In summary, this article has discussed the following:
Current is actually defined as being positive opposite the direction of electron flow. This is nothing but an unfortunate sign error that has remained permanently.
Electrons are not the only charge carriers; holes, or open spaces in bonding sites can also be used in conduction.
While free electrons exist in the Conduction Band, holes exist in the Valence Band, at bonding sites, and are viewed as having a positive charge.
Holes do not actually move, rather other electrons move in the opposite direction. As a result, in conduction, electrons and holes move in the opposite direction.
The law of Space Charge Neutrality tells us that a material must always contain the same charge. In other words, if we add a free electron, we must remove another electron. If we create a free electron through excitation, a corresponding hole will be created, whose positive charge will cancel the electron’s negative charge.
Recombination is the process by which a free electron falls back into a hole left at a bonding site, effectively ‘destroying’ the free electron and hole. Free electrons will always recombine when they can, as doing so represents returning to a lower energy state.
Doping is the process of introducing impurities into the silicon, whose presence creates additional charge carriers to be used in conduction.
Adding Group V elements (those with 5 valence electrons), such as Phosphorus, creates an excess of free electrons in the material. These are known as n-type materials.
Adding Group III elements (those with 3 valence electrons), such as Boron, creates an excess of holes in the material. These are known as p-type materials.
Materials with more of one type of charge carrier than the other are called extrinsic. The charge carrier present in greater numbers is called the Majority Carrier. Materials with the exact same number of both types of carrier are called intrinsic.
Remember to review the material, and make sure you understand it well. Refer back to Part 1 to refresh and reinforce your understanding, as it's key to developing a firm grasp on future concepts. As well, for those of you to whom this may seem elementary, there are numerous sources of more detailed and descriptive information around the web.
We hope you've enjoyed Part 2 of our series. Stay tuned for Part 3, when we'll delve into the world of the pn junction, the most Crucial element of modern microelectronics.
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