Investigation of simulated and measured program characteristics in 4-bit/cell charge-trap flash (CTF) memories

We investigate the simulated and measured program characteristics for 4-bit program operations in silicon-oxide-nitride-oxide-silicon (SONOS) devices by using two-dimensional (2-D) device simulations. For this calculation, the width of the region with charge trapped locally in the drain region is assumed to be as narrow as 44 nm to remove second-bit effects during the 2-bit and 4-bit operation. We determine the reverse read voltage for screening bit-1 to be 2.5 V and confirm that both 2-bit and 4-bit characteristics are successfully observed in our devices. From the threshold voltage shift, the densities of trapped charge are estimated to be 0 × 10¹⁹, 3 × 10¹⁹, 6 × 10¹⁹ and 9 × 10¹⁹ cm^-3 at 4-level states, respectively. We also find that these simulation results are reasonably consistent with the experimental results achieved in this work.