Cryogenic characterisation of 55 nm SONOS charge-trapping memory in AC and DC modes

The electrical responses of 55 nm silicon-oxide-nitride-oxide-silicon (SONOS) memory cells have been investigated under cryogenic conditions, and the changes of the read curves of SONOS in AC mode (programmed/erased with pulse voltage) and DC mode (programmed/erased with direct voltage sweeping) at low temperatures are compared. The experimental results show that with the decrease of temperature, the subthreshold swing of SONOS decreases, whereas the on-state current of SONOS increases. The difference in AC and DC operations causes the threshold voltage of the read curve to drift accordingly, leading to the different change in the memory window. However, in both modes of operations, the efficiencies of programming and erasing decrease at cryogenic temperatures. It is analysed that the reduction of programming efficiency at cryogenic temperatures is caused by the decrease in the quantity of pre-tunnelling electrons. The reduction in erasing efficiency is attributable to the suppression of the Poole-Frenkel effect at low temperatures, which makes it more difficult for electrons to be de-trapped.