Characterization and modeling of edge direct tunneling (EDT) leakage in ultrathin gate oxide MOSFETs

This paper examines the edge direct tunneling (EDT) of electron from n(+) p olysilicon to underlying n-type drain extension in off-state n-channel MOSF ET's having ultrathin gate oxide thicknesses (1.4-2.4 mm), It is found that for thinner oxide thicknesses, electron EDT is more pronounced over the co nventional gate-induced-drain-leakage (GIDL), bulk band-to-band tunneling ( BTBT), and gate-to-substrate tunneling, and as a result, the induced gate a nd drain leakage is better measured per unit gate width. A physical model i s for the first time derived for the oxide field fox at the gate edge by ac counting for electron subband in the quantized accumulation polysilicon sur face. This model relates fox to the gate-to-drain voltage, oxide thickness, and doping concentration of drain extension, Once fox is known, an existin g DT model readily reproduces EDT I-V consistently and the tunneling path s ize extracted falls adequately within the gate-to-drain overlap region. The ultimate oxide thickness limit due to EDT is projected as well.