Build-up time of an acousto-electric oscillator

Abstract: The effect of an external alternating voltage on the transient and steady-state characteristics of a CdS acousto-electric oscillator has been studied experimentally. The incubation time of free oscillations as well as the frequency of steady oscillations are significantly changed by the external signal.

Abstract: Results are presented which demonstrate the sensitive dependence of the onset time td for deviations from ohmic current values in thin acoustoelectrically active photoconducting CdS platelets, on the optical absorption of the photoexciting illumination used. Computations, based on a theoretical model of linear round-trip acoustoelectric gain in thin platelets having exponential conductivity profiles parallel to the current flow, give semiquantitive agreement with experimental curves obtained using narrow-band illumination with measured optical absorbance values. A number of experimental and theoretical reasons are provided in explanation for the lack of precise agreement. By the use of very weakly absorbed illumination and carefully prepared specimen contacts, excellent agreement is obtained between the experimental dependence on conductivity of threshold and cut-off voltages for acoustoelectric activity, and the predictions of the linear gain model with drift mobility as a `fitting' parameter. By the assumption of a reasonable value for the non-electronic acoustic loss, a reasonable fit is also obtained between experimental and theoretically predicted values of the voltage for minimum onset time.

Abstract: An ultrasonic wave traveling in certain directions in a piezoelectric semiconductor such as cadmium sulfide can be amplified or attenuated by application of a dc electric field. The direct current flowing through the medium in the presence of an ultrasonic wave creates a traveling ac field which interacts with the ultrasonic wave. Amplification occurs when the drift velocity of the electrons exceeds the velocity of sound. For strongly piezoelectric semiconductors, amplification of as much as several percent per wavelength of path is obtainable. Calculations show that for properly prepared material, significant amplification is expected up to the microwave frequencies. At high frequencies, gain is reduced because electron diffusion smooths out the electron bunching necessary for amplification. The dc power required increases rapidly with frequency, and at frequencies above one or two thousand megacycles only pulsed operation seems feasible. Applications as a tool in ultrasonic studies, and for devices such as delay lines and amplifiers, are also discussed.

Abstract: This letter describes some characteristics of an oscillator based upon the acoustoelectric interaction in piezoelectric semiconductors—the acoustoelectric oscillator. Particular reference is made to frequency modulation and to related-device applications.

Abstract: Observations show that the current through a thin platelet of photoconducting CdS forming an acoustic cavity is modulated at discrete frequencies (in the range 4 – 900 Mc/s), when the electron velocity exceeds the velocity of sound. This modulation is attributed to those thermally excited lattice waves for which there is round trip gain. The results are compared with White's small signal theory for acoustic amplification.

Abstract: A strong lens effect has been experimentally observed when the electric field inside a CdS plate is so adjusted that strong quasi‐standing ultrasonic waves are set up inside the crystal. Physical understanding of the observed phenomena and possible device applications are discussed.