New work in transducer engineering is directed toward the use of silicon technologies for the fabrication of tiny microphones. For such devices, a number of conventional transduction methods are suitable, for example the condenser, piezoelectric, and piezoresistive principles. Also under study are microphones based on sound-wave induced modulation of the drain current in a field-effect transistor or acoustic sensors, depending on the modulation of light propagation in a micromachined waveguide. Experimental silicon sensors according to all these principles have been built, with some of these already showing good acoustic properties. Most advanced are two-chip condenser microphones with external biasing; electret biasing and one-chip designs made with sacrificial-layer techniques are also under study. Future work will concern the improvement of the characteristics of all silicon microphones and the integration of amplifiers or signal-processing electronics on the microphone chip. Advantages of silicon microphones are small size (membrane areas ~ 1 mm2), low-vibration sensitivity due to small diaphragm thickness (< 1 mm ), possibility of integration of sensor and amplifier on a single chip, and potentially low cost due to batch processing.
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