Abnormal threshold voltage shifts in p-channel low temperature polycrystalline silicon TFTs under deep UV irradiation

Abstract

The electrical properties of p-channel low temperature polycrystalline silicon (LTPS) thin film transistors (TFTs) under deep ultraviolet (UV) irradiation were studied. Characteristics including threshold voltage (Vth), hole field effect mobility (μeff) subthreshold swing (SS), and leakage current (Ioff) were investigated as a function of UV irradiation doses from 4 to 20 J/cm2. With an increase in UV irradiation dosage, the Vth shift of the TFT showed a turnaround effect, which first shifted to the positive direction and then shifted back to the negative direction after a UV irradiation dose of 16 J/cm2. The continuous degradation of SS and μeff was discovered with the enhancement of UV irradiation dose. To better understand the physical mechanisms underlying these characteristic changes, UV induced traps at poly-Si grain boundaries, traps at the poly-Si and gate oxide insulator (poly-Si/SiO2) interface, and trapped charges in the gate oxide were studied with a function of UV irradiation doses.