Video Transcript
The diagram below shows only the
mobile charges in a semiconductor diode. Which of the following statements
is correct? (A) The n-side of the diode
contains equal negative charge to the p-side of the diode. (B) The p-side of the diode
contains greater negative charge than the n-side of the diode. (C) The n-side of the diode
contains greater negative charge than the p-side of the diode. (D) There is not enough information
to determine which side has greater negative charge.
The question is asking us how the
negative charges of the p-side of the diode and the n-side of the diode compare. The diagram shows many free
electrons on the n-side and many vacancies on the p-side. Free electrons are negatively
charged. Vacancies are actually not charged,
as they are not particles but just unoccupied positions in a shell of an atom that
an electron could fill and so be bound to the atom.
If this question is answered only
based on what is shown in the diagram, the n-side contains greater negative charge
than the p-side. This, however, is incorrect, as the
diagram does not show all the charged objects that are present in the p-side and the
n-side. Free electrons in the n-side of a
semiconductor diode are mainly produced by doping intrinsic semiconducting material
in the n-side with donor atoms. The donor atoms become positively
charged when they are added to the intrinsic semiconductor, as one electron from an
atom becomes a free electron. The donor atom so becomes a
positively charged ion.
For the p-side of a semiconductor
diode, donor atoms of a different element are added. These atoms have the opposite
effect to the donor atoms added to the n-side. A donor atom added to the p-side
removes a free electron from an intrinsic semiconductor and, in the process,
produces a vacancy. As a vacancy is produced, the donor
atom becomes a negatively charged ion. We see then that to know which side
of the semiconductor diode has more negative charge, we would need to compare the
concentration of free electrons on the n-side to the concentration of negative ions
on the p-side.
The concentrations of free
electrons and negative ions depend on the level of doping of each side of the
diode. It is not necessarily the case that
the concentration of dopants is the same in the n-side and the p-side. Therefore, we do not have enough
information to compare the concentrations of free electrons on the n-side and
negative ions on the p-side. That means we can identify the
correct answer as option (D). There is not enough information to
determine which side has greater negative charge.
