A diode is an electronic device made of semiconductor materials (silicon, selenium, germanium, etc.). The diode has two electrodes, the positive pole, also called the anode; the negative pole, also called the cathode, when a forward voltage is applied between the two stages of the diode, the diode is turned on, and when a reverse voltage is applied, the diode is turned off. The on and off of the diode is equivalent to the on and off of the switch.
The diode has unidirectional conductivity, and the direction of current is from the anode to the cathode through the tube when it is turned on. 
Diode is one of the earliest semiconductor devices, and its application is very wide. Especially in various electronic circuits, use diodes and resistors, capacitors, inductors and other components to make reasonable connections to form circuits with different functions, which can realize AC rectification, modulation signal detection, amplitude limiting and clamping, and power supply Various functions such as voltage regulation  
Diodes can be found in common radio circuits or in other household appliances or industrial control circuits