Investigation of the Determining Factors for the “Mobility Boost” in High-k-Gated Transparent Oxide Semiconductor Thin-Film Transistors

In metal-oxide thin-film transistors (TFTs), high-k gate dielectrics often yield a higher electron mobility than SiO₂. However, investigations regarding the mechanism of this high-k “mobility boost” are relatively scarce. To explore this phenomenon, solution-processed In₂O₃ TFTs are fabricated using eight different gate dielectrics (SiO₂, Al₂O₃, ZrO₂, HfO₂, and bilayer SiO₂/high-k structures). With these structures, the total gate capacitance can be varied independently from the semiconductor–dielectric interface to study this mobility enhancement. It is shown that the mobility enhancement is a combination of the effects of areal gate capacitance and interface quality for disordered oxide semiconductor devices. The ZrO₂-gated TFTs achieve the highest mobility by inducing more accumulation charge with higher gate capacitance. Surprisingly, however, when the gate capacitance is held constant, no mobility enhancement is observed with the high-k gate dielectrics compared to SiO₂.