Drift mobility determinations using surface-potential decay in insulators

Open-circuit dark surface-potential decay characteristics have been measured in positively corona-charged films of solution-cast low-density polyethylene. For initial fields of about 5×105 V/cm, the surface potential V (t) decays linearly for a period of time, after which it is curvilinear. As a result, the time derivative of the surface potential −dV/dt is constant for a period of time, then abruptly decays. This transition is marked by a cusp, and the time it takes this cusp to appear is interpreted as the transit time through the sample of the leading front of carriers, implying a drift mobility of 2×10−10 cm2/V sec. As the initial field is increased, the corresponding −dV/dt decreases and then increases before the cusp appears. Further, the mobility appears to decrease with increasing initial field. The latter two phenomena are interpreted in terms of a mathematical model in which field ionization of centers in the bulk creates trapped space-charge layers, through which the surface layer of carriers then moves. Additional effects due to succesive corona charging and exposure to laboratory air and vacuum are discussed.