Characterization of Low-Pressure Chemical Vapor Deposited Polycrystalline Silicon Thin-Film Transistors by Low-Frequency Noise Measurements

Low-frequency noise measurements are used to characterize low-pressure chemical vapor deposited polycrystalline silicon thin-film transistors (polysilicon TFTs). The drain current spectral density shows a 1/ f behavior at low frequencies ( f <100 Hz), which is attributed to deep levels with uniform energy distribution, and a 1/ f γ behavior at high frequencies, which is due to the presence of exponential band tails. The origin of noise is ascribed to mobility and/or carrier number fluctuations depending on the polysilicon deposition pressure and the drain current range. For the analysis of noise data, a theoretical model is proposed. This model is based on an existing one for monocrystalline silicon metal-oxide-semiconductor field-effect transistors (MOSFET) modified by relating the carrier number fluctuations with dynamic trapping and detrapping at the interface grain boundary traps. From the analysis of experimental data, the deep level density and the exponential band tails are determined in polysilicon films deposited at various pressures.