Semiconductors

Classification of Semiconductors

Semiconductors are materials whose electrical conductivity lies between that of conductors and insulators. They can be classified as:

Intrinsic Semiconductors

Definition: Pure semiconductors without any impurities. Examples: Silicon (Si), Germanium (Ge).

Conduction Mechanism:

• Electrons are excited from the valence band to the conduction band via thermal or electrical energy.
• This leaves holes in the valence band.
• Both electrons and holes act as charge carriers (equal in number).
• Intrinsic conduction: Conduction is due to carriers generated inside the material.

Extrinsic Semiconductors

Semiconductors doped with impurities to increase conductivity.

Process: Doping – the addition of small amounts of impurities.

Types:

1. n-type: Doped with pentavalent atoms (e.g., Phosphorus, Arsenic).
   • Extra electron → becomes a free carrier.
   • Electrons are majority carriers, holes are minority.
   • Donor atoms donate electrons.
2. p-type: Doped with trivalent atoms (e.g., Boron, Indium).
   • Lacks one electron → creates holes.
   • Holes are majority carriers, electrons are minority.
   • Acceptor atoms accept electrons.

The p-n Junction

When p-type and n-type semiconductors are joined:

Diffusion of carriers occurs:

• Electrons from n → p
• Holes from p → n

This creates a depletion region (no free carriers).

A potential barrier forms, preventing further diffusion.

Barrier voltage:

• Germanium: ~0.2 – 0.3 V
• Silicon: ~0.6 – 0.7 V

Biasing of a p-n Junction

1. Forward Bias:
   • p-side connected to positive, n-side to negative.
   • Reduces barrier → allows large current.
2. Reverse Bias:
   • p-side to negative, n-side to positive.
   • Increases barrier → only small leakage current from minority carriers.

I-V Characteristics

1. Forward Bias:
   • Low current at low voltage.
   • After "knee" voltage (~0.7V for Si, ~0.3V for Ge), current rises sharply.
2. Reverse Bias:
   • Very small current until breakdown voltage is reached.
   • After breakdown, current rises suddenly (Zener or Avalanche breakdown).

Zener Breakdown

Occurs in Zener diodes, which are heavily doped.

Operate in reverse bias beyond Zener voltage (Vz).

A strong electric field causes electrons to tunnel through the depletion region.

Used for voltage regulation.

Key Terms

• Intrinsic semiconductor: Pure material (Si, Ge).
• Extrinsic semiconductor: Doped for higher conductivity.
• Donor: Pentavalent atom in n-type.
• Acceptor: Trivalent atom in p-type.
• Depletion layer: Region with no free charge carriers.
• Biasing: Applying voltage across a p-n junction.
• Zener diode: Operates in reverse bias for voltage regulation.