  Fundamental physical properties limiting the performance of spin field effect
transistors are compared to those of ordinary (charge-based) field effect
transistors. Instead of raising and lowering a barrier to current flow these
spin transistors use static spin-selective barriers and gate control of spin
relaxation. The different origins of transistor action lead to distinct size
dependences of the power dissipation in these transistors and permit
sufficiently small spin-based transistors to surpass the performance of
charge-based transistors at room temperature or above. This includes lower
threshold voltages, smaller gate capacitances, reduced gate switching energies
and smaller source-drain leakage currents.
