  As recent additions to the catalog of 2D crystals, silicene and other
silicene-class crystals have numerous unique properties currently being
investigated and considered for use in novel device applications. In this
paper, we investigate electronic and transport properties of silicene in a
field effect transistor geometry. We find that the Berry curvature of
silicene-class crystals can be continuously tuned by a perpendicular electric
field. By direct calculation of the $Z_2$ invariant, we confirm that an
electronic phase transition from a topological insulator to a band insulator
occurs when the electric field passes a critical value. In a device setting
with asymmetric gate voltages, this field-tunable Berry curvature generates a
large spin current transverse to the charge current. When the electric field
strength surpasses the critical value, the bulk spin current is found to change
direction and greatly decrease in magnitude. This finding of a large magnitude,
switchable spin current suggests that the silicene family of 2D crystals could
be an attractive candidate for field-tunable charge-spin conversion. Such
field-tunable phase transitions between topologically distinct phases could be
useful for robust qubits as well.
