Under normal conditions, there is only a small current in the reverse biased PN junction. This leakage current is kept constant until the reverse voltage exceeds a certain value. After this value, the PN junction suddenly starts to have a large current conduction (Figure 1.15). Reverse breakdown typically sets the maximum operating voltage of the solid state device. However, the reverse breakdown can be used as a very stable reference voltage if appropriate precautions are taken to limit the current.
Figure 1.15 Reverse breakdown of the PN junction diode.
One mechanism that causes reverse breakdown is avalanche multiplication. Consider a reverse biased PN junction. The depletion region widens as the bias rises, but not fast enough to prevent the electric field from strengthening. A powerful electric field accelerates some carriers through the depletion region at very high speeds. When these carriers collide with atoms in the crystal, they strike loose valence electrons and generate additional carriers. Because a carrier can generate extra thousands of carriers by impact, it is like a snowball can produce an avalanche, so this process is called avalanche multiplication.
Another mechanism for reverse breakdown is tunneling. Tunneling is a quantum mechanism process that enables particles to move a small distance without any obstacles. The tunneling current is mainly determined by the width of the depletion region and the voltage difference across the junction. The reverse breakdown caused by Tunneling is called Zener breakdown.
The reverse breakdown voltage of the junction depends on the width of the depletion region. The wider the depletion region, the higher the breakdown voltage is required. As previously discussed, the lighter the doping, the wider the depletion region and the higher the breakdown voltage. When the breakdown voltage is less than 5 volts, the depletion region is too thin, mainly Zener breakdown. When the breakdown voltage is higher than 5 volts, it is mainly avalanche breakdown. The PN diodes designed to work in the reverse-conducting state are called Zener diodes or avalanche diodes according to the dominant working mechanism. The breakdown voltage of the Zener diode is less than 5 volts, while the breakdown voltage of the avalanche diode is higher than 5 volts. Engineers often refer to them as Zener tubes regardless of how they work. Therefore, the 7V Zener tube, which relies mainly on avalanche breakdown work, may be confusing.
Figure 1.16 is all the circuit symbols discussed above. The PN junction uses a straight line to represent the cathode, while the Schottky diode and Zener diode make some modifications to the cathode end. In all of these legends, the direction of the arrows indicates the direction of the current under forward bias of the diode. In Zener diodes, this arrow can be somewhat misleading because the Zener is normally operating in a reverse biased state. For the casual observer, this symbol should be inserted next to the phrase "the direction is reversed." 
Figure 1.16 Circuit diagram symbols for PN junction, Schottky, and Zener diodes. In some circuit diagram symbols, the arrows are hollow or half arrows.
Zener tubes generally have two uses (the following IZ is the operating current, UZ is the nominal regulated voltage, UW is the actual operating voltage):1 In normal operation, it is in the "on" state, IZ≥0.1mA level, at this time the Zener tube acts as a voltage regulator, UW≈UZ.
2 "Off" state during normal operation, ie UW
Commonly used Zener tubes are divided into two categories, one is usually called "regulator tube", and the second is TVS type device. The former is usually the first usage, the latter is usually the second usage. But it is not absolute, the two are only characteristic parameters. Ordinary Zener can also be used as a protection device, but the response speed is poor, and it is not suitable for occasions where it is necessary to suppress extremely high-speed pulse interference. Several mistakes that beginners often make:
1. Think of the Zener characteristics too good: when UW7V), the curve is still okay, change a low voltage, such as 3V, then the actual curve is really "soft", there is a large current at 1.5V, until IZ Increased to tens of mA, UZ lazily reached the nominal value. 2. With the Zener tube for protection, there is a price for not knowing everything in the world. The price here is the leakage current IR (IZ in the "off" state): IR>0; the second does not understand everything in the world, there is room for it, here The room is to ensure the "cutoff" voltage margin UM: UM = UZ-UW > 0 (IR → very small); three do not understand the world is all elastic (concession), the elasticity here is the UW with the conduction state The increment of IZ increase is UP: UP=UW-UZ>0 (IR→large). And even if you leave room and pay the price, you still have to make concessions. To reduce the IR, it is necessary to increase the ΔU, that is, to select the UZ tube, but this will reduce the "strength" of the protection.
3. I don't understand the dynamic internal resistance of the Zener tube dV/dI>0, that is, UZ will increase with IZ. From these few items, some principles can be summarized:
1. Try to avoid using low pressure Zener tubes.
2. Use Zener tube for protection, choose UZ reasonably, make UWMAX+UM
3. The design circuit must have the concept of "dynamic". The circuit is the same as everyone, and all the machines are unresponsive. The only difference is "slower" and "less slow".
Zener diodes (also known as Zener diodes), a semiconductor device that has a very high resistance until the critical reverse breakdown voltage. At the critical breakdown point, the reverse resistance is reduced to a small value. In this low resistance region, the current increases and the voltage remains constant. The Zener diode is binned according to the breakdown voltage. Due to this characteristic, The Zener diode is mainly used as a voltage regulator or voltage reference component. Zener diodes can be connected in series for use at higher voltages, and more stable voltages can be obtained by series connection. A Zener diode is different from a é”— diode in that if a reverse voltage, sometimes referred to as a "bias", is added to a particular value, a slight increase in current will cause a significant increase in current. The voltage that causes this effect is called the "breakdown voltage" or "Zener voltage." The breakdown voltage of the 2DW7 tube is between 5.8 and 6.5V, and the maximum current is 30mA.
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