Semiconductors and doping

Semiconductors form the basis for all modern electronics, with the resistors, capacitors and -- to a much lesser extent -- inductors that we have studied previously playing supporting roles. Semiconductors are used to make transistors, which are the building blocks of modern electronics. For example, the main processor chip in a personal computer contains a few million transistors. The basic physics of these transistors will be described shortly.

Semiconductors are materials that are neither conductors nor insulators. Rather they have resistivities intermediate between those of conductors like copper and insulators like plastic. The semiconductors used in practical devices are made mainly from silicon or germanium that has been "doped" by the addition of a small amount of an impurity. If the impurity adds some extra electrons that are free to move in the material, it is called n-type semiconductor because of the negative charge of the electrons. If the impurity removes some electrons, it creates "holes" in the crystal lattice: unoccupied places that could accommodate an electron. The holes are positively charged because an electron is missing from them. Holes can move through the crystal, since a nearby electron can jump into a hole, fillng it up, but leaving a new hole nearby. Such materials are called p-type semiconductors, because it is postive charge that can move freely.

For instance if a phosphorus atom is introduced into a silicon crystal, the phosphorous atom will shed one of the electrons from its outer shell in order to fit in with the silicon lattice. This electron is available to slide through the material, carrying current. This is an example of n-type doping. If on the other hand a boron atom is introduced into the silicon crystal, the boron site will grab an electron out of the lattice to fit neatly into its atomic structure. The site of the missing electron can move from one place to another in the crystal, forming a current of effectively positive charge. This is an example of p-type doping.

Useful devices are built by introducing both types of doping into a single piece of silicon, so that p-type and n-type regions are formed side by side, as is discussed in the following sections.

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