Positive charge trapping phenomenon in n-channel thin-film transistors with amorphous alumina gate insulators

In this work, we investigate the charge trapping behavior in InGaZnO (IGZO) thin-film transistors with amorphous AlO (alumina) gate insulators. For thicknesses ≤10 nm, we observe a positive charge generation at intrinsic defects inside the AlO, which is initiated by quantum-mechanical tunneling of electrons from the semiconductor through the AlO layer. Consequently, the drain current shows a counter-clockwise hysteresis. Furthermore, the de-trapping through resonant tunneling causes a drastic subthreshold swing reduction. We report a minimum value of 19 mV/dec at room temperature, which is far below the fundamental limit of standard field-effect transistors. Additionally, we study the thickness dependence for AlO layers with thicknesses of 5, 10, and 20 nm. The comparison of two different gate metals shows an enhanced tunneling current and an enhanced positive charge generation for Cu compared to Cr.