MP Board · Class 12 · Physics · Semiconductor ElectronicsWhat is an intrinsic semiconductor? Explain how doping with trivalent and pentavalent impurities leads to the formation of p-type and n-type extrinsic semiconductors with suitable energy band diagram concepts.

Definition of Intrinsic Semiconductor\nAn intrinsic semiconductor is a pure semiconductor material in its extremely refined form, containing no notable impurities. Examples include pure crystals of Germanium (Ge) and Silicon (Si). At absolute zero temperature, an intrinsic semiconductor behaves as an insulator because all valence electrons are tightly bound in covalent bonds. At room temperature, thermal agitation breaks some covalent bonds, generating free electrons in the conduction band and holes in the valence band. However, the intrinsic conductivity is very low and practically limited for electronic devices.

Concept of Doping and Extrinsic Semiconductors\nTo improve the electrical conductivity of intrinsic semiconductors, a controlled amount of suitable impurity atoms is added to the pure crystal. This process is known as doping, and the resulting material is called an extrinsic semiconductor. Extrinsic semiconductors are broadly classified into two categories:

1. n-Type Extrinsic Semiconductor

• Formation: When a pure semiconductor (silicon or germanium, belonging to Group 14) is doped with a pentavalent impurity (such as Phosphorus, Arsenic, or Antimony belonging to Group 15), four of the five valence electrons form covalent bonds with four neighboring silicon atoms. The fifth electron is loosely bound and requires very little energy to break free.
• Charge Carriers: These impurity atoms donate extra electrons and are known as donor impurities. In an n-type semiconductor, electrons are the majority charge carriers and holes are the minority charge carriers.
• Energy Band Representation: A donor energy level is formed just below the bottom of the conduction band, making it very easy for electrons to jump into the conduction band at room temperature.

2. p-Type Extrinsic Semiconductor

• Formation: When a pure semiconductor of Group 14 is doped with a trivalent impurity (such as Boron, Indium, or Aluminum belonging to Group 13), its three valence electrons form covalent bonds with three neighboring silicon atoms. A vacancy or missing electron exists at the fourth bond location, which is known as a hole.
• Charge Carriers: The impurity atoms accept electrons from neighboring bonds to complete the structure and are called acceptor impurities. In a p-type semiconductor, holes are the majority charge carriers and free electrons are the minority charge carriers.
• Energy Band Representation: An acceptor energy level is formed just above the top of the valence band, allowing electrons from the valence band to easily jump into this acceptor level, thereby creating mobile holes in the valence band.