By antimony doping tin oxide, SnO2:Sb (ATO), below 1.0% Sb concentration, controllable n-type doping was realized. Plasma-assisted molecular beam epitaxy has been used to grow high quality single crystalline epitaxial thin films of unintentionally doped (UID) and Sb-doped SnO2 on r-plane sapphire substrates. A UID thickness series showed an electron concentration of 7.9×1018 cm-3 for a 26 nm film, which decreased to 2.7×1017 cm-3 for a 1570 nm film, whereas the mobility increased from 15 to 103 cm2/V s, respectively. This series illustrated the importance of a buffer layer to separate unintentional heterointerface effects from the effect of low Sb doping. Unambiguous bulk electron doping was established by keeping the Sb concentration constant but changing the Sb-doped layer thickness. A separate doping series correlated Sb concentration and bulk electron doping. Films containing between 9.8×1017 and 2.8×1020 Sb atoms/cm3 generated an electron concentration of 1.1×1018–2.6×1020 cm-3. As the atomic Sb concentration increased, the mobility and resistivity decreased from 110 to 36 cm2/V s and 5.1×10-2 - 02;to 6.7×10-4 Ω cm, respectively. The Sb concentration was determined by secondary ion mass spectrometry. X-ray diffraction and atomic force microscopy measurements showed no detrimental effects arising from the highest levels of Sb incorporation. Temperature dependent Hall measurements established a lower limit for the Sb electron activation energy of 13.2 meV and found that films with greater than 4.9×1019 electrons/cm3 were degenerately doped. Within experimental uncertainties, 100% donor efficiency was determined for Sb-doped SnO2 in the range studied.