Theory of electro-acoustic effects in hexagonal piezoelectric semiconductors

Abstract

A theoretical model is presented to describe electro-acoustic effects in single crystalline materials. The calculations, which are essentially linear, are based on the assumption that the electro-acoustic effects can be described by the trapping of bunches of free charge carriers in two types of potential troughs. One type of trough is associated with acoustic waves, which are amplified from the thermal background, travelling in the direction of the drifting carriers; the second type is associated with waves with large amplitudes travelling in the opposite direction. It is assumed that the two types of troughs are independent, and that they are created and annihilated at random throughout the crystal. Expressions are derived for the IV-characteristic, the current noise, the ac impedance and the wave attenuation coefficients. In these calculations the anisotropy of the crystal is taken into account. When space charge, diffusion and the displacement current are neglected, the calculated noise spectra consist of two Lorentzians. The ac impedance, which is calculated without making these approximations, shows two low-frequency roll-offs and resonances that are related to the transit time of potential troughs. Finally some remarks on dispersion effects are made.