The method of erasing is the main differentiating factor between the various classes of reprogrammable nonvolatile memories. UV light renders oxide slightly conductive. Limited number of programming cycles - about 1000. Very dense - single transistor functions as both the programming and access device. Very thin oxide allows electrons to flow to and from the gate via Fowler-Nordheim tunneling.
Erasure is achieved by reversing the voltage applied during writing.
Erasure performed using Fowler-Nordheim tunneling. Monitoring control hardware checks the value of the threshold during erasure - making sure the unprogrammed transistor remains an enhancement device. Programming performed by applying 12V to gate and drain. See Weste and Eshraghian for a configuration that eliminates precharging but requires transistor ratioing and a sense amplifier. Also, row and column decoder layouts and options discussed.