Semiconductor Electronics: Materials, Devices And Simple Circuits 14.1 Introduction | Class 12 Physics Notes

14.4 EXTRINSIC SEMICONDUCTOR

Intrinsic semiconductors have low conductivity at room temperature, limiting their practical use in electronic devices. To enhance conductivity, small amounts of impurities (dopants) are deliberately introduced into the pure semiconductor, a process called doping. The resulting materials are called extrinsic semiconductors or impurity semiconductors.

Dopants must have atomic sizes similar to the host semiconductor atoms to fit into the crystal lattice without significant distortion. For tetravalent semiconductors like Si and Ge, two types of dopants are used:

(i) Pentavalent dopants (valency 5) such as Arsenic (As), Antimony (Sb), Phosphorus (P) create n-type semiconductors.

(ii) Trivalent dopants (valency 3) such as Indium (In), Boron (B), Aluminium (Al) create p-type semiconductors.

In n-type semiconductors, pentavalent dopant atoms replace Si or Ge atoms in the lattice. Four of their five valence electrons form covalent bonds with neighboring atoms, while the fifth electron is loosely bound and easily freed at room temperature, contributing to conduction. These dopants are called donor impurities. The ionization energy to free this electron (~0.01 eV for Ge, ~0.05 eV for Si) is much less than the intrinsic band gap energy, so doping greatly increases free electron concentration independent of temperature.

In n-type semiconductors, the majority carriers are electrons (n_e >> n_h), and holes are minority carriers. The total conduction electron concentration includes both donor electrons and thermally generated intrinsic electrons, while holes come only from intrinsic generation. Recombination reduces hole concentration further.

In p-type semiconductors, trivalent dopants replace Si or Ge atoms but have one less valence electron, resulting in a vacancy or hole in the covalent bond. This hole acts as a positive charge carrier. The dopant atom becomes negatively charged after accepting an electron from a neighboring atom. These dopants are called acceptor impurities. The holes generated by doping are majority carriers (n_h >> n_e), while electrons are minority carriers. The acceptor energy level lies slightly above the valence band, allowing electrons to jump into it with small energy, creating holes in the valence band.

The product of electron and hole concentrations in thermal equilibrium satisfies the relation:

n_e × n_h = n_i²

Doping shifts the balance of carriers, increasing majority carriers and reducing minority carriers due to recombination.

Energy band diagrams for n-type and p-type semiconductors show donor and acceptor levels near conduction and valence bands respectively.

This section explains how doping controls semiconductor conductivity and carrier type, enabling practical electronic devices.

📊 Diagram: Figure 14.7(a) shows a pentavalent donor atom doped in Si or Ge lattice forming n-type semiconductor; (b) schematic representation showing fixed positive cores and extra electrons. Figure 14.8(a) shows a trivalent acceptor atom doped in Si or Ge lattice forming p-type semiconductor; (b) schematic representation showing fixed negative cores and associated holes. Figure 14.9(a) shows energy band diagram of n-type semiconductor with donor level near conduction band; (b) shows p-type with acceptor level near valence band.

🧪 Activity: No specific activity mentioned in this section.

🔗 Connection: Prepares for understanding p-n junction formation and behavior.