Temperature Dependence of Performance and Hot-Carrier Immunity in Low-Temperature Polycrystalline Silicon Thin-Film Transistors

The temperature dependence of the performance and hot-carrier instability of excimer-laser-crystallized low-temperature polycrystalline silicon thin-film transistors (TFTs) is described. The dependence of the TFT's performance on temperature indicates three characteristics: 1) electron mobility decreases with decreasing temperature, whereas the hole mobility is temperature independent; 2) off-leakage current decreases with decreasing temperature; and 3) threshold voltage increases with decreasing temperature. To consider the origin of the temperature dependence of the threshold voltage, estimation using the trap state density obtained by capacitance–voltage analysis is carried out. The main origin is concluded to be the change in the density of occupied defect states owing to the temperature dependence of the Fermi level. Contrary to the mobility, current under the drain-avalanche hot-carrier stress is found to be higher in a cold environment. The more serious current degradation after the stress in a cold environment is attributable to the reduced scattering of hot carriers owing to less lattice vibration.