Improved Field-Effect Mobility Model for Amorphous In-Ga-Zn-O Thin-Film Transistors

Tze-Ching Fung and Jerzy Kanicki

The conventional approach to extract the electrical parameters from thin-film transistors (TFTs) I_DS/V_GS data is to apply the standard MOSFET drain current equation which assumes constant field-effect mobility (μ_eff). The μeff and threshold voltage (V_th) can be extracted from a simple linear fit of the on-region drain current data and this method is frequently used in most of the ionic metal oxide semiconductor TFT studies. However, when analyzing the IDS/VGS data for a-IGZO TFTs, we observed a deviation between the actual data and the theoretical prediction based on standard MOSFET equation. Inconsistent results might be obtained if the proper data range (V_GS range) is not specified.

Further analysis of the linear region transfer data reveals a non-linear I_DS/V_GS behavior observed in a-IGZO TFT at the higher V_GS: the I_DS seems to be higher than expected for a given V_GS. In other words, the apparent field effect mobility is dependent on VGS and the μ_eff extracted from standard MOSFET equation can only be considered as an average value. We propose an improved TFT I_DS/V_GS model for accurate device parameter extraction. A new drain current equation is derived by introducing a gate voltage dependent field-effect mobility (μ_eff(V_GS)) into the standard MOSFET equation. The a-IGZO TFT parameters were extracted based on the improved model. Two independent extraction routines were exercised. The first method determines the parameters by finding the minimum fitting error over a trial threshold voltage range, while the second method extract all parameters by a more straight-forward, two steps processes. Two methods give very similar results which demonstrate the consistency of the new approach. In addition, as compared to the tradition method, the improved model can reach a reduced fitting error with 10 times improvement. This project is supported by DARPA.

T.-C. Fung, K. Nomura, H. Hosono and J. Kanicki, Digest of Technical Papers, SID, 2008 Vehicles and Photons, pp. 117-123, 2008.

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