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


By: Dan Mepham

Welcome to Part 3 of our series explaining the basics of semiconductor physics. If you’re still with us after the first two installments, congratulations! Granted, we’ve provided only a basic peek, but the material is nevertheless rather difficult to grasp. In fact, it takes most physics students years to develop an in-depth understanding of the concepts, so pat yourself on the back!

In Part 1, we learned some basic properties of the atom, such as their structure, and how they bond together to form crystalline semiconductor materials. We saw some of the basic mechanisms that give rise to the conductive properties of semiconductors, and how, by altering those conditions, we can in turn alter the conductivity of the material. In Part 2, we took it a step further, introducing the concept of doping. By adding impurity atoms, called dopants, we can create small armadas of free charge carriers (remember, they can be either electrons or holes), which in turn drastically alters the conductivity as well.

But for all that, we haven’t really done anything. By doping our silicon, we’ve made it much more conductive, which, as interesting as it may be, is useless in and of itself. If all we wanted was a good conductor, we might just as well have started with a piece of copper, and saved ourselves some hassle. It certainly would have been cheaper than growing a silicon crystal and doping it!

Of course, this isn’t the end of our story. In Part 2, we mentioned that current technology allows us to dope materials to very specific concentrations, in very specific regions. Our control is so precise, in fact, that we could create an ultra-thin n-type layer hundreds of times thinner than a human hair. Likewise, we could create a p-type layer just as thin. And we can create these layers virtually anywhere we want; at the edge of a sample, in the middle, and even right beside one another.

In Part 3, we’ll explore the unique behavior observed when we create a p-type and n-type layer right next to one another. In other words, we’ll explore the pn junction.

Part 1 can be found here.
Part 2 can be found here.

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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