Spatial Depth and Density of Charge in Electrets

A new method for determining the mean spatial depth and the density of charges trapped in charged dielectrics (electrets) is suggested. The method is based on (i) a measurement of the initial induction charge on a metal plate in proximity to the electret and (ii) a measurement of the displacement current obtained for discharge of the electret between contacting electrodes. The method can be used if charge carriers of positive and negative polarity are trapped in proximity to the two surfaces of the electret, respectively (mean depth of the charges less than about 20% of the foil thickness), if the retrapping time of both types of charge carriers is short compared to their transit time through the dielectric, and if dipole polarization is negligible. The method offers the possibility of determining charge depths in a wide variety of materials for which such data could not be obtained before. In addition, the method should also be useful for obtaining information about the retrapping kinetics in various dielectrics.