Example embodiments relate to semiconductor devices including two-dimensional (2D) materials, and methods of manufacturing the semiconductor devices. A semiconductor device may be an optoelectronic device including at least one doped 2D material. The optoelectronic device may include a first electrode, a second electrode, and a semiconductor layer between the first and second electrodes. At least one of the first electrode and the second electrode may include doped graphene. The semiconductor layer may have a built-in potential greater than or equal to about 0.1 eV, or greater than or equal to about 0.3 eV. One of the first electrode and the second electrode may include p-doped graphene, and the other may include n-doped graphene. Alternatively, one of the first electrode and the second electrode may include p-doped or n-doped graphene, and the other may include a metallic material.