Semiconductor devices where networks of molecular nanowires (or nanofibers) are used as the semiconductor material. Field effect transistors are disclosed where networks of molecular nanowires are used to provide the electrical connection between the source and drain electrodes. The molecular nanowires have diameters of less than 500 nm and aspect ratios of at least 10. The molecular nanowires that are used to form the networks can be single element nanowires, Group III-V nanowires, Group II-VI nanowires, metal oxide nanowires, metal chalcogenide nanowires, ternary chalcogenide nanowires and conducting polymer nanowires.