diode - c Copyright 2010. W. Marshall Leach, Jr.,...

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c ° Copyright 2010. W. Marshall Leach, Jr., Professor, Georgia Institute of Technology, School of Electrical and Computer Engineering. The Diode Basic Operation The diode is fabricated of a semiconductor material, usually silicon, which is doped with two impurities. One side is doped with a donor or n-type impurity which releases electrons into the semiconductor lattice. These electrons are not bound and are free to move about. Because there is no net charge in the donor impurity, the n-type semiconductor is electrically neutral. The other s ideisdopedw ithanacceptororp-typeimpur itywhich imparts free holes into the lattice. A hole is the absence of an electron which acts as a positive charge. The p-type semiconductor is also electrically neutral because the acceptor material adds no net charge. Figure 1(a) illustrates the cross section of the diode. The junction is the dividing line between the n-type and p-type sides. Thermal energy causes the electrons and holes to move randomly. Electrons di f use across the junction into the p-type side and holes di f use across the junction into the n-type side. This causes a net positive charge to develop in the n-type side and a net negative charge to develop in the p-type side. These charges set up an electric f eld across the junction which is directed from the n-type side to the p-type side. The electric f eld opposes further di f usion of the electrons and holes. The region in which the electric f eld exists is called the depletion region. There are no free electrons or holes in this region because the electric f eld sweeps them out. Figure 1: (a) Diode cross section. (b) Reverse biased diode. (c) Forward biased diode. Figure 1(b) shows the diode with a battery connected across it. The polarity of the battery is such that it reinforces the electric f eld across the junction causing the depletion region to widen. The positive terminal pulls electrons in the n-type side away from the junction. The negative terminal pulls holes in the p-type side away from the junction. No current can F ow. The diode is said to be reverse biased. Figure 1(c) shows the diode with the battery polarity reversed. The
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This note was uploaded on 05/26/2011 for the course ECE 3050 taught by Professor Hollis during the Summer '08 term at Georgia Institute of Technology.

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diode - c Copyright 2010. W. Marshall Leach, Jr.,...

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