Micrometer and submicrometer dimension Si MOSFET's have been studied at liquid nitrogen temperature. The emphasis of the study has been on the changes in the minimum channel length required for long-channel behavior Lmindue to cooling. It is found that there is a reduction in Lminwhich is quite considerable in MOSFET's with low-channel doping. We have shown that this effect is due to a shorter lateral depletion width, and therefore longer effective channel length at low temperatures. A drastic decrease in punchthrough current has also been observed.