Question

The impurity to be doped in pure germanium to make it p-type semiconductor is: A. Aluminium B. Phosphorous C. Antimony D. Arsenic

Answer 1

Hint:The materials are classified into three types based on electricity conductivity, namely, conductors, insulators and semiconductors. The semi-conductors have electric conductivity, not as high as conductors, but better than insulators. To make a semiconductor conduct electricity, either it must be heated to a higher temperature or it must be doped with some other element.

Complete answer:
The electric conductivity of a semiconductor is lesser than that of conductors but higher than the insulators. This is because the energy gap between the conduction band and the valence band is very lesser than the insulators but not overlapping as in the case of conductors.

The conduction happens when the electrons in the valence band are excited to the conduction band. This can be done in semiconductors in two ways: i) Increasing the temperature of the material ii) Adding an impure element in the lattice structure

The first type is called intrinsic semiconductor and the second type is called extrinsic semiconductor. The former is pure crystal and the latter is doped with impurities in parts per millions.

The major semiconductors viz. silicon and germanium belong to the group 14^th of the periodic table.

This pure semiconductor is doped with elements of either the 13^th group or the 15^th group of the periodic table based on the type of extrinsic semiconductor to be produced.

The major conducting entities in the semiconductors are electrons and holes. The electrons are the valence electrons of the individual atoms and holes are gaps produced in the crystal lattice due to the displacement of electrons.

Based on the concentration of electrons and holes, there are two kinds of extrinsic semiconductors, mainly P-type and N-type.

In P-type, the concentration of holes is higher than the electrons. This can be ensured by doping the pure semiconductors with an element with one less valence electron than that of the germanium i.e. the element from the 13^th group table.

The elements of the 13^th group are: Boron, Aluminium, Gallium, Indium and Tellurium.

By doping the pure germanium with one of the above elements, the p-type semiconductor is obtained.

Hence, the correct option is Option A – Aluminium.

Note:In N-type semiconductor, the concentration of electrons is higher than holes. This is obtained by doping the germanium with elements in the 15^th group elements. If you see the other options in the question viz. Phosphorous, Antimony and Arsenic, they all belong to the 15^th group of the periodic table and hence, they form N-type semiconductor.