Diodes, electronic components, a device with two electrodes, only allows the current flow from a single direction, many of the use of its rectifier is the application of the function. The varactor diode is used as an electronic adjustable capacitor. Most of the diodes have the current direction we often call "rectifier" function. The most common function of diodes is to allow only current from a single direction (called forward bias), reverse the block (called reverse bias). Therefore, the diode can think of electronic version of the check valve.

Early vacuum electronic diodes; it is a kind of electronic devices that can unidirectionally conduct current. Inside the semiconductor diode there is a PN junction of two lead terminals, this electronic device in accordance with the direction of the applied voltage, with unidirectional current conductivity. In general, a crystal diode is a p-n junction interface formed by p-type semiconductor and n-type semiconductor sintering. In the interface on both sides of the formation of space charge layer, constitute a self-built electric field. When the applied voltage is equal to zero, due to the p-n junction carrier concentration difference caused by the diffusion current and self-built electric field caused by the drift current equal to the electrical balance, which is normal under the diode characteristics.

Early diodes consist of "cat whiskers" and vacuum tubes. Most of today's most common diodes are the use of semiconductor materials such as silicon or germanium.