Zener diode technology and principle

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Zener diode technology and principle [Copy link]

Under normal conditions, only a small current flows through a reverse-biased PN junction. This leakage current remains constant until the reverse voltage exceeds a certain value, after which the PN junction suddenly begins to conduct a large current (Figure 1.15). This sudden and significant reverse conduction is called reverse breakdown, and it can cause damage to the device if there is no external measure to limit the current. Reverse breakdown usually sets the maximum operating voltage of solid-state devices. However, if proper precautions are taken to limit the current, the reverse-broken junction can serve as a very stable reference voltage. 740)this.width=740" border=undefined> Figure 1.15 Reverse breakdown of a PN junction diode. One mechanism that leads to reverse breakdown is avalanche multiplication. Consider a reverse-biased PN junction. The depletion region widens as the bias increases, but not fast enough to prevent the electric field from strengthening. The strong electric field accelerates some carriers through the depletion region at very high speeds. When these carriers collide with atoms in the crystal, they knock loose valence electrons and create additional carriers. Because one carrier can create thousands of additional carriers by knocking them out just like a snowball can create an avalanche, the process is called avalanche multiplication. Another mechanism of reverse breakdown is tunneling. Tunneling is a quantum mechanical process that allows particles to move a short distance regardless of any obstacles. If the depletion region is thin enough, carriers can jump across it by tunneling. The tunneling current depends mainly on 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. Wider depletion regions require higher breakdown voltages. As discussed previously, the lighter the doping, the wider the depletion region, and the higher the breakdown voltage. When the breakdown voltage is below 5 volts, the depletion region is too thin and Zener breakdown is dominant. When the breakdown voltage is above 5 volts, avalanche breakdown is dominant. PN diodes designed to operate primarily in the reverse conduction state are called Zener diodes or avalanche diodes, depending on which operating mechanism is dominant. Zener diodes have a breakdown voltage below 5 volts, while avalanche diodes have a breakdown voltage above 5 volts. Often engineers call them Zener diodes regardless of their operating principle. Therefore, a 7V Zener diode that operates primarily by avalanche breakdown may be confusing. In practice, the breakdown voltage of a junction depends not only on its doping characteristics but also on its geometry. The above discussion analyzes a planar junction consisting of two uniformly doped semiconductor regions intersecting in a plane. Although some real junctions approximate this ideal, most junctions are curved. The curvature intensifies the electric field and reduces the breakdown voltage. The smaller the radius of curvature, the lower the breakdown voltage. This effect has a great impact on the breakdown voltage of thin junctions. Most Schottky diodes have a significant discontinuity at the edge of the metal-silicon interface. Electric field intensification can greatly reduce the measured breakdown voltage of Schottky diodes unless special measures are taken to weaken the electric field at the edge of the Schottky barrier. Figure 1.16 shows the circuit symbols for all of the above. The PN junction has a straight line representing the cathode, while the Schottky and Zener diodes have some modifications to the cathode end. In all of these illustrations, the direction of the arrow indicates the direction of current flow when the diode is forward biased. In the case of a Zener diode, the arrow can be somewhat misleading because Zeners are usually operated in a reverse biased state. To the casual observer, the symbol should be accompanied by the phrase "reversed direction". 740)this.width=740" border=undefined> Figure 1.16 Schematic symbols for PN junctions, Schottky, and Zener diodes. Some of the schematic symbols have hollow or half arrowheads. Source: Integrated Circuit Education Network (www.ICedu.net)