The forward and reverse current-voltage characteristics of amorphous silicon p-n junction diodes are presented as a function of doping level. A study of the forward characteristics with temperature show that at high doping levels the current is limited by tunnelling through the depletion region via gap states. At the lowest doping levels the diode current is determined by generation-recombination in the depletion region. A similar interpretation is applied to the reverse characteristics. These results are relevant to the use of these diodes both as nonlinear elements in matrix-addressed large-area liquid-crystal displays and thin-film solar cells.