Doping in Semiconductors: Introduction

Doping is the process of intentionally introducing impurities into a semiconductor crystal to modify its electrical properties. By doping a semiconductor, we can change its conductivity, making it either p-type or n-type.

P-type Semiconductor

A p-type semiconductor has a predominance of positive charge carriers known as holes. This is achieved by doping the semiconductor crystal with impurity atoms that have fewer valence electrons than the host semiconductor material.

Doping with Foreign Atoms

To obtain a p-type semiconductor germanium crystal, it must be doped with foreign atoms. These foreign atoms are also known as dopants. The dopants used for p-type doping have three valence electrons, which is one less than the four valence electrons of germanium.

Explanation of the Correct Answer

The correct answer is option 'B', which states that the valency of the foreign atoms used for p-type doping in germanium is 3. Let's understand why.

Germanium belongs to Group 14 of the periodic table, which means it has four valence electrons in its outer shell. To create a p-type semiconductor, we need to introduce impurities that have fewer valence electrons. This creates a deficiency of electrons, resulting in the formation of holes, which act as positive charge carriers.

To achieve this, we need to select dopants that have three valence electrons. These dopants replace some of the germanium atoms in the crystal lattice. The presence of the dopant atoms with three valence electrons creates an excess of holes in the crystal, making it p-type.

Common dopants used for p-type doping in germanium include boron (B), aluminum (Al), and gallium (Ga). These elements have three valence electrons, which makes them suitable for p-type doping.

Conclusion

To obtain a p-type semiconductor germanium crystal, it must be doped with foreign atoms that have three valence electrons. This creates a deficiency of electrons, resulting in the formation of holes and the predominance of positive charge carriers. Common dopants used for p-type doping in germanium include boron, aluminum, and gallium.