Low barrier (≲0.4 eV) microwave mixer diodes were fabricated by means of silicon molecular beam epitaxy (Si MBE). Schottky barrier lowering was achieved by using a thin highly doped epitaxial layer to modify the electric field near the surface. The advantages of silicon MBE for these devices arise from the ability to grow epitaxial layers which are very precisely controlled in thickness and in the spatial distribution of dopant. The resulting diodes have both a low barrier height and a very low forward series resistance.