The electrical characteristics of metal/amorphous Si/crystalline Si (M/a Si/c Si) structures have been investigated and found to depend strongly on whether the M/aSi contact is Ohmic or blocking. Evaporated 1000-Å-thick Cu and Sb films were found to form Ohmic contacts to aSi after proper annealing. With these contacts, aSi/p-Si junctions exhibited good rectification characteristics with breakdown voltages as high as 60 V. Good rectification was not observed on n-type substrates. Reversible threshold switching was found to occur with junctions formed on both n- and p-type substrates when the M/aSi contact was non-Ohmic. The scanning electron microscope was used to observe the structure of filaments in the a Si films subsequent to the forming process.