We demonstrate for the first time, a metal-semiconductor field-effect transistor (MESFET) fabricated with a self-assembled and high mobility <110> GaAs planar nanowire (NW) channel. The planar NWs were grown on GaAs (100) substrates with metalorganic chemical vapor deposition (MOCVD) through gold (Au) catalyzed vapor-liquid-solid (VLS) mechanism . Unlike conventional out-of-plane <111> NWs, these <110> planar NWs grow self-aligned in the [0-11] or [01-1] directions laterally and epitaxially on the surface and stay effectively "pinned" to the (100) substrate during growth, as illustrated in Fig. la-b. They are also free of twin-defects that are often found in conventional <111> III-V NWs. In addition, they can also be transfer-printed to other substrates such as Si using a smart release scheme and standard contact printing . All of these naturally make the planar NWs integratable and compatible with existing circuit design and process technology.