The p-n junction - 25

Doping silicon with either donors or acceptors can significantly change the conductivity of the silicon, but a mere change in conductivity is not enough for the construction of logic devices.  Transistors, diode (semiconductor) lasers, and many other useful devices rely upon the p-n junction.  The concept is simple enough:  dope one region of a silicon wafer with phosphorus and an adjacent region with gallium.  The behavior this apparently simple process produces, however, is subtle yet very important to modern technology.

Consider the image shown below.  The left-hand side of the image shows an n-type semiconductor; the right-hand side shows a p-type.  This nice delineation, however, is not present in physical devices.  Objects in nature tend to diffuse.  Just as red food coloring will spread until it is rather uniformly mixed throughout a glass of water, the electrons from the n-side of this boundary want to spread throughout the entire solid.  And holes from the p-side will spread to the n-side for the same reason.  So an electron just to the left of the boundary will, in the course of random motion, enter into the p-side.  It might then combine with one of the holes in the p-side, removing in one fell swoop two charge carriers from the system.  Similarly, a hole just to the right of the boundary can diffuse over to the left side and combine with one of the conduction electrons there.  This motion of holes into the n-region and electrons into the p-region is called the diffusion current.  Click twice on the Next button in the figure below to see an animation of this effect.

After a few electrons and holes have combined, we are left with a charge separation at the boundary.  The p-side has become negatively charged, and the n-side has become positively charged.  Eventually the buildup of negative charge on the right side of the junction creates a potential barrier large enough to prevent further diffusion across the junction.  In the band diagram, this potential barrier raises the energy of the p-side of the material.  Clicking on the Next button a third time will show you this effect.  (Why does the entire side rise rather than a localized hump appearing?  Click here for the answer.)  As you view the last sequence of the animation above, keep in mind that while a barrier for electrons is represented by a rise in the energy band diagrams, a barrier for holes is represented by a dip.

The very charge separation that stops the diffusion current by the majority carriers will attract the minority carriers:  holes in the n-side are attracted to the p-side, and electrons in the p-side are attracted to the n-side.  The result is a minority current, but this current is very small due to the paucity of minority carriers.  This small minority current takes charge carriers back to their "original" sides, thus decreasing the potential barrier.  The decrease in the barrier allows diffusion current to start back up.  Thus equilibrium is established:  the occasional minority carrier slips across the barrier, but the effect is offset by the occasional majority carrier energetic enough to overcome the barrier.    The net effect is two-fold:  a potential difference arises at the boundary, similar in cause to the contact potential which develops when two different metals are joined, and a region without charge carriers, called a depletion zone, is created around the boundary.
 
 

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That's it for this Module!
The next module will explore some of the applications of pn junctions!
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Copyright © 2003-2004 Doris Jeanne Wagner and Rensselaer Polytechnic Institute.  All Rights Reserved.