How Do Contact and Channel Contribute to the Dirac Points in Graphene Field-Effect Transistors?

The Dirac point(s) in graphene field-effect transistors (GFETs) are of great importance for electronic application. However, the lack of the effective means to distinguish the electrical properties of graphene at the contact and channel regions limits a clear understanding of their contributions to the Dirac point(s). A method, which can characterize the electrical properties of graphene under metal contact and in the channel, is developed, respectively. It is found that the Fermi levels of graphene at the contact and channel regions are quite different in the GFETs. The difference in Fermi levels results in the penetration of the doping effect under the contact into the channel with a length as long as 1 mu m. The difference also causes the double Dirac point feature in the long-channel GFET due to the combined graphene properties both in contact and channel. One of the two Dirac points diminished in short-channel GFET due to the penetration of doping effect under the contact. The study demonstrates that the electrical behavior of short-channel GFET is dominated by the contact region. This paces a way to deeply understand and further improve the performances of GFETs by controlling the Fermi levels in the whole devices.