Diode, a device with two electrodes in an electronic component that allows current to flow in a single direction, many of which are used for rectification purposes. The variable capacitance diode (Varicap Diode) is used as an electronic adjustable capacitor. Most diodes have a current directivity that we usually call Rectifying. The most common function of a diode is to allow the current to pass in a single direction (called forward bias) and to block it in reverse (called reverse bias). Therefore, the diode can be thought of as an electronic check valve.
An early vacuum electron diode; it is an electronic device that can conduct current in one direction. In the semiconductor diode there is a PN junction with two lead terminals. This electronic device has unidirectional current conductivity in the direction of applied voltage. Generally speaking, a crystal diode is a p-n interface formed by sintering of p-type and n-type semiconductors. A space charge layer is formed on both sides of the interface to form a self built electric field. When the applied voltage is equal to zero, the diffusion current and drift current caused by self-built electric field are equal to each other, which is also the characteristics of diodes under normal conditions.