A semiconductor device used to convert alternating current to direct current. The most important characteristic of a diode is its conductivity in one direction. In a circuit, current can only flow in from the positive side of the diode and out from the negative side. Usually it contains a PN junction with two terminals, positive and negative. Its structure is shown in the figure. Carriers in P region are holes and carriers in N region are electrons, which form a barrier in P region and N interval. When the applied voltage makes the P region positive relative to the N region, the barrier decreases, and the storage carrier is generated near both sides of the barrier, which can pass through the large current and has a low voltage drop (typical value is 0.7V), which is called the positive conduction state. If the opposite voltage is added to increase the barrier, it can withstand high reverse voltage and flow through a small reverse current (called reverse leakage current), which is called reverse blocking state. The rectifier diode has obvious unidirectional conductivity. Rectifier diodes can be made of semiconducting materials such as germanium or silicon. Silicon rectifier diode has high breakdown voltage, small reverse leakage current and good high temperature performance. Usually high voltage and high power rectifier diodes are made of high purity monocrystalline silicon (doped more easily reverse breakdown). The junction area of this device is large, can pass through a large current (up to thousands of amperes), but the working frequency is not high, generally in the tens of KHZ below. Rectifier diode is mainly used in a variety of low frequency half wave rectifier circuit, such as full wave rectifier need to be connected to the rectifier bridge.