A semiconductor structure for a DRAM is described having multiple layers of arrays of thyristor memory cells with silicon-germanium base regions. Memory cells in a vertical string extending through the layers have an electrical connection to one terminal of the memory cells in that string. Word lines couple the strings together. Each layer of the array also includes bit line connections to memory cells on that layer. Select transistors enable the use of folded bit lines. Methods of fabricating the array are described.