▎ 摘　　要

In this letter, we study the electrical characteristics of monolayer and bilayer graphene p-n junctions through the self-consistent solution of the 2-D Poisson and Schrodinger equations within the Non-Equilibrium Green's Function (NEGF) formalism. Negative differential resistance is observed in both devices at room temperatures, which opens the possibility of exploiting graphene in analog electronics. An analytical expression, which is suitable for a fast exploration along the parameter space, is provided and compared against the tight-binding model, showing good agreement.