A threshold voltage definition for modeling asymmetric dual-gate amorphous InGaZnO thin-film transistors with parameter extraction technique

Abstract

A physical definition of threshold voltage has been proposed for asymmetric dual-gate amorphous InGaZnO (a-IGZO) thin-film transistors. Based on the definition, a compact model for drain current has been derived with complete parameter extraction methods. According to the distribution characteristics of density of states (DOS) in the a-IGZO bandgap, the threshold voltage of bottom-gate (BG)-driven devices is defined by the ratio of the trapped-carrier density to the free-carrier density at the bottom surface of the channel layer. This definition has proven to give appropriate values of threshold voltage for devices with DOS parameters varying in a wide range. The trapped-charge density in the above-threshold region can be expressed based on the defined threshold voltage, with the form related to the BG voltage, the top-gate voltage, and the potential along the channel. The free-charge density and analytical drain current can thus be obtained in the above-threshold region. Finally, the continuous drain current expression is developed covering all operation regions. In the modeling process, the determination and extraction strategies for parameters are also presented.