A novel three-terminal switching device, fabricated only from metals, has recently been demonstrated. Somewhat similar to a semiconductor transistor, the physical principles of operation are quite different. It is an active device driven by a thermodynamic force associated with the effective Zeeman energy of the spin polarized electrons in the base. `Bipolar' has a double meaning: there are two polarities of carriers, upspin and downspin electrons; and the output can be a positive or negative voltage (or current). The three-terminal device does not have power gain, but because it shows a memory effect it is natural to use it as a storage element in a nonvolatile memory array. Power gain can be achieved in a five-terminal embodiment, allowing fanout and the linking of devices in logic operations. Switching times faster than 1 ns can be expected. Because all the layers are metals and charge carrier densities are high, fabrication of the device at submicron scales is quite possible and operation at the nanometer scale is conceivable. Thus, the possibility of achieving high packing densities is very plausible, and is not obviated by problems common to semiconductors such as heat dissipation and low carrier density.
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