The coupled modes of acoustic waves and drifting carriers in piezoelectric crystals

The amplification of acoustic waves in piezoelectric semiconductors is analyzed in terms of the normal modes of the uncoupled system. The characteristics of the growing wave are presented for a range of the crystal parameters in terms of the conduction frequency (σ/ε) and the diffusion frequency (v²/D). Boundary conditions are worked out to show that an acoustic wave at the input to the amplifying section will couple into the growing wave without appreciable initial loss. The experimental results with CdS at 500 Mc confirm the general features of the theory and serve to point out two problems which must be overcome. The first is one of eoupling into the acoustic waves from an external source. Present methods are quite inefficient. The second is one of internal oscillations within the crystal which occur when the drift velocity exceeds the velocity of sound. If these problems can be solved, it should be possible to use this gain mechanism well into the microwave region.