Nonvolatile memory devices include a plurality of nonvolatile memory cell transistors having respective channel regions within a semiconductor layer formed of a first semiconductor material and respective source/drain regions formed of a second semiconductor material, which has a smaller bandgap relative to the first semiconductor material. The source/drain regions can form non-rectifying junctions with the channel regions. The source/drain regions may include germanium (e.g., Ge or SiGe regions), the semiconductor layer may be a P-type silicon layer and the source/drain regions of the plurality of nonvolatile memory cell transistors may be P-type germanium or P-type silicon germanium.