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    Nov 26, 2018

    Lead: Among the diodes and electronic components, a device with two electrodes allows only current to flow in a single direction, and many of them use the function of rectification.


    Diode


    Diode, (English: Diode), a device with two electrodes that allows only current to flow in a single direction, many of which are applied to its rectification function. The varactor diode (Varicap Diode) is used as an electronically tunable capacitor. The current directionality of most diodes is often referred to as the "Rectifying" function. The most common function of a diode is to allow only current to pass in a single direction (called forward bias) and reverse in reverse (called reverse bias). Therefore, the diode can be thought of as an electronic version of the check valve.


    An early vacuum electronic diode; it is an electronic device that conducts current in one direction. Inside the semiconductor diode, there is a PN junction and two lead terminals. The electronic device has a unidirectional current conductivity according to the direction of the applied voltage. Generally, a crystal diode is a p-n junction interface formed by sintering a p-type semiconductor and an n-type semiconductor. 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 is equal to the drift current caused by the self-built electric field due to the difference in the concentration of carriers on both sides of the p-n junction, and is in an electric equilibrium state, which is also a diode characteristic under normal conditions.


    Early diodes included "Cat's Whisker Crystals" and vacuum tubes (known as "Thermionic Valves" in the UK). Most of the most common diodes today use semiconductor materials such as silicon or germanium.


    Characteristic


    Positive


    When the forward voltage is applied, the forward voltage is small at the beginning of the forward characteristic, which is insufficient to overcome the blocking effect of the electric field in the PN junction. The forward current is almost zero. This segment is called the dead zone. This forward voltage that does not turn the diode on is called the deadband voltage. When the forward voltage is greater than the deadband voltage, the electric field in the PN junction is overcome, the diode is forward-conducting, and the current rises rapidly as the voltage increases. In the current range of normal use, the terminal voltage of the diode remains almost constant during turn-on. This voltage is called the forward voltage of the diode. When the forward voltage across the diode exceeds a certain value, the internal electric field is quickly weakened, the characteristic current increases rapidly, and the diode is conducting. It is called the threshold voltage or threshold voltage, the silicon tube is about 0.5V, and the manifold is about 0.1V. The forward voltage drop of the silicon diode is about 0.6-0.8V, and the forward voltage drop of the germanium diode is about 0.2-0.3V.


    Reverse


    When the applied reverse voltage does not exceed a certain range, the current through the diode is the reverse current formed by the minority carrier drift motion. Since the reverse current is small, the diode is in an off state. This reverse current is also called reverse saturation current or leakage current, and the reverse saturation current of the diode is greatly affected by temperature. Generally, the reverse current of a silicon tube is much smaller than that of a tantalum tube. The reverse saturation current of a small power silicon tube is on the order of nA, and the low power tube is on the order of μA. When the temperature rises, the semiconductor is excited by heat, the number of minority carriers increases, and the reverse saturation current also increases.


    Breakdown


    When the reverse voltage exceeds a certain value, the reverse current suddenly increases. This phenomenon is called electrical breakdown. The threshold voltage that causes electrical breakdown is called the diode reverse breakdown voltage. The diode loses unidirectional conductivity during electrical breakdown. If the diode does not cause overheating due to electrical breakdown, the unidirectional conductivity may not be permanently destroyed. After the voltage is removed, the performance can be restored, otherwise the diode is damaged. Therefore, the reverse voltage applied to the diode should be avoided.


    A diode is a two-terminal device with unidirectional conduction. It has an electronic diode and a crystal diode. Because the heat loss of the filament is lower than that of the crystal diode, the electronic diode is rarely seen. It is more common and commonly used. It is a crystal diode. The unidirectional conduction characteristics of diodes, semiconductor diodes are used in almost all electronic circuits. It plays an important role in many circuits. It is one of the earliest semiconductor devices, and its application is also very extensive.


    The tube voltage drop of the diode: the silicon diode (non-illuminated type) has a forward voltage drop of 0.7V, and the forward tube voltage drop of the neon tube is 0.3V. The forward tube voltage drop of the LED varies with different illuminating colors. There are three main colors. The specific voltage drop reference values are as follows: the voltage drop of the red LED is 2.0--2.2V, the voltage drop of the yellow LED is 1.8-2.0V, and the voltage drop of the green LED is 3.0-3.2V, the rated current is about 20mA when it is normally illuminated.


    The voltage and current of the diode are not linear, so the resistors should be connected when the different diodes are connected in parallel.


    Characteristic curve


    Like the PN junction, the diode has unidirectional conductivity. Typical volt-ampere characteristics of silicon diodes. When the diode is applied with a forward voltage, when the voltage value is small, the current is extremely small; when the voltage exceeds 0.6V, the current begins to increase exponentially, which is usually referred to as the turn-on voltage of the diode; when the voltage reaches about 0.7V When the diode is fully turned on, this voltage is usually referred to as the turn-on voltage of the diode, indicated by the symbol UD.


    For a germanium diode, the turn-on voltage is 0.2V and the turn-on voltage UD is about 0.3V. A reverse voltage is applied to the diode, and when the voltage value is small, the current is extremely small, and the current value is the reverse saturation current IS. When the reverse voltage exceeds a certain value, the current begins to increase sharply, which is called reverse breakdown. This voltage is called the reverse breakdown voltage of the diode and is represented by the symbol UBR. The breakdown voltage UBR values of different types of diodes vary widely, from tens of volts to several thousand volts.


    Reverse breakdown


    Zener breakdown


    Reverse breakdown is divided into two cases: Zener breakdown and avalanche breakdown. In the case of high doping concentration, the width of the barrier region is small, and when the reverse voltage is large, the covalent bond structure in the barrier region is destroyed, and the valence electrons are decoupled from the covalent bond to generate an electron-hole pair. , causing a sharp increase in current, this breakdown is called Zener breakdown. If the doping concentration is low, the barrier region is wide, and it is not easy to cause Zener breakdown.