Video Transcript
Which of the following best
completes the sentence? The addition of impurities composed
of antimony atoms to a pure semiconductor crystal increases the number of blank. (A) bound electrons, (B) free
electrons, (C) holes, (D) all of the above.
Here, we’ve been asked to recognize
an effect of adding impurities to a pure semiconductor. This process is called doping. And in this question, we’re
considering doping a semiconductor with antimony atoms. To consider the effects of doping
with antimony atoms, let’s start by recalling some basic properties of
semiconductors. First, recall that a semiconductor
conducts electricity by means of free charge carriers: free electrons and electron
holes, or vacancies.
Increasing the amount of either
type of charge carrier increases the conductivity of a semiconductor. One way we can do this is through
doping. To dope a semiconductor, we add
impurity atoms with either three or five outermost electrons to a pure semiconductor
lattice. Recall that an atom with three
outermost electrons is referred to as “trivalent” and an atom with five outermost
electrons is referred to as “pentavalent.”
The effects of doping depend on the
type of dopant used. For instance, trivalent dopant
atoms are often called acceptor ions, since they accept an electron from the lattice
into their outermost electron shell. This increases the number of holes,
or vacancies, in the lattice. On the other hand, pentavalent
dopant atoms are often called donor ions, since they essentially donate free
electrons to the lattice. This increases the number of free
electrons in the lattice.
Now, the key to answering this
question is recognizing that a neutral antimony atom has five electrons in its
outermost shell, so antimony is pentavalent. We′ve already established that
doping with pentavalent atoms increases the number of free electrons in the
semiconductor, so it seems that (B) is the correct answer. To be sure, though, let’s
double-check the other answer choices.
Option (A) suggests that doping
with antimony would increase the number of bound electrons in the semiconductor. While this is technically true,
there’s no reason that we would want or need to purposefully increase the number of
bound electrons in a semiconductor, since bound electrons themselves do not conduct
electricity. Since we’re not really concerned
with the number of bound electrons in a semiconductor, option (A) is not the best
answer choice.
Option (C) suggests that doping
with antimony would increase the number of holes. This isn’t true. We saw earlier that when we dope a
semiconductor to increase the number of holes in the lattice, we should use a
trivalent impurity, which antimony is not. Option (C) is therefore
incorrect.
Based on our answers to options (A)
and (C), option (D) cannot be correct either. This leaves only option (B), which
is the correct answer. The addition of impurities composed
of antimony atoms to a pure semiconductor crystal increases the number of free
electrons.
