Question

A pentavalent impurity has __________ valence electrons:
A. 3
B. 4
C. 5
D. 6

Answer 1

Hint: Pentavalent impurities are added to semiconductors like Carbon, Silicon and Germanium (which have 4 valence electrons) to provide for additional free charge carriers in the form of free electrons. The word ‘Penta’ comes from the Greek language and means five in number. For example, a pentagon is a two-dimensional figure with 5 sides.

Complete Step-by-Step solution:
Semiconductors like Silicon and Germanium are the basis of semiconductor materials and devices. They have 4 electrons in their outer valence shell and thus a total of four valence electrons.
Semiconductors do not conduct electricity too well in their pure state by themselves, so they are mixed with impurities which can provide free charge carriers in the form of electrons or holes.
Pentavalent impurities are elements that have 5 electrons in their valence shells. They are mixed with the semiconductors for providing better conductivity. Four of the five valence electrons of the pentavalent element combine with the 4 valence electrons of the semiconductor and the remaining one free electron acts as a free charge carrier and aids in conduction of electricity.
Pentavalent impurities form an n-type semiconductor.
Some common pentavalent impurities are Phosphorous (P) and Arsenic (As).
Hence, pentavalent impurities have 5 valence electrons.
So, the correct option is C) 5.

Note: There are also trivalent impurities mixed with a semiconductor. They have 3 valence electrons. They combine with the four valence electrons of the semiconductor, resulting in a deficit of electrons and thus provide a positive charge carrier (a hole).
Some trivalent impurities are Gallium (Ga) and Indium (In).
Pentavalent impurities form a p-type semiconductor.
An easy way to remember which type of impurity forms which type of semiconductor is to keep in mind that the ‘n’ in n-type stands for negative ( an n-type semiconductor has an excess of negative charge carriers due to extra electrons from the pentavalent impurity)
The ‘p’ in p-type semiconductors stands for positive (a p-type semiconductor has a deficit of negative charge carriers or excess of positive charge carriers-holes- due to a deficit of electrons from the trivalent impurities).