electrical parameters of the ISFET by introducing the floating gate structure, which modifies the threshold voltage and operating characteristics. This parameter change makes the sensor less sensitive to fabrication variations and improves reliability.
3Area of moving object
If pixel size is reduced to increase sensor density, then manufacturing precision requirements increase, but conventional processes cannot achieve this
Solution Approach 1:
The floating gate structure changes the electrical parameters of the chemFET, allowing for optimized operating conditions that enable smaller pixel sizes. By adjusting the biasing characteristics, the sensor can maintain performance with reduced physical dimensions, facilitating higher sensor density.
Solution Approach 2:
The pixel employs a composite structure combining the chemFET element with the floating gate structure and integrated source/drain regions. This composite design allows for compact packaging of functional elements, reducing overall pixel area while maintaining all necessary components.
4Measurement precision
If conventional ISFET arrays are used, then the body effect causes non-linearity, but eliminating it requires complex circuitry
Solution Approach 1:
The floating gate structure converts the harmful body effect into a beneficial feature by providing a controlled biasing mechanism. The floating gate automatically adjusts the threshold voltage to compensate for body effect variations, improving linearity without requiring complex external circuitry.
Solution Approach 2:
The floating gate structure provides self-adjusting biasing that automatically compensates for the body effect. The sensor circuit serves itself by using the floating gate to maintain optimal operating conditions, eliminating the need for complex external correction circuitry.
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 design enables a very large-scale, high-density chemFET array capable of rapid, accurate measurements of analytes, particularly for DNA sequencing, with improved pixel density and reduced complexity, while mitigating performance issues from fabrication processes.
Implementation Method 1
an ion-sensitive field effect transistor, often denoted in the relevant literature as ISFET (or pHFET). ISFETs conventionally have been explored, primarily in the academic and research community, to facilitate measurement of the hydrogen ion concentration of a solution (commonly denoted as 'pH')
Implementation Method 2
a floating gate structure with no electrical connection between the source and body
Implementation Method 3
incorporates microfluidics for high-speed analyte delivery
Data Source
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AI summary
Methods and apparatus relating to very large scale FET arrays for analyte measurements. ChemFET (e.g. ISFET) arrays may be fabricated using conventional CMOS processing techniques based on improved FET pixel and array designs that increase measurement sensitivity and accuracy, and at the same time facilitate significantly small pixel sizes and dense arrays. Improved array control techniques provide for rapid data acquisition from large and dense arrays. Such arrays may be employed to detect a presence and/or concentration changes of various analyte types in a wide variety of chemical and/or biological processes. In one example, chemFET arrays facilitate DNA sequencing techniques based on monitoring changes in hydrogen ion concentration (pH), changes in other analyte concentration, and/or binding events associated with chemical processes relating to DNA synthesis.