  The concept of a magnetic bipolar transistor (MBT) is introduced. The
transistor has at least one magnetic region (emitter, base, or collector)
characterized by spin-splitting of the carrier bands. In addition,
nonequilibrium (source) spin in MBTs can be induced by external means
(electrically or optically). The theory of ideal MBTs is developed and
discussed in the forward active regime where the transistors can amplify
signals. It is shown that source spin can be injected from the emitter to the
collector. It is predicted that electrical current gain (amplification) can be
controlled effectively by magnetic field and source spin.
