Depletion Layer High-Frequency Transducers

Abstract

The depletion layer transducer is a new ultrasonic transducer made from piezoelectric semiconductors, such as gallium arsenide, and is best suited for use at microwave frequencies. The electromechanically active region is a thin, flat, high-resistance depletion layer such as a p-n junction or the surface barrier at a metal semiconductor contact. Since the transducer is a thin piezoelectric layer, its behavior is more like that of a flat piezoelectric transducer plate used at low frequencies than other microwave transducers. Advantages expected when the thickness is comparable to a half wavelength of sound are large electromechanical coupling and large bandwidth. The major difficulty is the low impedance due to the extreme thinness. It is also possible to change the thickness, hence the resonant frequency, by a dc bias. The first models utilized metal semiconductor contacts to form the depletion layers. Frequencies above 1000 Mc were generated and detected, and bandwidths of 5% were observed. The insertion loss was much higher than theoretically predicted. Current efforts are directed toward transducers formed by p-n junctions. It is anticipated that improvements in fabrication techniques and circuitry will extend the frequency range and lower the insertion loss.