1. A signal amplifier comprising: a piezoelectric substrate propagative of acoustic surface waves; an input transducer on said substrate responsive to an input signal for launching acoustic surface waves along a predetermined path at a given velocity; an output transducer on said substrate responsive to said acoustic surface waves for developing an output signal; a film of semi-conductive material on said substrate between said input and output transducers responsive to an applied unidirectional potential for conducting charge carriers alongside said path; means for applying a unidirectional potential to said film with a magnitude adjusted to effect movement of said charge carriers at a velocity slightly greater than said given velocity to achieve amplification of said acoustic surface waves; and control means for applying a unidirectional field transversely through said film and said substrate for controlling the density of said charge carriers in said semiconductive material and the amplification of said acoustic surface waves.

7. In a signal transmission device in which acoustic surface waves propagate along a piezoelectric substrate and co-act with charge carriers traveling in a semi-conducting medium for the amplification of signal energy, the improvement comprising: a unidirectional potential coupled transversely of said medium for establishing a unidirectional electric field extending through said medium in a direction transverse to the direction of travel of said charge carriers to decrease the density of said charge carriers.