A micromachined low frequency microphone based on a field effect transistor and an electret

Recently, several Internet of Things (IoT) devices using low-frequency acoustic sound have emerged as promising sensor applications. Unfortunately, the detection of low-frequency sound using miniaturized microphones is restricted due to the low cut-off frequency of capacitive type transduction. To overcome this limitation, a micromachined microphone based on a field-effect transistor (FET) and an electret was reported and its feasibility as a low-frequency microphone was demonstrated in 2015. However, the proposed microphone was realized by bonding two chips mechanically and the FET was embedded in a membrane, which was disadvantageous for sensitivity enhancement. To realize stable highly sensitive modulation, we devised and fabricated a structure in which the electric field due to an electret embedded in the membrane modulates the channel of the FET. The acoustic signal causes the electret mounted on the membrane to vibrate, which changes the distance between the channel of the FET and the electret. The resulting change in the electric field modulates the conductivity of the channel. The use of an electret embedded on the membrane makes it possible to detect the displacement of the membrane directly and enhances the sensitivity of the microphone. Its feasibility as a low-frequency microphone will be examined experimentally.