Question

What type of semiconductor will form, when a pentavalent impurity is doped in a crystal lattice of germanium?

Answer 1

Semiconductors can be dopped with trace elements of higher or lower valency.
Germanium belongs to the carbon family having four valence electrons. Each germanium is bonded to four other germanium atoms.

When germanium is doped with small amount of group -15 elements such as P, As or Sb (pentavalent) its electrical conductivity increases sharply.
When these pentavalent elements are added, four of its five valence electrons form bond with four germanium atoms. The fifth electron is free and can enhance the electrical conductivity of germanium.
Such semiconductors are called as n-type semiconductors.

At absolute zero or low temperature, the fifth electron is localized and do not conduct electricity.

At normal temperatures, some of these free electrons are easily excited into the conduction band, where they can carry current quite readily. This is called Extrinsic conduction.

Hence, option (b) is correct.