Polysilicon-oxide-nitride-oxide-silicon nonvolatile memory devices are characterized by a high concentration of traps in the silicon nitride layer with a well-defined trapping distance, corresponding to the tunnel oxide thickness. A "breakpoint" is observed at a particular frequency with an inverse equivalent to the trap-to-trap tunneling time constant by variable frequency charge pumping technique. from which the tunnel oxide thickness can be decided. By examining the charge transport and trapping properties of nitride films utilizing charge separation technique. an experimental method to extract the silicon nitride storage layer thickness has been developed. Excellent agreement is obtained between the ellipsometry results and extracted values. (C) 1001 Elsevier Science Ltd. All rights reserved.