  Quantum and thermal fluctuations are fundamental to a plethora of phenomena
within quantum optics, including the Casimir effect that acts between closely
separated surfaces typically found in MEMS and NEMS devices. Particularly
promising for engineering and harnessing these forces are systems out of
thermal equilibrium. Recently, semiconductors with external bias have been
proposed to study the nonequilibrium Casimir force. Here, we explore systems
involving moderately biased semiconductors that exhibit strong repulsive
Casimir forces, and we determine the effects of bias voltage, semiconductor
bandgap energy, and separation for experimentally accessible configurations.
Modes emitted from the semiconductors exert a repulsive force on a near surface
that overcomes the attractive equilibrium Casimir force contribution at
submicron distances. For the geometry of two parallel planes, those modes
undergo Fabry-P\'erot interference resulting in an oscillatory force behavior
as a function of separation. Utilizing the proximity-force approximation, we
predict that the repulsive force exerted on a gold sphere is well within the
accuracy of typical Casimir force experiments. Our work opens up new
possibilities of controlling forces at the nano- and micrometer scale with
applications in sensing and actuation in nanotechnology.
