Beam instability in a metal-insulator-semiconductor microstructure with periodical field electrodes is considered. The instability appears as a consequence of the inversion of electron distribution function in gallium arsenide in the strong electric field. Space charge wave amplification in the two-dimensional electron gas of a GaAs(AlGa)As heterostructure is also considered. The amplification of a space charge wave in such microstructures seems to be a basis for new microwave amplifiers.