Electrical contacts between subcells in a-Si tandem solar cells are n(+)p(+) junctions in which a highly conductive path is provided by charge-carrier tunneling transitions between extended states and the states in the gap of a-Si. A tunneling-assisted capture-emission model of carriers is developed in n(+)p(+) a-Si junctions with a high built-in electrical field in the depletion region. The pure thermal and the tunneling-assisted thermal capture and emission of carriers determine the occupancy function in the mobility gap and generation-recombination rates in the space-charge region. The developed model is applied to calculate the current-voltage characteristics of an n(+)p(+) a-Si junction, and a good agreement with measured characteristics is obtained. (C) 2001 Elsevier Science B.V. All rights reserved.