Retention performance of the ultrafast nonvolatile memory. (a) Transfer characteristic curves of the flash memory at different time intervals. The device was reset to state 1 and programmed to state 0 by a 22 V pulse with a 50 ns pulse width. (b) Transfer characteristic curves of the flash memory at different time intervals. The device was reset to state 0 and erased to state 1 by a −26 V pulse with a 50 ns pulse width. (c) Time variation of the channel current read at V_gs = 0 V. We expect that the device possesses the capability of a 10-year retention. After 5 months, the retention characteristics have been tested again. (d) The multibit storage of the nonvolatile flash memory. By applying a sequence of pulses with a duration of 50 ns and ranging from −18 to −27 V with steps of 1 V, transfer curves corresponding to the 10 states were obtained (black dashed line indicates the initial state-0). (e) Comparison of the operational speed and retention time of the reported memories.