ELECTRICAL CONDUCTIONS OF HIGH-MOBILITY POLY-SI THIN-FILM TRANSISTORSAT LOW-TEMPERATURES

N-channel, polycrystalline silicon thin-film transistors (poly-Si TFTs ) with high field-effect mobility of 160 cm2/V.s are fabricated from s putter-deposited Si film irradiated with laser light. Electrical prope rties, including Hall effects, of the poly-Si TFTs are measured at tem peratures from 300 K to 4.2 K. At high gate voltages, the conductiviti es of the poly-Si TFTs increase with decreasing temperature down to 22 0 K, but below 200 K, they decrease with decreasing temperature even a t high gate voltages, which produce low potential barriers at grain bo undaries. At temperatures from 300 K to 100 K, the carriers show activ ation-type conduction. Below about 40 K, the conductivity obeys a vari able-range hopping mechanism between localized states at grain boundar ies. Hall effect measurements show that carrier density is nearly inde pendent of temperature and that carrier velocity plays a more importan t role rather than carrier density in the electrical properties of pol y-Si TFTs over the wide range of temperature.