The effects of random dopant fluctuations (RDFs) on the performance of Germanium-source tunnel field-effect transistors (TFETs) is studied using 3-D device simulation. The RDF in the source region is found to have the most impact on threshold voltage variation (σ VTH) if the source is moderately doped (1019 cm-3) such that vertical tunneling within the source is dominant. If the source is heavily doped (1020 cm-3) such that lateral tunneling from the source to the channel is dominant, the impact of RDF in the channel region is also significant. RDF-induced threshold voltage variation (σ VTH) for an optimally designed Ge-source TFET is relatively modest (σ VTH < 20 mV at Lg = 30 nm), compared with a MOSFET of similar gate length. Supply voltage scaling is not beneficial for reducing TFET σV TH. © 2011 IEEE.