Determination of charge density and charge centroid location in electrets with semiconducting substrates

A method for determining the amount and the mean spatial depth of charge in electret-semiconductor systems is presented. It consists of two nondestructive measurements, from which the internal and external electric fields can be calculated independently. First, the surface potential at the electret-air interface is measured by an electrostatic voltmeter. Second, using a contacting front electrode, a metal-insulator-semiconductor structure is formed, on which capacitance or conductance voltage measurements are carried out. With these results, one obtains the surface potential at the electret-semiconductor interface. The charge centroid and the charge density can now be calculated. For SiO₂-silicon structures, where the 1-μm-thick thermally wet-grown SiO₂ layer acts as an electret, it was found that the charge centroid can be influenced strongly by temperature treatment prior to charging