▎ 摘　　要

We propose a spin field effect transistor using a graphene nanoribbon as the channel. The control and manipulation of the electron spin in the channel modulate the spin-polarized current. The modulation is carried out by the magnetic exchange field which arises from the electron exchange interaction with ferromagnetic gate and quantum confinement effect. Numerical estimation indicates that a substantial magnetic exchange field can generate a phase difference on the order of pi within a timeframe far below the spin lifetime and timescale between successive collisions, which ensures both the spin coherence and ballistic conduction during the electron transport. A graphene ribbon with armchair boundaries has the desired Dirac point and metallic character. This Dirac-like dispersion relation, together with negligible spin-orbit coupling, makes large on-off ratio feasible even in the presence of thermal dispersion.