FET Based Sensory Systems for Miniaturized MEMS Microphones
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Solution Overview
Problem
Conventional MEMS microphones face challenges with reduced sensitivity and increased noise due to miniaturization, which limits their frequency response and sensitivity, especially with decreasing air gap capacitance.
Innovation Solution
The development of MEMS based vertical and horizontal moving gate Field Effect Transistor (FET) sensors that utilize a moving gate structure over the channel region, allowing for increased sensitivity and dynamic range by directly coupling mechanical motion with an active electrical device, eliminating the need for a backplate and enhancing signal detection through sub-threshold transistor operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If MEMS microphones are miniaturized to reduce device size, then device dimensions are reduced, but sensitivity decreases and noise increases
Solution Approach 1:
The patent replaces the conventional capacitive transduction mechanism with a FET-based transduction mechanism. Instead of measuring capacitance changes between a backplate and diaphragm, the invention uses a FET where the diaphragm serves as the gate, directly converting mechanical displacement into electrical current through the field effect transistor's current modulation capability. This substitution enables miniaturization while maintaining high sensitivity because the FET's transconductance provides amplification that compensates for the reduced mechanical displacement in smaller devices.
2Volume of moving object
If the air gap capacitance is decreased through miniaturization, then device size is reduced, but sensitivity and dynamic range deteriorate
Solution Approach 1:
The invention replaces the capacitive sensing mechanism with a FET-based mechanism. In conventional capacitive microphones, sensitivity depends on the air gap capacitance which deteriorates with miniaturization. The FET-based approach uses the transistor's current-voltage characteristics where the gate (diaphragm) voltage modulates the channel current. This mechanism is not limited by air gap capacitance and provides inherent signal amplification through the FET's transconductance, maintaining sensitivity even with reduced air gap dimensions.
Solution Approach 2:
The patent exploits the FET's operating parameters, particularly operating the transistor in the subthreshold region where small gate voltage changes produce exponential changes in drain current. This parameter exploitation provides high gain and sensitivity that is independent of the air gap capacitance magnitude, allowing the device to maintain performance with reduced dimensions.
3Device complexity
If conventional capacitive transduction is used, then device structure is simple, but noise performance is poor and sensitivity is limited
Solution Approach 1:
The patent replaces the passive capacitive transduction with an active FET-based transduction system. Conventional capacitive microphones suffer from high noise due to the need for charge amplification and the inherent noise of capacitive sensing. The FET provides low-noise current output directly proportional to the acoustic signal, eliminating the need for complex charge amplification circuits and reducing overall system noise.
Solution Approach 2:
The FET structure inherently provides signal amplification through its transconductance property, eliminating the need for separate amplification stages that would add noise. The device essentially amplifies its own signal through the FET's natural operation, providing low-noise performance without requiring additional active components that would contribute to noise.
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 achieves lower noise levels and higher dynamic range, maintaining sensitivity even with smaller sizes, and allows for the detection of small mechanical motions and acoustic vibrations with improved frequency response.
Implementation Method 1
a movable gate metal-oxide-semiconductor field effect transistor (MOSFET) that directly couples mechanical motion with an active electrical device
Implementation Method 2
The mechanical movement, such as deflection, is then converted into an electric signal through the use of piezo-resistive/electric material
Implementation Method 3
The mechanical movement, such as deflection, is then converted into an electric signal through the use of piezo-resistive/electric material
Implementation Method 4
The mechanical movement, such as deflection, is then converted into an electric signal through changing capacitance between the moving part and a reference plate
Data Source
AI summary
This invention describes the structure and function of an integrated multi-sensing system. Integrated systems described herein may be configured to form a microphone, pressure sensor, gas sensor, multi-axis gyroscope or accelerometer. The sensor uses a variety of different Field Effect Transistor technologies (horizontal, vertical, Si nanowire, CNT, SiC and III-V semiconductors) in conjunction with MEMS based structures such as cantilevers, membranes and proof masses integrated into silicon substrates. It also describes a configurable method for tuning the integrated system to specific resonance frequency using electronic design.