Simple theory of acoustic amplification

Carriers moving in a semiconductor can impart gain or loss to an acoustic wave traveling through piezoelectric material. A physical picture of this familliar interaction process is given that applies to piezoelectric semiconductors as well as to layered structures in which acoustic wave and movmg earners are phystcally separated. In any of these systems, an acoustic wave is accompanied by a space charge wave, a sinusoidal pattern of electric charges. Such a charge pattern has a relaxation time that depends strongly on system geometry. It is shown that once the relaxation time is known, gain or loss as a function of frequency, carrier drift velocity, and electromechanical coupling factor can be found by a stmple method based on physical intuition.