This chapter introduced you to a representative selection of solid-state devices that have special
properties. The basic operating principles of the devices discussed in this chapter are summarized in the
following paragraphs for you to use as a review and a future reference.
The ZENER DIODE is a PN junction that is designed to operate in the reverse-bias breakdown
mode. When the applied voltage reaches the breakdown point, the Zener diode, for all practical purposes,
becomes a short circuit. The reverse bias and breakdown mode of operation cause the Zener diode to
conduct with (in the direction of) the arrow in the symbol as shown.
Two theories are used to explain the breakdown action of Zener diodes. The ZENER EFFECT
explains the breakdown of diodes below 5 volts. The heavy doping used in these diodes allows the
valence band of one material to overlap the energy level of the conduction band of the other material.
This situation allows electrons to tunnel across the PN junction at the point where the two energy bands
overlap. Zener diodes that operate above 5 volts are explained by the AVALANCHE EFFECT in which
free electrons colliding with bound electrons cause an ever-increasing number of free current carriers in a
multiplying action. The Zener diode is used primarily as a voltage regulator in electronic circuits.
The TUNNEL DIODE is a heavily doped PN junction that exhibits negative resistance over part of
its range of operation, as can be seen in the curve in the illustration. The heavy doping causes the tunnel
diode to have a very narrow depletion region and also causes the valence band of one of the
semiconductor materials to overlap the energy level of the conduction band of the other semiconductor
material. At the energy overlap point, electrons can cross from the valence band of one material to the
conduction band of the other material without acquiring any additional energy. This action is called
tunneling. Tunnel diodes are used as amplifiers, oscillators, and high-speed switching devices.