Transient charge-transport in MNOS memory devices

This paper presents the results of an experimental technique. Its salient features are (1) direct display of the waveform representing the charging of the traps in the insulator during the application of a voltage pulse, (2) simultaneous display of both the decay of the injected charge ΔQ and the instantaneous flat-band voltage immediately after the pulse, (3) determination of the centroid x of ΔQ, and (4) self-consistent determinations of the separate geometric capacitances of the nitride and the thin-tunneling-oxide as they exist in the device. This direct technique has revealed valuable insight into the MNOS charge-transport mechanisms that could not be provided by the C-V plot approach currently used. Direct recordings of the initial decays of ΔQ in thin-tunneling-oxide (∼25Å) devices show that a large fraction of ΔQ is already lost before any meaningful C-V plots can be taken. Only for pulse amplitudes and durations below certain values will coincide with the oxide-nitride interface. Even for some excitations commonly used in MNOS applications, can be as much as 100 Å or more into the nitride from the dielectric interface. A device model is developed to include the transient movement of charge in the nitride. The predictions of the model are found to be in remarkably good agreement with the experimental results.