We have found that heavy Si doping is efficient for large field emission (FE) from AlN. For Si-doped AlN and Al_xGa_1−xN (0.38≤x<1), from a linear relation between the applied bias and the anode-sample distance, we clarified the band-gap (the Al content) and Si-dopant-density dependences of the electric field necessary for FE, E_FE. As the band gap (the Al content) increases, E_FE decreases due to the lower electron affinity. As the Si-dopant density increases, E_FE decreases due to spontaneous formation of ridges with nanometer-order sharpness. Heavily Si-doped AlN showed a large FE current density of 0.22 A/cm² and high stability in FE current over time (fluctuation: 3%).