A transistor comprises a planar layer of a topological material located between a gate electrode and a dielectric layer. The topological material exhibits a topological phase transition between a trivial state and a non-trivial state at a critical electric field strength on application of an electric field in a direction perpendicular to the planar layer. The topological material exhibits a change in bandgap, in the presence of the electric field, having a Rashba spin-dependent bandgap contribution that is at least three times as large as a non-spin-dependent bandgap contribution.