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

Doping Continued

By: Dan Mepham

In the last section, we discussed what happens when we implant a Phosphorus atom into our sample. Now we shall consider what happens if we were to implant a Boron atom. Boron is number 5 on the periodic table, having 5 protons, and 5 electrons. Boron has only 3 valence electrons; not enough to give one to each one of the bonds with its four neighbors. The result is that one bond does not get an electron from the Boron, and is left incomplete. And what do we have when we have a bond missing an electron? We have a hole." alt="Semiconductor Physics">
Fig. 8 - Another form of impurity doping, this time with a Group III element. Having only three valence electrons - not enough to fill all four bonds - it creates an excess hole that can be used in conduction.

The rest follows from the previous section – instead of adding an extra electron, for each Boron atom we implant, we’re effectively removing an electron, or creating a hole that can be used in conduction. As with Phosphorus, we can easily implant huge numbers of Boron atoms, thus increasing the number of charge carriers exponentially. Implanting Boron atoms makes the material extrinsic.

We’ll now stop to explore some terminology and address some questions that may have arisen from the previous sections. Implanting either Phosphorus (known as a Group V element), or Boron (a Group III element) atoms creates an extrinsic material, one with many more free electrons than holes, and the other with many more holes than free electrons. These are known as n-type and p-type materials, respectively. The n- and p- stand for Negative and Positive, as the dominant charge carrier in n-type materials are the negative electrons. Conversely, the dominant charge carriers in p-type materials are the positively charged holes. The dominant carriers in each case are called the Majority Carriers, while the others are known as the Minority Carriers. In an n-type material, for example, electrons are the majority carriers, while holes are the minority carriers.

Recalling the third section of this article, we said that electrons have a higher mobility than holes. That is, they ‘move’ more easily. An interesting corollary to this assertion is the observation that, given a p-type and n-type sample, each doped to exactly the same concentration of excess carriers, the n-type sample will conduct more easily, and offer less resistance than the p-type material. Since electrons move more easily, it is easier to pass current through the sample whose majority carriers are electrons than through the sample whose majority carriers are holes.

Current technology gives us the ability to dope and re-dope materials to virtually any degree desired, and thus we have extremely precise control over the properties of the material. In addition, we have the ability to mask off and dope only specific regions of a sample. We could create, for example, a thin n-type region surrounded by a p-type region. These combinations, and specifically, pn junctions (regions where p-type and n-type materials intersect), are the basis of transistors, and the subject of the upcoming installments in our series. Stay tuned.

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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