An acoustoelectric transistor

A new kind of three-terminal amplifying device that works on the principle of nonlinear acoustic wave amplification is described. The device has an advantage of simple transistor-like operation while maintaining the long delay times of an acoustic wave amplifier. A piezoelectric semiconductor with three ohmic contacts forms the basic unit; transducers are not required. An applied voltage to the "emitter" contacts results in the generation of acoustoelectric flux which is then injected into the "collector" region. The collector current is controlled by the injected flux, through the acoustoelectric effect. Large current gains can be obtained by adjusting the dimensions and resistivities of the emitter and collector regions. The time delay between input and output signals is determined by the sound velocity and device dimensions. It is possible to separate the emitter and collector by a centimeter or more, thus making possible microseconds of delay. There is gain at frequencies from dc to an upper cutoff fcwhich depends on the electromechanical coupling constant k²and the small-signal frequency of maximum acoustic gain . When there is strong acoustoelectric interaction (large K²) fccan be in the microwave region. A theory of device operation is described along with analytical expressions for the dc and r-f gain, and cutoff frequency, Experimental results are given for a test device made of epitaxial GaAs and operated at 77°K. Since GaAs has a small coupling constant, fcwas only 3 MHz; however a net current gain of 10 was obtained, and all key points of the analytical model were confirmed. In theory, room-temperature microwave performance can be obtained with strong acoustic materials such as ZnO or CdS, or with combinations of materials such as LiNbO3and Si. More experimental work must be done before we can say whether or not this type of acoustoelectric amplifier will be a practical high-frequency device.