Diode, a device with two electrodes in electronic components, allows current to flow only in a single direction, and many uses use its rectifier function. Varicap Diode is used as an electronic adjustable capacitor. Most diodes have current directionality which we usually call "Rectifying" function. The most common function of a diode is to allow only current to pass in a single direction (called forward bias) and to block in reverse (called reverse bias). Therefore, the diode can be thought of as an electronic version of the check valve.
Early vacuum electronic diodes; they were electronic devices capable of conducting unidirectional currents. There is a PN junction and two lead terminals inside the semiconductor diode. The electronic device has unidirectional current conductivity in accordance with the direction of applied voltage. Generally speaking, a crystal diode is a p-n junction interface formed by sintering of p-type semiconductor and 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 equals zero, the diffusion current caused by the carrier concentration difference between the two sides of the p-n junction is equal to the drift current caused by the self-built electric field, which is also the characteristic of the diode under normal conditions.
Early diodes included "Cat's Whisker Crystals" and vacuum tubes (known in Britain as "Thermionic Valves"). Most of today's most common diodes use semiconductor materials such as silicon or germanium.
When the forward voltage is applied, the forward voltage in the initial part of the forward characteristic is very small, which is not enough to overcome the blocking effect of the electric field in PN junction. The forward current is almost zero, which is called dead zone. This forward voltage, which does not turn on the diode, is called dead-time voltage. When the forward voltage is greater than the dead-time voltage, the electric field in PN junction is overcome, the diode is conducting forward, and the current rises rapidly with the increase of the voltage. Within the normal current range, the terminal voltage of the diode remains almost unchanged when it is turned on. This voltage is called the forward voltage of the diode. When the forward voltage at both ends of the diode exceeds a certain value, the internal electric field is weakened quickly, the characteristic current increases rapidly, and the diode is conducting forward. Called threshold voltage or threshold voltage, the silicon tube is about 0.5V and the germanium tube is about 0.1V. The forward voltage drop of silicon diode is about 0.6-0.8V, and that of germanium diode is about 0.2-0.3V.
When the applied reverse voltage does not exceed a certain range, the current passing through the diode is the reverse current formed by the drift movement of a few carriers. Because the reverse current is very small, the diode is in the cut-off state. This reverse current is also called reverse saturated current or leakage current. The reverse saturated current of the diode is greatly affected by temperature. Generally, the reverse current of silicon tube is much smaller than that of germanium tube. The reverse saturation current of low power silicon tube is in the order of nA and that of low power germanium tube is in the order of Mu A. As the temperature increases, the number of minority carriers increases and the reverse saturated current increases.
When the applied reverse voltage exceeds a certain value, the reverse current will suddenly increase. This phenomenon is called electrical breakdown. The critical voltage that causes electrical breakdown is called the diode reverse breakdown voltage. The diode loses its unidirectional conductivity when it breaks down. If the diode is not overheated due to electrical breakdown, the unidirectional conductivity may not be permanently destroyed. After removing the applied voltage, the performance of the diode can be restored, otherwise the diode will be damaged. Therefore, excessive reverse voltage applied to the diode should be avoided.
Diode is a kind of two-terminal device with single conductivity, which is divided into electronic diode and crystal diode. Because of the heat loss of filament and the lower efficiency than crystal diode, electronic diode is seldom seen now. Crystal diode is more common and commonly used. Semiconductor diodes are used in almost all electronic circuits because of their unidirectional conductivity. They play an important role in many circuits. They are one of the earliest semiconductor devices and are widely used.
Diode tube voltage drop: silicon diode (non-luminescent type) forward tube voltage drop 0.7V, germanium tube forward tube voltage drop 0.3V, LED forward tube voltage drop will vary with different luminescent color. There are three main colours. The specific reference values of voltage drop are as follows: red LED voltage drop is 2.0-2.2V, yellow LED voltage drop is 1.8-2.0V, green LED voltage drop is 3.0-3.2V, and normal rated current is about 20mA.
The voltage of the diode is not linear with the current, so when different diodes are connected in parallel, the corresponding resistance should be connected.
Like PN junctions, diodes have unidirectional conductivity. Typical Voltammetry of Silicon Diode
Characteristic curve (figure). When the voltage of the diode is small, the current is very small; when the voltage exceeds 0.6V, the current increases exponentially, which is usually called the opening voltage of the diode; when the voltage reaches about 0.7V, the diode is in a fully conducting state, which is usually referred to as the conducting voltage of the diode, which is expressed by the symbol UD.
For germanium diodes, the turn-on voltage is 0.2V and the turn-on voltage UD is about 0.3V. A reverse voltage is added to the diode. When the voltage value is small, the current is very small, and its current value is reverse saturated current IS. When the reverse voltage exceeds a certain value, the current begins to increase sharply, which is called reverse strike.