Video Transcript
A pure semiconductor crystal is
being doped to become an n-type semiconductor. Which of the following elements can
be used for this doping process? (A) Boron, (B) aluminum, (C)
phosphorus, (D) copper.
To begin, let’s recall some
information about n-type semiconductors. First, they’re called n-type
because the density of free electrons, represented by n, is greater than the density
of vacancies. Recall that any n-type
semiconductor must contain some number of positive donor ions in the semiconductor
lattice. These are dopant atoms that were
originally neutral when added to the lattice but became positively charged ions as
they donated one of their outermost electrons to the lattice as a free electron.
To achieve this effect, a
semiconductor must be doped with pentavalent impurity atoms. Recall that a pentavalent atom is
one that has five electrons in its outermost electron shell.
So, returning to the question at
hand, we need to identify which of these elements has five outermost electrons in a
neutral atom. To help visualize this information,
for each option, let’s draw an atomic diagram showing the type of element and its
outermost occupied electron shell. Note that we don’t need to be
concerned with the inner electron shells in order to answer this question. So we won’t include them in the
diagrams.
Starting with answer choice (A), we
see that boron has three outermost electrons. So we say that it’s trivalent, not
pentavalent. Doping with trivalent atoms does
not result in an n-type semiconductor, so let’s eliminate this option.
Similarly, aluminum has three
outermost electrons, meaning it’s not pentavalent. And so we should eliminate option
(B) as well.
Next, we can see that a neutral
phosphorus atom does, in fact, have five outermost electrons. Thus, phosphorus is a pentavalent
atom. So if we use phosphorus to dope a
pure semiconductor, it will become an n-type doped semiconductor. Option (C) must be correct.
But still, let’s check out answer
choice (D). The diagram shows that copper has
only one electron occupying its outermost electron shell. Since it’s not pentavalent then, we
cannot use copper as a dopant to produce an n-type semiconductor. So (D) is incorrect.
Thus, we can be sure that option
(C) is the correct choice. Of all the elements listed here,
phosphorus is the only one that can be used as a dopant to result in an n-type
semiconductor.
