Analysis of Effective Channel Length in Amorphous Silicon Thin-Film Transistors

This paper analyzes the effective channel length in amorphous silicon thin-film transistors (a-Si TFTs) by examining the parasitic channel length, ΔL, which causes the apparent dependence of the field-effect mobility of TFTs on the nominal channel length. The parasitic channel length is found to depend on the a-Si thickness, La, at the source path and the drain path of TFTs if there is no etching stopper insulator on the a-Si layer. An analytical formula for the parasitic channel length is derived by assuming that the space-charge-limited current is the dominant conduction mechanism through these paths. The validity of the analysis is confirmed by applying the formula to the experimental results on the ΔL-La relationship for a range of La between 0.18 and 0.98 µm.