Matched ISFET Pixel Sampling for Offset and Mismatch Cancellation
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Solution Overview
Problem
Existing ion-sensitive field effect transistor (ISFET) arrays face challenges with transistor mismatch and offset issues due to fabrication tolerances, which affect signal uniformity and require additional bits for accurate digital conversion, especially in sensors without reset capabilities.
Innovation Solution
The implementation of a matched transistor pair within each pixel, where a chemically-sensitive sensor is paired with a select transistor, allows for delta double sampling techniques to cancel out offset and mismatch signals, reducing the dynamic range requirements of analog-to-digital converters and enhancing signal uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If delta double sampling is implemented to reduce transistor mismatch and offset artifacts, then signal uniformity and conversion accuracy are improved, but device complexity increases due to additional circuit components and processing steps
Solution Approach 1:
The pixel is divided into two separate transistors: a chemically-sensitive sensor transistor and a select transistor. This segmentation allows the select transistor to serve as a dedicated reference element for characterizing and canceling offsets and mismatches in the sensor transistor, thereby improving signal uniformity without requiring complex external calibration circuits
Solution Approach 2:
The select transistor acts as an intermediary reference element that captures the electrical characteristics (offsets and mismatches) of the sensor transistor. By using the select transistor as a mediator, the system can characterize and cancel these artifacts through delta double sampling, improving measurement precision without direct complex intervention
2Measurement precision
If matched transistor pairs are used within each pixel for delta double sampling, then offset and mismatch cancellation is improved, but manufacturing precision requirements increase due to the need for closely matched transistor characteristics
Solution Approach 1:
The sensor transistor and select transistor are merged into a single pixel structure with shared electrical connections and proximity placement. This merging ensures that both transistors experience similar fabrication conditions and environmental factors, improving their matching characteristics while maintaining standard manufacturing tolerances
Solution Approach 2:
The select transistor is positioned locally within the same pixel as the sensor transistor, ensuring local matching of electrical characteristics. This local quality approach allows the transistors to share common process variations and environmental conditions, improving offset cancellation without requiring global manufacturing precision
3Productivity
If dynamic range requirements of analog-to-digital converters are reduced through offset cancellation, then conversion efficiency is improved, but loss of information may occur due to aggressive signal processing
Solution Approach 1:
The offset and mismatch characterization is performed preliminarily through the select transistor before the actual chemical sensing measurement. By capturing the electrical artifacts in advance and canceling them through delta double sampling, the system prepares the signal path to handle only the genuine sensor output, improving conversion efficiency while preserving signal integrity through controlled differencing
Data Source
AI summary
An array of sensors arranged in matched pairs of transistors with an output formed on a first transistor and a sensor formed on the second transistor of the matched pair. The matched pairs are arranged such that the second transistor in the matched pair is read through the output of the first transistor in the matched pair. The first transistor in the matched pair is forced into the saturation (active) region to prevent interference from the second transistor on the output of the first transistor. A sample is taken of the output. The first transistor is then placed into the linear region allowing the sensor formed on the second transistor to be read through the output of the first transistor. A sample is taken from the output of the sensor reading of the second transistor. A difference is formed of the two samples.