Transport-map analysis of ionic liquid-gated ambipolar WSe₂ field-effect transistors

We present the electrical characteristics of ionic liquid-gated ambipolar WSe₂ field-effect transistors (FET) and analyze them graphically using a transport map. With the transport map, the ambipolar I-V characteristics, which have either electrons or holes as the majority charge carriers depending on the bias conditions, were easily divided into the electron and hole conduction regimes. The effect of the drain bias (V _d) on trapped charges is inferred from the decreased offset voltage, which indicates the downward deflection of the channel energy band with the V _d. We confirmed the change of the majority carrier types shown in the transport map by analyzing the output curves, which show a transition from rectifying to saturated behavior. The existence of a relatively thin Schottky barrier for hole conduction is supported by the degradation in the transconductance curves and the variation in V _th with the V _d. These results provide important information for understanding the operating mechanism of ambipolar WSe₂ FETs gated by ionic liquids and can be extended to FETs based on other two-dimensional semiconducting materials.