1. A three-dimensional mask-programmable read-only memory, comprising: a semiconductor substrate including transistors; a plurality of mask-programmable read-only memory levels stacked above and coupled to said substrate, each memory level comprising a plurality of memory cells including a first cell and a second cell; each memory cell comprising a quasi-conduction layer, said conduction layer conducting more current in one direction than in the other direction; said memory cells having N (N>2) states, cells in different states having different current-voltage characteristics; wherein, said first cell has the smallest threshold voltage among all states; said second cell has a larger threshold voltage than said first cell, said second cell comprises at least a first resistive layer more than said first cell.