We’ve thus far talked only of silicon, but we’re now ready to introduce another element into our studies; Phosphorus. Phosphorus is right beside Silicon on the periodic table, element number 15. As such, it has 15 protons, and 15 electrons, the key difference being that Phosphorus has 5 valence electrons (where Silicon had only 4). Suppose we took our Silicon crystal lattice, plucked out one of the Silicon atoms, and replaced it with a Phosphorus atom. We’d end up with something that looks as follows.
http://media.hardwareanalysis.com/articles/small/10653.gif" alt="Semiconductor Physics">Fig. 7 - A silicon lattice with a single impurity atom added. The Phosphorus impurity has one extra valence electron which, after all bonds are made, is free to roam about the material and aid in conduction, even without any form of carrier generation, such as the application of heat or light. It will always exist because it cannot recombine; there is no corresponding hole for it to fall back into.
The Phosphorus atom assumes the place of a Silicon atom, and makes four bonds, one with each of its neighbors. Like the Silicon it replaced, the Phosphorus must give one of its valence electrons to each of the four bonds. The difference is that Phosphorus has five
valence electrons, so after making all four bonds, it still has one left over. This remaining electron is not trapped in any bond, and is thus free to roam about the material in the Conduction Band, and participate in conduction. By implanting more Phosphorus atoms, we can create a small army of free electrons that will always be in the Conduction Band, and always available to participate in electrical conduction. Further, because the free electron did not come from excitation, it was not accompanied by the creation of a hole (it did not break out of a bond; it was never in one). As a result, the sample now has one more free electron than it does holes. This is called an Extrinsic
material (a material with more of one type of charge carrier than the other). Each Phosphorus atom added into the sample produces another free electron without generating a corresponding hole, thus making the material more and more extrinsic.
To reinforce the usefulness of this type of doping, consider the following figures: intrinsic silicon at room temperature has on the order of 1010
charge carriers per cubic centimeter available at all times, simply due to thermal generation. Using common modern technology, we can easily implant in excess of 1017
Phosphorus atoms per cubic centimeter, thereby increasing the number of charge carriers from 1010
, an increase of 10,000,000 times. Clearly, this will very drastically impact the conductivity of the sample.