# 1.6 Summary

In this chapter we have examined the structure and functioning of the PN junction. A PN junction produces a depletion region which is an area devoid of free charges. This leads to an energy hill or barrier voltage, the precise value of which depends on the material used as well as other factors such as temperature. The PN junction is the basis for most diodes. Its current-voltage characteristic is described by the Shockley equation and shows a logarithmic characteristic (i.e., the voltage is proportional to the log of the current).

The terminals of a diode are identified as the anode (P material) and the cathode (N material). If a positive potential which is greater than the barrier voltage is applied from anode to cathode, the diode will conduct current. If the polarity is reversed, the diode will not conduct. Therefore a simple model of the diode is a polarity sensitive switch. Improved models include the forward barrier voltage and the bulk resistance of the diode. Another refinement includes the effect of reverse breakdown, that is, the tendency of a diode to suddenly begin conducting if the reverse-bias potential is large enough. For ordinary diodes, the reverse potential should not be allowed to reach breakdown.

Besides the common switching and rectifying diodes, other types are also available. These include the Zener which is normally used in reverse-bias mode. It is commonly used to set or limit a specific voltage. In forward-bias, a Zener behaves like an ordinary diode. LEDs produce light from an electrical input. Their forward potentials tend to be in the neighborhood of a few volts. The photodiode is the complement of the LED and produces a current or voltage that scales with incident light. The Schottky diode is notable for its fast switching speeds and low barrier potential. Finally, the varactor is used as an electrically controlled capacitance. It is used in reverse-bias mode.

1.6.1: Review Questions

1. What is a depletion region?
2. Draw and explain the energy diagram for a PN junction, including the Fermi level.
3. Describe and compare the three diode models.
4. Explain the difference between the effective DC resistance and AC resistance of a diode.
5. List some of the practical differences between switching diodes, Zener diodes and LEDs.