Piezoelectric, piezoresistive and capacitive are the common micro microphone technologies. Piezoelectric microphone uses piezoelectric material to output current or voltage, converts sound signal into electrical signal, and outputs it after amplification. Piezoresistive microphone uses piezoresistive material to change resistance characteristics after being stressed. Generally, the above two microphones have low sensitivity to sound pressure and high system noise. Due to the excellent characteristics of high sensitivity and low power consumption, capacitive micro microphone is the mainstream of current market development.
　　The structure of capacitive microphone mainly uses two conductive plates and the insulating air layer between the two plates to form a basic capacitance structure. The two conductive plates are usually called "membrane" and "backplate" respectively. The ideal diaphragm is a very soft elastic membrane, which will vibrate under the action of sound pressure, resulting in a small distance change, resulting in a dynamic micro displacement between the diaphragm and the backplane, so that the capacitance value of the structure will also change.
　　Structure principle of capacitive microphone
　　The sensor chip structure of MEMS microphone is usually composed of a thin and low stress polycrystalline silicon or silicon nitride to form a diaphragm, and a thick polycrystalline silicon or metal layer to form a back plate with porous structure to form a group of micro electric containers with air as the dielectric layer. In addition to the necessary MEMS sensors, there is usually another circuit chip in the package of MEMS microphone to provide the stable bias voltage required for the normal operation of the MEMS chip, and the signal is amplified and then output, which is generally called application specific IC (ASIC).
　　Structure diagram of MEMS microphone sensing chip
　　The ASIC used in MEMS microphones can be divided into analog type and digital type due to different application categories. The basic architecture of analog ASIC is composed of three functional blocks: charge pump, voltage regulator and amplifier.
　　The purpose of the voltage doubling circuit is to boost the input power supply to provide a higher operating voltage for MEMS chips. The function of the amplifier circuit is to amplify and stabilize the input signal. The function of the voltage stabilizer is to provide voltage stabilization at the input of ASIC power supply, so that all circuit blocks in the chip can operate normally. In addition to the above three basic functional blocks, digital ASIC also adds the so-called "sigma delta modulator" circuit to take charge of signal sampling and noise suppression.