Impact of the fringing capacitance at the back of thin-film transistors

Analysis of the fringing capacitance at the “back” surface of the semiconducting film in thin-film transistors (TFTs) indicates that this capacitance contributes to the enhancement of the channel current by two components, ΔID and ΔIL. Because of similarities in the output characteristics, the component ΔID is compared to the channel length modulation in the TFT operating in the saturation regime at high drain bias voltages VD. Similarly, the other component ΔIL is compared to space-charge limited conduction (SCLC) in the leakage current of the TFT at low gate bias. In this work, a derivation method and simple analytical expressions are provided to illustrate the impact of the fringing capacitance on the DC characteristics of TFTs. The expressions were obtained by utilizing results from conformal mapping of the TFT planar structure, along with a characterization procedure for evaluation of the characteristic unit-area “back”-film capacitance CL. The most practical result is for the saturation regime, in which the channel current is composed of the saturation current of the “ideal” TFT, the additional modulation current ΔID, and the component ΔIL that can be ascribed to leakage current. The quantitative analysis of experimental data for organic TFTs indicates that the impact of fringing capacitance is of the same order of magnitude as the channel length modulation and SCLC. The impact of the fringing capacitance at the “back” surface of the semiconducting film can be distinguished only quantitatively from the effects of the channel length modulation and SCLC.