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Electrical Conduction in Semiconductors
17 questions By Tony R. Kuphaldt
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Question 13 of 17
In extrinsic semiconductors, what are majority carriers and how do they differ from minority carriers?
Reveal answer“Majority carriers” are those charge carriers existing by the purposeful addition of doping elements to the material. “Minority carriers” are the opposite type of charge carrier, inhabiting a semiconductor only because it is impossible to completely eliminate the impurities generating them.
Notes:We speak of pure semiconductor materials, and of “doping” pieces of semiconductor material with just the right quantity and type(s) of dopants, but the reality is it is impossible to assure perfect quality control, and thus there will be other impurities in any semiconductor sample.
Ask your students to specifically identify the majority and minority charge carriers for “P” and “N” type extrinsic semiconductors. In each case, are they electrons, or holes?
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Question 14 of 17
What effect does temperature have on the electrical conductivity of a semiconducting material? How does this compare with the effect of temperature on the electrical conductivity of a typical metal?
Reveal answerSemiconducting materials have negative temperature coefficient of resistance (α) values, meaning that their resistance decreases with increasing temperature.
Notes:The answer to this question is a short review on temperature coefficients of resistance (α), for those students who may not recall the subject from their DC circuit studies. As always, though, the most important point of this question is why conductivity increases for semiconductors. Ask your students to relate their answer to the concept of charge carriers in semiconducting substances.
An interesting bit of trivia you could mention to your students is that glass - normally an excellent insulator of electricity - may be made electrically conductive by heating. Glass must be heated until it is red-hot before it becomes really conductive, so it is not an easy phenomenon to demonstrate. I found this gem of an experiment in an old book:
Demonstration Experiments in Physics, first edition (fourth impression), copyright 1938, by Richard Manliffe Sutton, Ph.D.
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Question 15 of 17
Explain what the Fermi level is for a substance.
Reveal answerThe “Fermi level” is the highest energy level that electrons will attain in a substance at a temperature of absolute zero.
Notes:It is sometimes helpful to use analogies for illustrative purposes. An analogy for Fermi level is to imagine a pot of boiling water, where water molecules represent electrons and height represents energy level. Under ambient temperature conditions, there are many water molecules (electrons) leaving the liquid surface, and some that are returning to it. Cool the pot below the boiling point, however, and all the water molecules return to the liquid where the uppermost level represents the Fermi level in a substance.
