Vacuum fluorescent displays (VFD) are voltage-controlled devices that provide high brightness and wide viewing angles. These relatively low-cost displays consist of a cathode, grid and anode sealed in a high-vacuum glass envelope. Heating the cathode provides the electrons with energy for emission. If both the grid and the anode are at a positive potential with respect to the cathode, the emitted electrons pass through the grid and onto the anode. Typically, the anode terminal is coated with a fluorescent material that emits light when bombarded by electrons. The anode areas correspond to individual segments of the display, and the grid is used to switch on and off whole digits. Brightness is a function of the voltage difference between the cathode and the anode. Vacuum fluorescent displays require high-voltage drivers that are capable of large output voltage swings between the on and off conditions. Most devices can produce output swings of at least 50 V and use metal oxide semiconductor (MOS) and bipolar technologies.