Diodes are dual terminal electronic components that conduct current mainly in one direction (asymmetric conductivity); It has low resistance in one direction (ideally zero) and high resistance in the other direction (ideally infinite). A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode, and a plate, in which electrons can only flow from the cathode to the plate in one direction. A semiconductor diode, the most commonly used type today, is a semiconductor material with a pn junction connected to two electrical terminals in a crystal. Semiconductor diodes were the earliest semiconductor electronic devices. In 1874, German physicist Ferdinand Braun discovered the phenomenon of asymmetric conductivity between crystalline minerals and metals. Nowadays, most diodes are made of silicon, but other semiconductor materials such as gallium arsenide and germanium are also used. The most common function of a diode is to allow current to pass in one direction (referred to as the forward direction of the diode), while preventing current from passing in the opposite direction (reverse direction). In this way, the diode can be viewed as an electronic version of the check valve. This unidirectional behavior is called rectification, which is used to convert alternating current (AC) into direct current (DC). The form of rectifiers and diodes can be used for tasks such as extracting modulation from radio signals in radio receivers. However, due to the nonlinear current voltage characteristics of diodes, their behavior may be more complex than this simple switching action. Only when there is a certain threshold voltage or cut in voltage in the forward direction (the diode is referred to as a forward biased state), does the semiconductor diode begin to conduct electricity. The voltage drop at both ends of the forward bias diode varies only slightly with current and is a function of temperature. This effect can be used as a temperature sensor or reference voltage. In addition, when the reverse voltage at both ends of the diode reaches a value called breakdown voltage, the high resistance of the diode to reverse flow suddenly drops to low resistance. The current voltage characteristics of semiconductor diodes can be customized by selecting semiconductor materials and introducing impurities in the materials during the manufacturing process. These technologies are used to create specialized diodes that perform many different functions. For example, diodes are used to regulate voltage (Zener diodes), protect circuits from high-voltage surges (avalanche diodes), electronically tune radio and television receivers (varactor diodes) to generate RF oscillations (tunnel diodes), Gunn diodes, IMPATT diodes, and generate light (light emitting diodes). Tunnel diodes, Gunn diodes, and IMPATT diodes have negative resistance, which is useful in microwave and switching circuits. Both vacuum diodes and semiconductor diodes can be used as shot noise generators