  Solid-state topological insulating phases, characterized by spin-momentum
locked edge modes, provide a powerful route for spin and charge manipulation in
electronic devices. We propose to control charge and spin transport in the
helical edge modes by electrically switching the topological insulating phase
in a HgTe/CdTe double quantum well device. We introduce the concept of a
topological field-effect-transistor and analyze possible applications to a spin
battery, which also realize a set up for an all-electrical investigation of the
spin-polarization dynamics in metallic islands.
