Floating-Gate ISFET Array Layout for Linear pH Sensing
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Solution Overview
Problem
Conventional ISFETs face challenges in achieving high signal linearity and large array sizes due to the body effect, which compromises pH measurement accuracy and scalability, particularly in nucleic acid applications.
Innovation Solution
The design incorporates a floating gate structure with a passivation layer sensitive to hydrogen ions, coupled with a Kelvin bridge configuration and separate n-wells for n-channel MOSFETs, ensuring accurate pH measurements over a wide range and enabling larger arrays.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional ISFET structure is used, then device simplicity is maintained, but pH measurement accuracy and linearity deteriorate due to body effect
Solution Approach 1:
The device is segmented into separate n-wells for n-channel MOSFETs, isolating the sensing element from the transistor body to eliminate the body effect. This segmentation allows independent optimization of the ISFET sensing region while maintaining transistor functionality elsewhere in the device.
Solution Approach 2:
A floating gate structure is introduced as an intermediary element between the sensing region and the transistor body. The floating gate acts as a mediator that transfers the pH-sensitive signal while isolating the transistor from body effect interference, enabling accurate measurements without direct coupling.
2Productivity
If ISFET array size is increased, then productivity and throughput are improved, but measurement precision deteriorates due to body effect and signal linearity issues
Solution Approach 1:
The array is divided into multiple independent pixel elements, each with its own separated n-well structure. This segmentation allows each pixel to maintain accurate pH measurements independently while the overall array provides high throughput through parallel processing of multiple samples simultaneously.
Solution Approach 2:
The device transitions from a planar two-dimensional layout to a three-dimensional structure with vertically stacked n-wells. This dimensional change allows overlapping of sensing regions and transistor regions in the vertical dimension, enabling higher pixel density and array size without compromising measurement precision in the horizontal plane.
3Area of stationary object
If pixel size is reduced for larger arrays, then array capacity increases, but signal sensitivity deteriorates
Solution Approach 1:
The sensing structure utilizes the vertical dimension with stacked n-wells and floating gate layers, allowing the sensing volume to extend in the depth direction. This enables smaller footprint pixels to maintain adequate signal sensitivity by increasing the effective sensing volume vertically rather than horizontally.
Solution Approach 2:
The device employs a composite structure combining multiple semiconductor layers with different properties - the floating gate layer, the n-well regions, and the substrate - to enhance signal generation and detection efficiency in compact pixel areas, maintaining sensitivity despite reduced planar dimensions.
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 approach enhances pH sensitivity and linearity, allowing for precise hydrogen ion concentration measurements across a wide pH range (1-14) while enabling larger ISFET arrays with improved pixel size and functionality.
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
Implementation Method 2
the operation of an ISFET is based on the modulation of charge concentration (and thus channel conductance) caused by a MOS (Metal-Oxide-Semiconductor) capacitance constituted by the polysilicon gate 64, the gate oxide 65 and the region 60 of the n-type well 54
Data Source
Figure 1~2
Figure 2A
Figure 3~4
AI summary
An apparatus may include an array of sensors, a plurality of microwells, a row addressing circuit, a column addressing circuit, and analog-to-digital converters. Each sensor includes a two-transistor circuit with a chemically-sensitive field effect transistor (chemFET) and a row select transistor and each chemFETs has a floating gate structure and a passivation material located over at least a subset of the floating gate structures. The microwells define a plurality of cavities disposed over at least a portion of the floating gate structures and are of a size and shape capable of retaining analyte therein. The column addressing circuit is suitable for supplying output from each sensor in the array of sensors. The analog-to-digital converters are used for each column in the sensor array to convert an analog output of the column addressing circuit to digital form.