  We introduce a fabrication method for gate-all-around nanowire field-effect
transistors. Single nanowires were aligned perpendicular to underlying bottom
gates using a resist-trench alignment technique. Top gates were then defined
aligned to the bottom gates to form gate-all-around structures. This approach
overcomes significant limitations in minimal obtainable gate length and
gate-length control in previous horizontal wrap-gated nanowire transistors that
arise because the gate is defined by wet etching. In the method presented here
gate-length control is limited by the resolution of the
electron-beam-lithography process. We demonstrate the versatility of our
approach by fabricating a device with an independent bottom gate, top gate, and
gate-all-around structure as well as a device with three independent
gate-all-around structures with 300 nm, 200 nm, and 150 nm gate length. Our
method enables us to achieve sub-threshold swings as low as 38 mV/dec at 77 K
for a 150 nm gate length.
