Graphene nanoribbons combine the unique electronic and spin properties of graphene with a transport gap that arises from quantum confinement and edge effects. This makes them an attractive candidate material for the channels of next-generation transistors. Nanoribbons can be made in a variety of ways, including lithographic, chemical and sonochemical approaches, the unzipping of carbon nanotubes, the thermal decomposition of SiC and organic synthesis. However, the reliable site and alignment control of nanoribbons with high on/off current ratios remains a challenge. Here we control the site and alignment of narrow (∼23 nm) graphene nanoribbons by directly converting a nickel nanobar into a graphene nanoribbon using rapid-heating plasma chemical vapour deposition. The nanoribbons grow directly between the source and drain electrodes of a field-effect transistor without transfer, lithography and other postgrowth treatments, and exhibit a clear transport gap (58.5 meV), a high on/off ratio (>10(4)) and no hysteresis. Complex architectures, including parallel and radial arrays of supported and suspended ribbons, are demonstrated. The process is scalable and completely compatible with existing semiconductor processes, and is expected to allow integration of graphene nanoribbons with silicon technology.