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Semiconductor and solid-state devices have been used extensively to measure and generate stresses and stress waves in solids, liquids, and gases. Semiconductor devices have been used as strain gauges to measure strains and to respond to acoustic pressures, p-type silicon has a gauge factor of over 100 times that of a metallic strain gauge. By using the amplifying regimes of Esaki diodes and transistors, a great increase in sensitivity is obtained and microphones have been constructed that are more sensitive than the carbon microphone. With the advent of monolithic transistor on single chips and films, it appears likely that the most stable and economical devices are obtained by separating the pickup and amplifying functions. p-n junctions have a region (the depletion region) where electrons are excluded and if the semiconductor is piezoelectric, very high-frequency transducers for generating mechanical waves in solids can be produced. Evaporation techniques can also be used to produce frequencies from lower ranges to very high-frequency ranges. With the advent of more highly coupled piezoelectric crystals and ceramics, this technique may be extended further. Solid-state transducers have also been constructed using the Hall effect and the magnetoresistance effect. These have been applied in measuring static and dynamic fluxes, as variable resistors without sliding contacts and as displacement meters and microphones.