A study was made of various parameters that influence the sensitivity of transistor microphones, namely: the emitter junction depth; the position of the stressing member on the emitter of the transistor; the n-p-n vs the p-n-p transistors; the magnitude of the collector-to-emitter voltage VCE. The highest sensitivity of the microphone is obtained for a transistor which has a shallow emitter (less than 1 micron) and in which the indenter is positioned where the emitter-base junction meets the surface of the transistor, i.e., at the emitter boundary. The p-n-p and n-p-n transistors behave similarly under stress when the stress is applied at the boundary of the emitter. An increase in VCE enhances the sensitivity, but not by a large amount.
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