A Modeling and Feasibility Study of a Micro-Machined Microphone Based on a Field-Effect Transistor and an Electret for a Low-Frequency Microphone

Abstract

Miniaturized capacitive microphones often show sensitivity degradation in the low-frequency region due to electrical and acoustical time constants. For low-frequency sound detection, conventional systems use a microphone with a large diaphragm and a large back chamber to increase the time constant. In order to overcome this limitation, an electret gate on a field-effect transistor (ElGoFET) structure was proposed, which is the field-effect transistor (FET) mounted diaphragm faced on electret. The use of the sensing mechanism consisting of the integrated FET and electret enables the direct detection of diaphragm displacement, which leads its acoustic senor application (ElGoFET microphone) and has a strong ability to detect low-frequency sound. We studied a theoretical model and design for low-frequency operation of the ElGoFET microphone prototype. Experimental investigations pertaining to the design, fabrication, and acoustic measurement of the microphone were performed and the results were compared to our analytical predictions. The feasibility of the microphone as a low-frequency micro-electromechanical system (MEMS) microphone, without the need for a direct current bias voltage (which is of particular interest for applications requiring miniaturized components), was demonstrated by the flat-band frequency response in the low-frequency region.

Recent Trends in Structures and Interfaces of MEMS Transducers for Audio Applications: A Review

“…Typical capacitive microphones suffer from decreased performance at low frequencies, due to the reduction in air gap capacitance that results from the microphone's miniaturization. In order to overcome this limitation, the Electret Gate of Field-Effect Transistor (ElGoFET) microphone was introduced [22,75]. The ElGoFET device combines a field-effect transistor (FET), embedded in the diaphragm, and an electret; a displacement of the diaphragm due to the acoustic pressure leads to a change in the separation distance between the FET and the electret, which results in a change in the electric field across the air gap and, therefore, in a change in the FET drain-source current.…”

“…The ElGoFET device combines a field-effect transistor (FET), embedded in the diaphragm, and an electret; a displacement of the diaphragm due to the acoustic pressure leads to a change in the separation distance between the FET and the electret, which results in a change in the electric field across the air gap and, therefore, in a change in the FET drain-source current. As the sensitivity in ElGoFET transduction is dependent on the ratio of capacitive components in the transduction structure, high sensitivity can be achieved also at low frequency, even with a smaller air gap capacitance due to the miniaturization of the microphone [22,75].…”

“…MEMS microphones are widely employed in mobile phones [1][2][3][4][5][6][7][8][9][10][11] and wearable devices [1,2,5,6,12,13] to capture high-quality audio for calls and recordings, whereas in the automotive field [10,[14][15][16], they are used for hands-free calling, voice control, or even pedestrian detection [17]. They are also exploited for medical applications [13,14,18] such as smart stethoscopes [19,20], blood pressure monitoring, or to detect abnormal heartbeats [21]; in addition, these devices are a full-fledged part of the Internet of Things (IoT) world [2,5,18,22]. Currently, one of the applications of major interest in the consumer market is Voice Activity Detection (VAD), which exploits voice as a vector for human/machine interface; indeed, MEMS microphones are employed within smart voice assistants [7,10,23,24] such as Amazon Alexa and Google Home that operate according to the user's voice commands, are embedded in the remote controls of smart TVs, and could even be installed in the rooms of smart homes [5].…”

Highly Sensitive Low-Frequency-Detectable Acoustic Sensor Using a Piezoresistive Cantilever for Health Monitoring Applications

“…In non-resonant photoacoustic-based gas sensors 21,34 , because the photoacoustic pressure increases as the modulation frequency of the incident light decreases 35 , improving the SNR of sound detection at low frequencies results in a high SNR of these sensors. Unfortunately, for frequencies below 20 Hz, the SNR of conventional MEMS-based microphones decreases significantly as the sound frequency decreases [36][37][38][39] .…”

Highly sensitive low-frequency-detectable acoustic sensor using a piezoresistive cantilever for health monitoring applications

“…of these sensors. Unfortunately, for frequencies below 20 Hz, the SNR of conventional MEMS-based microphones decreases significantly as the sound frequency decreases [36][37][38][39] .…”