depletion region in the normal PN junction diode, but thicker than that in the Zener effect
diode. The thicker depletion region is achieved by decreasing the doping level from the
level used in Zener effect diodes. The breakdown is at a higher voltage because of the
higher resistivity of the material. Controlling the doping level of the material during the
manufacturing process can produce breakdown voltages ranging between 2 and 200 volts.
3-12. The mechanism of the Avalanche breakdown is different from that of the Zener
effect. In the depletion region of a PN junction, thermal energy is responsible for the
formation of electron-hole pairs. The leakage current is caused by the movement of
minority electrons that is accelerated in the electric field across the barrier region. As the
reverse voltage across the depletion region is increased, the reverse voltage eventually
reaches a critical value. Once the critical or breakdown voltage has been reached, sufficient
energy is gained by the thermally released minority electrons. This enables the electrons to
rupture covalent bonds as they collide with lattice atoms. The released electrons are also
accelerated by the electric field, resulting in the release of further electrons, and so on, in a
chain or Avalanche effect. Figure 3-3 shows this process.
3-13. For reverse voltage slightly higher than breakdown, the Avalanche effect releases
an almost unlimited number of carriers so that the diode essentially becomes a short circuit.
The current flow is limited in this region only by an external series current-limiting
resistor. Operating a diode in the breakdown region does not damage it, as long as the
maximum power dissipation rating of the diode is not exceeded. Removing the reverse
voltage permits all carriers to return to their normal energy values and velocities.
3-14. See Figure 3-4, views (A) through (E) for some of the symbols used to represent
Zener diodes. Notice that the polarity markings indicate electron flow is with the arrow
symbol instead of against it as in a normal PN junction diode. This is because breakdown
diodes are operated in the reverse-bias mode, which means that the current flow is by
minority current carriers.
3-15. Zener diodes of various sorts are used for many purposes. Their most widespread
use is as voltage regulators. Once the breakdown voltage of a Zener diode is reached, the
voltage across the diode then remains almost constant regardless of the supply voltage.
Therefore, they hold the voltage across the load at a constant level. This characteristic
makes Zener diodes ideal voltage regulators and they are found in almost all solid state
circuits in this capacity.
23 June 2005