Video Transcript
Fill in the blanks. A p–n junction consists of two
different types of semiconductors: a p-type semiconductor and an n-type
semiconductor. The p-type semiconductor has blank
free electrons than a pure semiconductor, and the n-type semiconductor has blank
free electrons than a pure semiconductor. Where the p-type and n-type
semiconductors are joined, electrons from the blank semiconductor recombine with
electron holes from the blank semiconductor, creating a region between the two
materials which acts as a or an blank. (A) More, fewer, n-type, p-type,
conductor. (B) Fewer, more, p-type, n-type,
conductor. (C) Fewer, more, n-type, p-type,
insulator. (D) More, fewer, n-type, p-type,
insulator. Or (E) fewer, more, p-type, n-type,
insulator.
In this question, we need to recall
some information about p-type and n-type semiconductors, as well as the effects of
placing them in contact with each other to create a p–n junction.
A p–n junction is what gives some
diodes their specialized electrical ability to allow a current through it in one
direction but not in the opposite direction. Interestingly, understanding this
will allow us to fill in the final blank of the question first. To fill in this blank, we need to
recognize whether the region between the two semiconductors, which refers to the p–n
junction, acts as a conductor or insulator.
Recall that a conductor allows for
charge to flow freely, while an insulator does not. So, in order to allow the flow of
charge in one direction, but not in the other direction, which is what a diode does,
the p–n junction must act as an insulator in some ways.
We can think about it like
this. If we were to attempt to create a
diode from only one type of semiconductor, either n-type or p-type, it would allow
for the flow of charge in both directions across the component. This is because a doped
semiconductor is made of a substance that has been altered to increase its ability
to conduct electricity. So, to create an actual functional
diode through which current can only pass in one direction, some insulating
properties must be introduced, which is done by forming a p–n junction.
Thus, we know that the final blank
should be filled in with the word insulator. So we’re able to eliminate options
(A) and (B) since they say conductor.
Now let’s return to the first two
blanks of the question. Here it says, “The p-type
semiconductor has blank free electrons than a pure semiconductor, and the n-type
semiconductor has blank free electrons than a pure semiconductor.” The options for either of these
blanks are more or fewer. Thus, we need to consider the
amount of free electrons in a p-type semiconductor and n-type semiconductor as
compared to a pure semiconductor.
Recall that in a pure
semiconductor, the amount of free electrons is equal to the amount of vacancies, or
electron holes. We can choose to dope a pure
semiconductor to become either p-type or n-type, depending on the kind of impurity
used for doping. Then, once a semiconductor is
doped, the number of free electrons and electron holes are no longer equal. This is because the process of
doping introduces either more free electrons or holes to the semiconductor
lattice.
Recall that the amount of free
electrons in a semiconductor is represented by the letter 𝑛. The amount of electron holes in a
semiconductor is represented by the letter 𝑝. This is where the names n-type and
p-type come from.
In a p-type semiconductor, more
electron holes have been introduced, which increases the value of 𝑝. Increasing the amount of holes
decreases the amount of free electrons. Thus, a p-type semiconductor has
fewer free electrons than a pure semiconductor, so we should fill in the first blank
with the word fewer.
In an n-type semiconductor, more
free electrons have been introduced, which increases the value of 𝑛. Thus, an n-type semiconductor has
more free electrons than a pure semiconductor. So we should fill in the second
blank with the word more. With these additions, we know that
option (D) is incorrect, so let’s eliminate it.
So it comes down to the third and
fourth blanks. Here it says, “Where the p-type and
n-type semiconductors are joined, electrons from the blank semiconductor recombine
with electron holes from the blank semiconductor.” And the options for either blank
are p-type and n-type. To fill in these blanks, we can use
the information that we recalled in order to fill in the first and second
blanks.
Here we have free electrons
recombining with electron holes, or vacancies. We need to recognize which type of
semiconductor provides either of these charge carriers. Logically, because the n-type
semiconductor has an abundance of free electrons, it makes sense that the n-type
semiconductor provides the majority of free electrons for recombination. Thus, for the third blank, we
should write n-type.
Likewise, because the p-type
semiconductor has an abundance of vacancies, it makes sense that the p-type
semiconductor provides the majority of vacancies for recombination. Thus, for the fourth blank, we
should write p-type.
These additions allow us to
eliminate option (E). This leaves us with option (C),
which corresponds to how we chose to fill in all the blanks.
Thus, option (C) is correct. A p–n junction consists of two
different types of semiconductors: a p-type semiconductor and an n-type
semiconductor. The p-type semiconductor has fewer
free electrons than a pure semiconductor, and the n-type semiconductor has more free
electrons than a pure semiconductor. Where the p-type and n-type
semiconductors are joined, electrons from the n-type semiconductor recombine with
electron holes from the p-type semiconductor, creating a region between the two
materials which acts as an insulator.
