diode
In electronic components, a device with two electrodes that only allows current to flow in a single direction is often used for its rectification function. And varactor diodes are used as electronic adjustable capacitors. The current directionality possessed by most diodes is commonly referred to as the “rectification” function. The most common function of a diode is to allow current to pass only in a single direction (known as forward bias), and to block it in reverse (known as reverse bias). Therefore, diodes can be thought of as electronic versions of check valves.
Early vacuum electronic diodes; It is an electronic device that can conduct current unidirectionally. There is a PN junction with two lead terminals inside the semiconductor diode, and this electronic device has unidirectional current conductivity according to the direction of the applied voltage. Generally speaking, a crystal diode is a p-n junction interface formed by sintering p-type and n-type semiconductors. Space charge layers are formed on both sides of its interface, forming a self built electric field. When the applied voltage is equal to zero, the diffusion current caused by the concentration difference of charge carriers on both sides of the p-n junction and the drift current caused by the self built electric field are equal and in an electric equilibrium state, which is also the characteristic of diodes under normal conditions.
Early diodes included “cat whisker crystals” and vacuum tubes (known as “thermal ionization valves” in the UK). The most common diodes nowadays mostly use semiconductor materials such as silicon or germanium.

characteristic
Positivity
When a forward voltage is applied, at the beginning of the forward characteristic, the forward voltage is very small and not enough to overcome the blocking effect of the electric field inside the PN junction. The forward current is almost zero, and this section is called the dead zone. The forward voltage that cannot make the diode conduct is called the dead zone voltage. When the forward voltage is greater than the dead zone voltage, the electric field inside the PN junction is overcome, the diode conducts in the forward direction, and the current rapidly increases with the increase of voltage. Within the normal range of current usage, the terminal voltage of the diode remains almost constant during conduction, and 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 conducts in the forward direction. It is called threshold voltage or threshold voltage, which is about 0.5V for silicon tubes and about 0.1V for germanium tubes. The forward conduction voltage drop of silicon diodes is about 0.6-0.8V, and the forward conduction voltage drop of germanium diodes is about 0.2-0.3V.
Reverse polarity
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 motion of minority carriers. Due to the small reverse current, the diode is in a cut-off state. This reverse current is also known as reverse saturation current or leakage current, and the reverse saturation current of a diode is greatly affected by temperature. The reverse current of a typical silicon transistor is much smaller than that of a germanium transistor. The reverse saturation current of a low-power silicon transistor is in the order of nA, while that of a low-power germanium transistor is in the order of μ A. When the temperature rises, the semiconductor is excited by heat, the number of minority carriers increases, and the reverse saturation current also increases accordingly.

breakdown
When the applied reverse voltage exceeds a certain value, the reverse current will suddenly increase, which is called electrical breakdown. The critical voltage that causes electrical breakdown is called the diode reverse breakdown voltage. When an electrical breakdown occurs, the diode loses its unidirectional conductivity. If the diode does not overheat due to electrical breakdown, its unidirectional conductivity may not be permanently destroyed. Its performance can still be restored after removing the applied voltage, otherwise the diode will be damaged. Therefore, excessive reverse voltage applied to the diode should be avoided during use.
A diode is a two terminal device with unidirectional conductivity, which can be divided into electronic diodes and crystal diodes. Electronic diodes have lower efficiency than crystal diodes due to the heat loss of the filament, so they are rarely seen. Crystal diodes are more common and commonly used. The unidirectional conductivity of diodes is used in almost all electronic circuits, and semiconductor diodes play an important role in many circuits. They are one of the earliest semiconductor devices and have a wide range of applications.
The forward voltage drop of a silicon diode (non luminous type) is 0.7V, while the forward voltage drop of a germanium diode is 0.3V. The forward voltage drop of a light-emitting diode varies with different luminous colors. There are mainly three colors, and the specific voltage drop reference values are as follows: the voltage drop of red light-emitting diodes is 2.0-2.2V, the voltage drop of yellow light-emitting diodes is 1.8-2.0V, and the voltage drop of green light-emitting diodes is 3.0-3.2V. The rated current during normal light emission is about 20mA.
The voltage and current of a diode are not linearly related, so when connecting different diodes in parallel, appropriate resistors should be connected.

characteristic curve
Like PN junctions, diodes have unidirectional conductivity. Typical volt ampere characteristic curve of silicon diode. When a forward voltage is applied to a diode, the current is extremely small when the voltage value is low; When the voltage exceeds 0.6V, the current begins to increase exponentially, which is commonly referred to as the turn-on voltage of the diode; When the voltage reaches about 0.7V, the diode is in a fully conductive state, usually referred to as the conduction voltage of the diode, represented by the symbol UD.
For germanium diodes, the turn-on voltage is 0.2V and the conduction voltage UD is approximately 0.3V. When a reverse voltage is applied to a diode, the current is extremely small when the voltage value is low, and its 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 greatly, ranging from tens of volts to several thousand volts.

Reverse breakdown
Zener breakdown
Reverse breakdown can be divided into two types based on the mechanism: Zener breakdown and Avalanche breakdown. In the case of high doping concentration, due to the small width of the barrier region and the large reverse voltage, the covalent bond structure in the barrier region is destroyed, causing the valence electrons to break free from covalent bonds and generate electron hole pairs, resulting in a sharp increase in current. This breakdown is called Zener breakdown. If the doping concentration is low and the width of the barrier region is wide, it is not easy to cause Zener breakdown.

Avalanche breakdown
Another type of breakdown is avalanche breakdown. When the reverse voltage increases to a large value, the applied electric field accelerates the electron drift speed, causing collisions with the valence electrons in the covalent bond, knocking them out of the covalent bond and generating new electron hole pairs. The newly generated electrons holes are accelerated by an electric field and collide with other valence electrons, causing an avalanche like increase in charge carriers and a sharp increase in current. This type of breakdown is called avalanche breakdown. Regardless of the type of breakdown, if the current is not limited, it may cause permanent damage to the PN junction.