 Charge trap (CT) memory was introduced with MNOS devices in the late 1960s.It had a device structure and operating principles similar to floating-gate (FG) memory, but the main difference is that the charges are stored in a conducting material (typically a doped polysilicon layer) in FG memory, whereas CT memory stored charges in localized traps within a dielectric layer (typically made of silicon nitride).SONOS was first conceptualized in the 1960s.