  It is shown that magnetic bipolar transistors (MBT) can amplify currents even
in the saturation regime, in which both the emitter-base and collector-base
junctions are forward biased. The collector current and the current gain can
change sign as they depend on the relative orientation of the equilibrium spin
in the base and on the nonequilibrium spin in the emitter and collector. The
predicted phenomena should be useful for electrical detection of nonequilibrium
spins in semiconductors, as well as for magnetic control of current
amplification and for current switching.
