In atomically thin p–n junctions, traditional strategies such as doping and implantation for realizing a p- or n-region will fail at the nanoscale, and the Schottky barrier and Fermi level pinning effect taking place in metal–semiconductor contacts seriously suppress the transport properties. In this work, based on first-principles calculations, we propose a new strategy for realizing an ultrathin p–n junction by vertically stacking nonstoichiometric Ca₂N and Na₂N monolayers, which represents a kind of donor–acceptor heterostructure with a natural Ohmic contact. It is of great interest to find that the tunneling barrier can be eliminated and the charge transfer quantity is one order of magnitude higher than that between polar monolayers by adjusting the interlayer distance. In addition, at equilibrium the interlayer tunneling can be turned into resonant transport due to the quasi-bonding, thus enabling excellent transmission performance. In accordance with the results, we believe that our new concept of an atomically thin p–n junction will provide an unprecedented possibility for the development of nanodevices.