Effect of Traps on Acoustoelectric Current Saturation in CdS

Abstract

Because of acoustoelectric interaction, the current-voltage ($I\ensuremath{-}V$) 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 $I\ensuremath{-}V$ curve and of the Hall drift velocity over the temperature range 300 to 15\ifmmode^\circ\else\textdegree\fi{}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 ${10}^{9}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. At very low temperatures, impact ionization of the donors is observed. The resulting enhanced current still flows at the sound velocity.