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

Putting It Together


By: Dan Mepham

We’ll now combine our p-type and n-type materials. When we do so, we create a diffusion gradient, or an unevenness in the distribution of electrons and holes. We have a huge concentration of holes on the left side, with few electrons, and a huge concentration of electrons on the right side, with few holes. It only makes sense that the holes are going to start to diffuse from the p region into the n region, and the electrons will diffuse from the n region into the p region.

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Just like a cup of coffee, if we leave the sample for a while, the holes (diffusing to the right) and electrons (diffusing to the left) will eventually disperse such that they’ve spread themselves evenly throughout the material, creating a single, uniform sample, right?

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Wrong. Let’s take a look at the process one step at a time. We’ll look at the n-type side first: When we attach the p-type region, the electrons, which are in extremely heavy concentration in the n region will want to diffuse, or spread out into the p region. And they do! But consider what happens when the first electron leaves the n region: we now have a charge imbalance. That free electron that diffused into the p region left behind its donor atom, a donor atom with fifteen protons, and, now, only fourteen electrons. That donor atom now has a positive charge. Every other electron that leaves the n region will likewise leave behind another positively charged acceptor ion.

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Fig. 4 - An electron diffuses from the n- to p-type side, leaving behind a positively-charged donor ion.

As more and more electrons leave, more and more positive charge is accumulated near the junction. As we all know, opposite charges attract, and so that immense positive charge begins to pull at the electrons. Like a powerful magnet, the positive ions start to haul some of the electrons, against their will to diffuse, back into the n region. This is called electron drift.

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Fig. 5 - As electrons diffuse into the p-type region, the positively charged ions left behind act to pull them back, causing an electron drift current in the opposite direction.

Eventually, an equilibrium will be reached whereby electrons are diffusing out of the n region, and drifting (pulled by the huge positive charge left behind) back in at exactly the same rate. The result is an apparent balance of electrons outside the n region. In other words, the electric field caused by the positive ions left behind prevents all of the electrons from diffusing out of the n region. The majority stays put; only a few are able to diffuse out at any given moment before the field strength becomes so strong that they’re pulled back.



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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