Fig. 1: Fermi-level tuning to maximize the energetic distance to oxide defect bands.
From: Improving stability in two-dimensional transistors with amorphous gate oxides by Fermi-level tuning
a, Schematic (left) shows a top-gated GFET with an Al2O3 gate oxide. For a cut through the GFET along the indicated arrow, the energetic alignment of the Fermi level to the defect band in the aluminium gate oxide is shown. In the band diagram (left), the device is electrically unstable with respect to variations in the threshold voltage as the Fermi level is aligned within the defect band. In the band diagram (right), the Fermi level has been shifted downwards, rendering the device more stable. b, Schematic of the charge transfer of electrons flowing through the WS2 channel to traps in the HfO2 gate oxide (left). This situation is depicted in the left band diagram where the Fermi level is aligned close to the conduction band edge, rendering the device unstable. If the Fermi level is instead aligned close to the valence band edge, the FET is stable. c, In this band diagram, the possible range of the graphene Fermi levels, which is currently achievable by doping, is shown as a grey-shaded region. The Fermi level can be continuously tuned within this region. d, Injection of electrons and holes from the band edges of WS2. In a layered semiconductor, the number of layers modifies the bandgap and doping determines whether electrons or holes will be the majority carriers and thus govern device stability.
