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.

Most commercially available microphones use a capacitive method, and their structure and performance are saturated to some extent. However, due to the limitation of the capacitive transduction method, the roll-off at the low frequency is inevitable, and there is a limit in reducing the mechanical thermal noise caused by the squeeze film damping that occurs between the membrane and backplate structure. Proposed FET (field effect transistor) based MEMS microphone detects a change in the source-drain current according to the gate voltage change of the FET induced by vibrating the membrane with the electric field. In the case of a MEMS microphone using the proposed FET, low frequency roll-off according to the energy conversion does not occur, and the size of the backplate can be drastically reduced as compared with the conventional MEMS microphone, thereby further reducing the mechanical thermal noise, leading to the possibility of achieving the higher SNR (signal to noise ratio). In this study, design and fabrication, performance test of the proposed FET based MEMS microphone are conducted. [Work supported by CMTC, UM15304RD3.]

ARTICLE INFO ABSTRACTArticle history:The demand of high cycle fatigue behavior on plate material is increasing because of its various applications. However, the high-cycle fatigue life data of the plate material is very rare compared to the rod material. Thus, in this study, a plate specimen is designed for the ultrasonic fatigue test because it is time efficient as compared to the conventional fatigue test. To apply the ultrasonic fatigue test, the specimen design is required to resonate at 20 kHz. Therefore, the dynamic elastic modulus was determined by measuring the resonance frequency with a piezoelectric element and laser doppler vibrometer (LDV). As a result, the plate specimen is designed and demonstrated using the ultrasonic fatigue testing machine.

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The MEMS microphone, which is widely used for mobile devices, includes electro-mechanical energy conversion as well as acousto-mechanical conversion through the structures such as the back plate with many holes and an extremely flexible membrane with nonlinear characteristics. Hence, it is a time-consuming job to build an appropriate finite element model to predict the dynamic behaviors of the microphone. The equivalent circuit models are frequently used to check the linear behaviors of the MEMS microphone. However, it is sometimes inadequate to predict the nonlinear behaviors due to the nonlinear deformation of the sensing membrane, which usually determine the acoustic overload point. In this study, a nonlinear four-degree-of-freedom model is developed to design the MEMS microphone. Instead of the conventional equivalent circuit model, a model based on mechanical analysis is constructed and the state equations for the model is derived in the form of a set of ordinary differential equations. The responses of the model can be easily predicted in the time and frequency domain both by solving the equations numerically. With the time domain analysis used, the sensitivity can be easily shown as a function of frequency even including the nonlinear dynamic behaviors. [Work supported by DB Kim Jun-ki Cultural Foundation.]