Acoustic amplification in semiconductors

If the application of semiconductors is to be extended to microwave frequencies, the limitations due to electrode capacitance and transit time must be overcome. To a certain extent this can be done by reducing junction size and using majority carriers where possible. However a muchmore sophisticated way of overcoming these difficulties is to try to realize in semiconductors the type of continuous interaction obtained in travelling-wave tubes. In the paper the present state of the art on acoustic amplification in semiconductors is reviewed. Although net gain at microwave frequencies has not yet been obtained, the experiments at lower frequencies have given some general indications about the feasibility of achieving this. The main problems lie in coupling into the crystal in such a way that the insertion losses are minimized and the shape of the acoustic wavefront within the crystal is not disturbed. The amplifiers described use quartz transducers to couple into and out of a CdS crystal. Recently, deposited CdS transducers have been operated at X-band for this type of application. This suggests that there are reasonable chances of making low loss delay lines and ultimately amplifiers for microwave applications. The importance of the acoustic amplifier as a diagnostic tool leading to a better understanding of other microwave phenomena in semiconductors is discussed briefly and the current instabilities in CdS which show similarities to the Gunn effect are described.