**broken link removed**
From this site, it said something like that......
"There are other mechanisms that affect the saturated current and the threshold voltage of small devices (L < 0.9 micrometer). Substrate current induced body effect (SCBE) is usually caused by impact ionization, where hot carriers collide with the silicon atoms and generate electron-hole pairs. The holes into the bulk appear as substrate current. This will increase the drain current and reduce the threshold voltage. Threshold voltage reduction also occurs due to drain-induced barrier lowering (DIBL). If the channel is very short, the applied drain bias affects the channel potential. It lowers the gate bias and this will decrease the threshold voltage. There is also a strong dependence of the threshold voltage on the channel length, width, substrate doping, and the drain voltage"