ISFET Bridge Circuit Body Effect Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing ion-sensitive field effect transistors (ISFETs) face challenges with temperature dependence, time drift, and body effect issues, which limit their precision in measuring ion concentrations, particularly when integrated with read-out interfaces in standard CMOS technology, requiring complex hardware and calculations for compensation.
Innovation Solution
A bridge-type floating source read-out circuit with a current mirror and differential amplifying circuit is used, featuring an ISFET with a grounded reference electrode and a third transistor connected to the same p-type base as the ISFET, allowing operation under constant current and voltage conditions, thereby reducing body effects and compensating for temperature and time drift without additional hardware or calculations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If ISFET is integrated with read-out interface in standard CMOS technology, then device integration and miniaturization are improved, but body effect influences measurement precision
Solution Approach 1:
The invention divides the ISFET structure into separate components: the sensing element and the read-out circuit are physically separated, with the ISFET placed in a well region isolated from the substrate. This segmentation eliminates the body effect caused by substrate potential while maintaining integration benefits, as the read-out circuit can still be implemented in CMOS technology without directly coupling to the ISFET substrate.
Solution Approach 2:
The invention introduces a well region as an intermediary structure between the ISFET and the substrate. This well acts as a buffer that isolates the ISFET from substrate potential variations, preventing body effect while allowing the integrated circuit architecture to function. The well region mediates the interaction between the sensing element and the substrate, enabling both integration and measurement precision.
2Measurement precision
If complex compensating circuits are added to eliminate body effect, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The invention extracts the body effect problem from the read-out circuit design by physically separating the ISFET from the substrate through a well structure. Instead of using complex compensating circuits to counteract body effect, the solution removes the source of body effect (substrate coupling) entirely, simplifying the overall device architecture while maintaining measurement precision.
Solution Approach 2:
The invention converts the potential harm of substrate coupling (body effect) into a benefit by using the well structure to create an isolated sensing region. The well acts as a protective barrier that prevents substrate potential from affecting the ISFET, thereby eliminating the need for complex compensation circuits while actually improving the stability and precision of measurements.
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
The solution enables precise ion concentration measurements with reduced body effects, temperature, and time drift, improving the accuracy of ISFET sensors without the need for complex hardware or redundant calculations, making it suitable for integration in standard CMOS technology.
Implementation Method 1
Ion-sensitive field effect transistor (ISFET), a kind of micro-sensing device invented by Bergveld in 1970 and developed quickly thereafter, is a solid state element consisting of a chemical sensing film and a field effect transistor. The ISFET, capable of selective measurements of concentration (activity) of certain ions in the electrolyte, is an impedance converting element
Implementation Method 2
a current mirror is coupled with the bridge-type floating source circuit and consists of a first transistor and a second transistor, for providing a current to the bridge-type circuit
Implementation Method 3
a differential amplifying circuit, wherein one input terminal of the amplifying circuit is an input for a reference voltage, and the other input terminal is coupled with the output of the bridge-type circuit, to generate a differential voltage provided to the input of the bridge-type circuit
Data Source
AI summary
An electronic circuit for ion sensor with the body effect reduction includes a bridge-type floating source circuit provided with an input terminal, an output terminal reflecting the change in the potential dependent on ion concentration, and an ion-sensitive field effect transistor (ISFET) wherein one terminal of the ISFET is coupled with the output terminal; a current mirror for providing a current to the bridge-type circuit; a third transistor for receiving the operating current provided by the current mirror, identical to the current provided to the ISFET; a differential amplifying circuit, wherein one input terminal of the amplifying circuit is input with a reference voltage, and the other input terminal is coupled with the output of the bridge-type readout circuit; and a third amplifier to generate a differential output voltage compensated for the body effect, temperature and time drift effects.