Modeling and Feasibility Study of a Micro-Machined Microphone Based on the Piezoresistive Effect of a Field-Effect Transistor

In this study, we propose a novel piezoresistive micro-electromechanical systems (MEMS) microphone that exploits the piezoresistive effect of a field-effect transistor (PrFET microphone). This study is the first attempt to demonstrate that the piezoresistive effect of FET channels can be applied as a microphone sensing method. The PrFET microphone features a single diaphragm with an FET embedded in its support structure for stress concentration. It is, therefore, relatively free from stiction and particle issues, and the entire process is compatible with complementary metal-oxide semiconductors (CMOS). Unlike other types of microphone in which the sensitivity is fixed by the bias voltage, the sensitivity of the PrFET microphone can be adjusted directly through the gate of the FET. To confirm the adjustable sensitivity, we derived a sensitivity model for the PrFET microphone. Then, CMOS and MEMS processes were used to fabricate the microphone chip. Finally, the voltage output was obtained using a PrFET with a trans-impedance amplifier. The measured sensitivity ranged from –75.82 to –63.79 dB depending on the gate voltage, which was within about 1.1 dB of the calculated results. The PrFET microphone exhibited higher sensitivity compared to conventional piezoresistive microphones, and we anticipate that further improvements in performance can be achieved through optimization and signal-processing techniques.