in integrated circuit technology. This subject is now called vacuum nanoelectronics. The most common design uses a cold cathode in the form of a large-area field electron source (for example a field emitter array). With these devices, electrons are field-emitted from a large number of closely spaced individual emission sites. Such integrated microtubes may find application in microwave devices including mobile phones, for Bluetooth and Wi-Fi transmission, and in radar and satellite communication. , they were being studied for possible applications in field emission display technology, but there were significant production problems. As of 2014, NASA's Ames Research Center was reported to be working on vacuum-channel transistors produced using CMOS techniques. Characteristics Space charge of a vacuum tube When a cathode is heated and reaches an operating temperature around 1050° Kelvin (780° Celsius), free electrons are driven from its surface. These free electrons form a cloud in the empty space between the cathode and the anode, known as the space charge. This space charge cloud supplies the electrons that create the current flow from the cathode to the anode. As electrons are drawn to the anode during the operation of the circuit, new electrons will boil off the cathode to replenish the space charge. The space charge is an example of an electric field. Voltage - Current characteristics of vacuum tube All tubes with one or more control grids are controlled by an AC (Alternating Current) input voltage applied to the control grid, while the resulting amplified signal appears at the anode as a current. Due to the high voltage placed on the anode, a relatively small anode current can represent a considerable increase in energy over the value of the original signal voltage. The space charge electrons driven off the heated cathode are strongly attracted by the positive anode. The control grid(s) in a tube mediate this current flow by combining the small AC signal current with the grid's slightly negative value. When the signal sine (AC) wave is applied to the grid, it rides on this negative value, driving it both positive and negative as the AC signal wave changes. This relationship is shown with a set of Plate Characteristics curves, (see example above,) which visually display how the output current from the anode () can be affected by a small input voltage applied on the grid (), for any given voltage on the plate(anode) (). Every tube