A branch of microelectronics in which the active devices use the principles of vacuum tubes for their operation: they are known as solid-state valves. The cathode is a field emitter constructed from a fine point of semiconductor material, usually silicon. The anode can be a metallic contact that collects the electrons emitted from the field-emission cathode. The flight of the electrons between the electrodes can be modified or modulated by interspersing other electrodes or grids to control the anode current. The complete structure may be only a few micrometres to a few millimetres in size, much smaller than conventional tubes; it is thus able to operate at very high frequencies, including microwave frequencies of around 10 gigahertz. These devices can be made using the standard processing techniques of silicon planar technology, and are therefore less expensive and more reliable than their glass tube counterparts.