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

Diffusion, How Do You Take Your Coffee?


By: Dan Mepham

Let’s start with two individual pieces of silicon, each doped to a fairly heavy concentration. The first is a p-type sample, and the second is an n-type sample. As a refresher, we’ll review the properties of each. The p-type material is doped with elements containing only three valence electrons, and thus has an abundance of free holes which can aid in conduction. From this point forward, think of a ‘hole’ as serving the exact same function as a free electron (i.e. aiding in conduction), but doing so in the opposite manner. Instead of a negative charge, it has a positive charge, and it moves in the opposite direction of the free electrons. The n-type material, in contrast, is doped with materials with five valence electrons, resulting in an abundance of free electrons. The impurity atoms in the p-type material are known as acceptor atoms, because they create a free hole (or spot to accept an electron), while the impurity atoms in the n-type material are called donor atoms, because they donate a free electron.

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Before we combine our two samples to form a pn junction, we’ll pause for a moment to briefly discuss the concept of diffusion. Diffusion says, simply, that molecules will tend to flow from areas of high concentration to areas of low concentration (assuming they’re free to move). That is, if we had a high concentration of one type of molecule or atom in one corner of a room, those molecules would tend to want to spread out, or diffuse from being confined in the corner to being dispersed throughout the whole room. This does not apply only to molecules floating in the air; consider what happens when you add cream to a cup of coffee. The cream molecules, as opposed to staying confined together where you poured them, will spread out, and diffuse throughout the liquid. If you leave it long enough, you’ll be left with a uniform liquid -- the cream molecules have equally distributed themselves throughout the entire cup.

The concept of diffusion holds whenever molecules are free to move about, and applies to subatomic particles like electrons and holes just as it does to atoms and molecules. You may now see where we’re going with this explanation. We’re now ready to combine our p- and n-type samples.



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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