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  Semiconductor Physics, Part 2 
  Feb 12, 2003, 07:30am EST 

Charge Carriers, Understanding Conduction

By: Dan Mepham

We’re first going to take a moment to expand upon a concept we first introduced in Part 1 of this series. In Part 1, we talked about electrons as being necessary to conduct current, remembering that current is defined as the flow of electrons. Specifically, we said that we needed electrons in the Conduction Band in order for our material to conduct. At this point in time, we’ll introduce a new term, called the Charge Carrier. Electrons are a charge carrier. In order to conduct, then, charge carriers have to be free to move through the material. Furthermore, we’ll introduce the idea of Space Charge Neutrality. Space charge neutrality is the idea that a sample of material will always contain the same total amount of charge, or in other words, the same number of electrons (remember that protons cannot move, so the only way to change the amount of charge in a substance is to add or remove electrons).

Consider a thin piece of silicon wire, having a few free electrons in its Conduction Band. We then connect it to a battery, which pushes an extra electron into one end of the wire. Space charge neutrality says that the amount of charge (i.e. number of electrons) inside the wire will remain the same; so if we push one electron into one end, another electron must be forced out of the other end. The one that emerged from the other end is not the same one we pushed in, thus we can picture the idea of free electrons as charge carriers in a chain, that are free to be pushed along from one end or another by other charge carriers. This is the concept behind electrical conduction." alt="Semiconductor Physics">
Fig. 2 - A thin sample of semiconductor material. If we force an electron in one end of the material, one must emerge from the other in order to maintain space charge neutrality. This is the process of conduction.

Two important observations follow from this example. First, if there are no free electrons inside the material, then there would be no free electron to emerge from the other end. Thus we would not be able to force an electron into one end in the first place (if one goes in, one must come out to obey space charge neutrality). This reinforces the idea that if there are no free charge carriers in the Conduction Band, the material cannot conduct; it will not allow an electron to be pushed in, thus it is an insulator. Secondly, consider the instance when the other end of the wire is not connected to anything. Then, even if there was a free charge carrier to be ‘pushed’ out, it could not be pushed out, because it has nowhere to go. Thus, again, the material would not conduct. This is an example of an open circuit, a gap in the loop preventing charge carriers from moving, thus preventing current.

Astute readers will notice that in Part 2, we have gone from defining conductive materials as those with free electrons, to those with free charge carriers. Once again, there’s a method to our madness; there’s another type of charge carrier.

1. Introduction
2. Charge Carriers, Understanding Conduction
3. Electrons and Holes
4. Excess Charge Carriers
5. Doping
6. Doping Continued
7. Summary

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