  We present a theoretical model that describes electrical spin-detection at a
ferromagnet/semiconductor interface. We show that the sensitivity of the spin
detector has strong bias dependence which, in the general case, is dramatically
different from that of the tunneling current spin polarization. We show that
this bias dependence originates from two distinct physical mechanisms: 1) the
bias dependence of tunneling current spin polarization, which is of microscopic
origin and depends on the specific properties of the interface, and 2) the
macroscopic electron spin transport properties in the semiconductor. Numerical
results show that the magnitude of the voltage signal can be tuned over a wide
range from the second effect which suggests a universal method for enhancing
electrical spin-detection sensitivity in ferromagnet/semiconductor tunnel
contacts. Using first-principles calculations we examine the particular case of
a Fe/GaAs Schottky tunnel barrier and find very good agreement with experiment.
We also predict the bias dependence of the voltage signal for a Fe/MgO/GaAs
tunnel structure spin detector.
