Acoustoelectric Current Saturation in CdS as a Fluctuation Process

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Abstract
The current through a CdS crystal saturates above a critical electric field at which the electron drift velocity equals the sound velocity, and an internal acoustic flux is generated. At the same time the fluctuations in the current increase enormously. By combining the concept of carrier bunching in the locally generated incoherent acoustic waves with a form of generation‐recombination noise process, a two‐state, two‐parameter phenomenological model has been constructed. The observed noise power and an analysis of the shape of the I‐V curve under various conditions are used to derive the parameters of the theory. Dependence on applied voltage, spectral density, temperature, light level, and crystal length for semiconducting and photoconducting crystals support the general expectations of the model. The parameter defining local acoustic wave growth, hence formation of electron bunches, is related to the linear acoustoelectric gain theory. The parameter defining wave decay, hence destruction of electron bunches, is related to the sound transit time. From the noise data and the two‐state phenomenological theory, the effective angular frequency of the acoustic waves is found to be about 3×109 sec−1 in a typical semiconducting sample.