mc66 wrote on Jan 13th, 2007, 3:22am:it's because halo implant,which is use to prevent punch through
when L decrease,and before SCE take effect,the halo implant will make the effect doping of sub rise,so vt rise
Perhaps I can clarify...and see if I understand...
There is a p-type halo implant around the s/d. This high p-doping decreases the width of the depletion region, thus making it harder for the depletion regions of the s/d to touch, which is important when the channel is short.
For long-channel devices, the halo implant is a pretty insignificant portion of the p-type substrate doping. As the channel shrinks, more of the channel consists of this high p implant, and thus the threshold voltage increases (the charge concentration is higher).
Makes sense to me.