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  Semiconductor Physics, Part 1 
  Jan 21, 2003, 08:30am EST 

Before the 'How', Ask Yourself 'Why?'

By: Dan Mepham

Before we dive in, we’ll take a moment to explain exactly why we’re so interested in, and dependent on semiconductors in the first place. Semiconductors are so important to our electronic devices simply because their properties can be easily and substantially manipulated.

All materials can be arranged into three basic classes; Insulators, Conductors, and Semiconductors (we’ll leave out Superconductors for the time being, as they’re slightly outside the scope of this article). Insulators, as the name implies, are materials that do not allow the flow of electrical current. Most rubber compounds, for example, are good insulators. In contrast, conductors are materials that allow current to pass easily. Metals, such as copper, are good conductors. Semiconductors lie somewhere in between. They’re not insulators – they do conduct – but they conduct very little. So little that they are generally not useful as conductive materials in their pure form.

In effect, we can think of semiconductors as being ‘on the edge’. They lie in a grey area between conductors and insulators whose boundaries are somewhat unclear, and it is this ambiguity itself that is useful. Because they’re effectively in the middle, with only slight modification, we can cause semiconductors electrical behavior to change very dramatically. That is, with only minor alterations, we can make a semiconductor hundreds to thousands of times more or less conductive. This high order of flexibility is only possible with semiconductors (we can make metals more conductive as well, for example, but certainly not thousands of times more conductive). Using today’s technology, we can alter the properties of the material virtually atom by atom, allowing us to create the microscopic electronic devices that are so Crucial to our lives.

Now that we know why semiconductors are so useful, we can begin to explore the physics that makes it all work.

1. Introduction
2. Before the 'How', Ask Yourself 'Why?'
3. Crystal Structure, Forming the Bonds
4. Electron Energy Levels
5. Band Formation, Things Get Complicated
6. Electron Excitation, Making the Leap
7. Summary

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