Amplification of Ultrasonic Waves in Piezoelectric Semiconductors

TLDR

Ultrasonic waves in piezoelectric semiconductors such as cadmium sulfide can be amplified or attenuated by a dc electric field. The study explores applications of this effect as a tool for ultrasonic studies and for devices such as delay lines and amplifiers. Direct current flowing through the medium in the presence of an ultrasonic wave generates a traveling ac field that interacts with the wave, and amplification occurs when the electron drift velocity exceeds the sound velocity. Amplification of several percent per wavelength is achievable in strongly piezoelectric semiconductors, with significant gain up to microwave frequencies, but at high frequencies electron diffusion reduces gain and the required dc power rises sharply, making pulsed operation necessary above one or two thousand megacycles.

Abstract

An ultrasonic wave traveling in certain directions in a piezoelectric semiconductor such as cadmium sulfide can be amplified or attenuated by application of a dc electric field. The direct current flowing through the medium in the presence of an ultrasonic wave creates a traveling ac field which interacts with the ultrasonic wave. Amplification occurs when the drift velocity of the electrons exceeds the velocity of sound. For strongly piezoelectric semiconductors, amplification of as much as several percent per wavelength of path is obtainable. Calculations show that for properly prepared material, significant amplification is expected up to the microwave frequencies. At high frequencies, gain is reduced because electron diffusion smooths out the electron bunching necessary for amplification. The dc power required increases rapidly with frequency, and at frequencies above one or two thousand megacycles only pulsed operation seems feasible. Applications as a tool in ultrasonic studies, and for devices such as delay lines and amplifiers, are also discussed.