PN Junction Diodes

A Brief Description Definitions The Depletion Region Varying VA Diode Currents
The Equation Derivations from the Ideal Let's Draw! Related Topics

The Depletion Region:

When analyzing the pn junction, we are most interested in the depletion region, or space charge region (SCR).  This is where all the interesting stuff goes on.  When we talked about band bending, one of the reasons that it occurred was because of non-uniform doping or an applied electric field.  Well, an electric field can result because of non-uniform doping, as in a pn junction, and that is why the energy band diagram for a pn junction has band bending even when it is in equilibrium.

As soon as you connect a p-type region with an n-type region, carriers will begin diffusing from regions of high concentration to regions of lower concentration.  That is, holes from the p region will diffuse to the n region, where there aren't as many holes, and electrons from the n region will diffuse to the p region where there aren't as many electrons.  As they leave, they leave behind the ionized impurities (dopants) that created them.  This migration of majority carriers stops when the electric field created by the ionized impurities causes carriers to drift back at the same rate as they diffulse away.

The space charge region is the region around the metallurgical junction where all the ionized acceptors and donors were uncovered and remain.  The number of ionized acceptors on the p-side equals the number of ionized donors on the n-side.  When we look at the depletion region like this, we ae using the depletion approximation and we are assuming that the carrier concentrations (n or p) are negligible compared to the net doping concentration (ND or NA).  We are also assuming that the charge density is zero outside the space depletion region (see definitions).

For a more visual explanation, take a look at the demo of a pn junction.  You will need shockwave plug-in for your browser in order for the demo to run.  It should run properly in the Vectras using Internet Explorer, but it will not run on the Suns.  Use the "full screen" option for better viewing.  In order to return to this page you must use the "back" button on your browser.

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