Extrinsic Semiconductors

Introduction

Extrinsic semiconductors are created by intentionally adding impurity atoms (dopants) to intrinsic semiconductors. This process, called doping, dramatically increases conductivity by introducing either extra electrons (N-type) or extra holes (P-type). Doping is the fundamental technique that makes practical semiconductor devices possible.

N-Type Semiconductor

Donor Doping

Adding Group V elements (5 valence electrons) to silicon creates N-type material:

Energy Band Diagram (N-type)

Carrier Concentrations in N-type

P-Type Semiconductor

Acceptor Doping

Adding Group III elements (3 valence electrons) creates P-type material:

Energy Band Diagram (P-type)

Carrier Concentrations in P-type

Comparison Table: N-type vs P-type

Fermi Level Position

N-type

P-type

Compensation Doping

When both donors and acceptors are present:

Numerical Examples

Example 1: N-type Silicon

Problem: Silicon is doped with phosphorus at a concentration of 10¹⁶/cm³. Find the electron and hole concentrations, conductivity, and Fermi level position at 300K.

Solution:

Step 1: Carrier concentrations

Step 2: Verify n >> p

Step 3: Conductivity

Step 4: Fermi level position

Example 2: P-type Silicon

Problem: Silicon is doped with boron at 5×10¹⁷/cm³. Calculate resistivity and minority carrier concentration.

Solution:

Example 3: Compensated Semiconductor

Problem: A silicon sample has Nd = 3×10¹⁶/cm³ and Na = 1×10¹⁶/cm³. Determine the type and carrier concentrations.

Solution:

Temperature Effects on Extrinsic Semiconductors

Three regions:

1. Freeze-out (low T): Not enough thermal energy to ionize all donors
2. Extrinsic (normal operating T): All donors ionized, n ≈ Nd
3. Intrinsic (high T): Thermal generation dominates, n ≈ nᵢ >> Nd

Interview Questions

1. Why does doping make a semiconductor more conductive?

Doping introduces energy levels very close to the band edges (within ~0.05 eV). These levels are easily ionized at room temperature, providing orders of magnitude more carriers than thermal generation across the full band gap.

1. Is a doped semiconductor electrically charged?

No. A doped semiconductor remains electrically neutral overall. Each donor atom contributes one free electron AND one fixed positive ion. Each acceptor contributes one hole AND one fixed negative ion. Total positive charge = total negative charge.

1. What is the freeze-out temperature and why does it matter?

Below the freeze-out temperature, dopant atoms are not fully ionized because thermal energy (kT) is insufficient compared to ionization energy. Device operation becomes unreliable. For Si with phosphorus doping, freeze-out occurs below ~50K.

1. Explain the mass action law and its significance.

np = nᵢ² states that increasing one carrier type necessarily decreases the other. This is because adding electrons provides more partners for recombination, reducing hole lifetime. This relationship is fundamental to junction behavior.

1. How does doping level affect the Fermi level position?

Higher N-type doping pushes Eᶠ closer to Ec (more electrons at higher energies). Higher P-type doping pushes Eᶠ closer to Ev. The Fermi level position directly indicates the majority carrier concentration.

Summary

Extrinsic semiconductors form the basis of all practical semiconductor devices. N-type doping (Group V atoms) provides excess electrons, while P-type doping (Group III atoms) provides excess holes. The mass action law ensures that increasing one carrier type reduces the other, enabling controlled junction behavior that underlies diodes, transistors, and integrated circuits.