N-Type Semiconductor Formation

Semiconductors like Silicon (\(\text{Si}\)) are materials whose electrical conductivity lies between that of conductors and insulators. Their conductivity can be significantly altered by adding a small amount of impurities in a process called doping. When a semiconductor is doped, its electrical properties change, leading to either an n-type or a p-type semiconductor.

An n-type semiconductor is formed when the majority charge carriers are electrons. The 'n' stands for negative, indicating the presence of excess negative charge carriers (electrons). This is achieved by doping a pure (intrinsic) semiconductor with specific types of impurities.

Doping Silicon with Phosphorous for N-Type Semiconductors

Silicon (\(\text{Si}\)) is a Group 14 element in the periodic table, meaning it has 4 valence electrons. In its pure crystalline form, each Silicon atom forms four covalent bonds with its neighboring Silicon atoms, resulting in no free electrons for conduction at room temperature.

To create an n-type semiconductor, Silicon is doped with a pentavalent impurity. A pentavalent impurity is an element from Group 15 of the periodic table, possessing 5 valence electrons. Common pentavalent impurities include:

• Phosphorous (\(\text{P}\))
• Arsenic (\(\text{As}\))
• Antimony (\(\text{Sb}\))

When an atom of a pentavalent impurity like phosphorous is introduced into the Silicon crystal lattice, four of its five valence electrons form covalent bonds with the four surrounding Silicon atoms. The fifth valence electron is left unbound and becomes a free electron. This free electron is loosely bound to the phosphorous atom and can easily move through the crystal lattice, becoming a charge carrier.

Since each phosphorous atom donates one free electron, these impurities are also known as 'donor' impurities. The introduction of these donor impurities significantly increases the number of free electrons in the Silicon, making electrons the majority charge carriers and thus forming an n-type semiconductor.

Understanding Other Dopants and P-Type Semiconductors

Let's consider why the other options would not form an n-type semiconductor when doping Silicon:

• Boron (\(\text{B}\)), Aluminium (\(\text{Al}\)), Indium (\(\text{In}\)): These elements are from Group 13 of the periodic table and have 3 valence electrons. When a trivalent impurity (3 valence electrons) like boron, aluminium, or indium is doped into Silicon, it forms three covalent bonds with the surrounding Silicon atoms. This leaves one bond incomplete, creating a 'hole' or a vacancy for an electron. These holes can accept electrons from neighboring bonds, thereby moving through the lattice and acting as positive charge carriers.
• Semiconductors formed by doping with trivalent impurities are called p-type semiconductors (where 'p' stands for positive), as holes are the majority charge carriers. These impurities are also known as 'acceptor' impurities.

Therefore, to form an n-type semiconductor from Silicon, it must be doped with a pentavalent impurity like phosphorous.