A micromachined microphone based on the electret membrane and field-effect-transistor mechano-electrical transduction

Abstract

Most commercially available MEMS (Micro-Electro-Mechanical-System) 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 membrane and backplate structure. Proposed electret membrane and 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 fixed charges. 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) than 65 dBA. Conventional metal-oxide-semiconductor fabrication process and micromachining process were used. In this study, design, fabrication and performance test of the proposed FET based MEMS microphone are conducted. [Work supported by CMTC, UM15304RD3.]