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By providing in a crystal, carriers of such an energy and such a momentum that at least one of their three main effective masses is negative, it should be possible to obtain wave amplification in such a crystal. The amplifier should work up to about 1000 kmc (0.3 mm wavelength), with a large bandwidth. To obtain carriers of sufficient energy and proper momentum, acceleration by a high field seems most feasible at present. Negative effective masses for relatively low energies may be obtained if the energy contours are re-entrant near the band edge, as is the case for the heavy holes in germanium and, as may be the case for other semiconductors with degenerate band edges. The optical-phonon collision cross-section should also be high in order to obtain sufficient concentration in k-space. If the latter is the case for germanium a verifiable microwave amplifier using the principle would consist of a wafer of p-type germanium with a strong bias field applied in a crystallographic (100) direction, and inserted into a waveguide or a cavity such that the electric vector of the microwave field is perpendicular to the bias field. Such a bulk amplifier has no critical dimensions, receives its power from a dc battery, and has essentially no frequency dependence over the entire radio spectrum. The principle is not restricted to germanium, but should also work with certain other semiconductors. Low-frequency amplifiers and bistable devices are also possible. Some design problems are discussed.