Abstract: In this paper, a method for determining the mean charge depth and equivalent surface charge density in an electret-semiconductor system is presented. It consists of two nondestructive measurements. Firstly, the surface potential at the electret-air interface is measured by the compensation method. Secondly, using a contacting front electrode, a metal insulator semiconductor structure (MIS) is formed, on which C-V measurement is carried out. The mean charge depth and the charge density can be calculated. The method was applied to the electret double layers of Si₃N₄/SiO₂ based on silicon substrate. It was found that the annealing temperature influences the mean charge depth markedly. The mean charge depth of the sample positively charged at room temperature shifted to the interface of Si₃N₄ and SiO₂ from the free surface after annealed at 400℃ for 20 minutes.

Key words: electret, film, charge density, mean charge depth