NAND flash memory technology utilizing fringing electric field

In this review article, basic properties of NAND flash memory cell strings which consist of cells with virtual source/drain (S/D) (or without S/D) were discussed. The virtual S/D concept has advantages of better scalability, less cell fluctuation due to effectively longer channel length at the same technology node, and less program disturbance. The fringing electric field from the control-gate and/or the floating-gate is essential to induce the virtual S/D (charges) in the space region of the body between control-gates and becomes effective as cell size shrinks. A cell string consisting of planar channel silicon-oxide-nitride-oxide-silicon (SONOS) cells formed in bulk Si substrate needs to have a bit-line body doping of ∼5 × 1017 cm−3 in the channel and a less doping in the space region to keep high bit-line read current. The floating gate (FG) flash memory cell string gives larger bit-line current compared to that of SONOS flash memory cell string at given similar body doping. Non-planar channel cells like arch and fin-type body structures were more effective to focus the fringing electric field on the space region. The virtual S/D concept is also useful in 3-dimensional (3-D) stacked NAND flash memory where thin film (or nanowire, nanotube) body is adopted.