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  Semiconductor Physics, Part 3 
  May 09, 2003, 10:00am EDT 

It Works Both Ways

By: Dan Mepham

The exact same effect is observed if we consider the p-type side. The positively charged holes, as they diffuse out of the p region, leave behind negatively charged acceptor ions. As more and more holes leave, the amount of negative charge increases correspondingly, and serves to tug on the holes, pulling them back toward the p region. An equilibrium will again be reached when the rate at while holes diffuse out becomes equal to the rate at which they’re pulled back in." alt="Semiconductor Physics">
Fig. 6 - Hole drift and diffusion currents.

When we combine the two identical, but opposite effects in the form of a pn junction, we obtain something that looks as follows." alt="Semiconductor Physics">
Fig. 7 - As electron and hole drift and diffusion currents come into equilibrium, a region centered around the metallurgical junction is formed known as the depletion region.

Note the directions of the particle flow marked on the diagram. Electrons diffuse from the n region into the p region, but the electric field causes them to drift (or rather, be pulled) back into the n region. Conversely, holes diffuse from the p into the n region, but the same electric field causes them to drift back into the p region.

We’ll once again pause to discuss some terminology and properties related to the previous discussion. First, the actual, physical junction between the p- and n-type regions (the exact middle, if you will) is known as the metallurgical junction. We refer to it as the metallurgical junction simply to distinguish it in discussion from the pn junction, which is technically the whole diagram above.

Secondly, the area from which electrons and holes diffuse into the opposing material, leaving behind charged donor and acceptor atoms, is known as the depletion region. For the purposes of our review, we shall say that absolutely no recombination occurs within the depletion region. This makes sense, if we look at the image above. Consider an electron that diffuses out of the n region. When it enters the p region, it is met with a layer of negatively charged ions, and no holes (they all diffused into the n region). So there’s nothing for the electron to recombine with. Instead, it must wait until it passes through the depletion region, and into the p-type region, where it is met with a huge concentration of holes, and can recombine. The same holds true for holes; a hole that diffuses into the n region is met immediately with a region devoid of electrons, and full of positive ions. It can’t recombine, it must wait until it reaches the n-type area, with its abundance of free electrons, to do so.

1. Introduction
2. Diffusion, How Do You Take Your Coffee?
3. Putting It Together
4. It Works Both Ways
5. A Look at What's Inside
6. Breaking Down the Walls
7. Summary

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