Effect of Traps on Acoustoelectric Current Saturation in CdS

Abstract

Because of acoustoelectric interaction, the current-voltage ($I-V$) curve in the piezoelectric semiconductor CdS exhibits current saturation when the trap-controlled drift velocity equals the velocity of sound, and an internal acoustic flux is generated. By simultaneous measurement of the trap-controlled drift velocity from the $I-V$ curve and of the Hall drift velocity over the temperature range 300 to 15°K, the effect of the traps in slowing down the velocity and frequency response of the electron-charge stream with respect to the internal acoustic flux is demonstrated. Comparison with a dynamical theory of trap occupation and space-charge bunching derived from the acoustic amplifier shows that the normal ionized donor states are the trapping agents, and that the effective angular frequency of the internal flux is ${10}^8$ to ${10}^9$ ${\sec }^{-1\mathrm{}}$. At very low temperatures, impact ionization of the donors is observed. The resulting enhanced current still flows at the sound velocity.