Pinning-induced pn junction formation in low-bandgap two-dimensional semiconducting systems

A model is presented for $pn$ junction formation near metal-semiconductor contacts in two-dimensional semiconducting systems such as graphene. Carrier type switching occurs in a region near the metal-semiconductor junction when energy band bending leads to a crossing between the junction Fermi level and the Dirac energy. A bias-dependent depletion region occurs due to the minimization of carrier density, which is shown to act as an additional parasitic resistance in devices. The $pn$ junction resistance is demonstrated by its implementation in a transfer length structure.