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Year 12 Science

Semiconductors and Electronics

Understand band theory for conductors, insulators, and semiconductors. Explore how doping creates n-type and p-type semiconductors, and learn how p-n junctions form the basis of diodes.

Band Theory

In solids, atomic energy levels merge into continuous energy bands. The valence band contains the outermost electrons, while the conduction band is where electrons can move freely and conduct electricity. The energy gap between them is the band gap.

Material Classification by Band Gap

Conductor

e.g., Copper, Silver

Conduction Band

No gap / Overlapping

Valence Band

Semiconductor

e.g., Silicon, Germanium

Conduction Band

Small gap (~1 eV)

Valence Band

Insulator

e.g., Diamond, Glass

Conduction Band

Large gap (>5 eV)

Valence Band

Key point: Semiconductors have a small band gap. At absolute zero, they behave as insulators. At room temperature, some electrons gain enough thermal energy to jump to the conduction band. Their conductivity increases with temperature (opposite to metals).

Doping: n-Type and p-Type Semiconductors

Doping is the deliberate addition of impurity atoms to a pure (intrinsic) semiconductor to increase its conductivity. This creates an extrinsic semiconductor with either excess electrons (n-type) or excess "holes" (p-type).

Types of Doping

n-Type (Negative)

  • • Group V element added (e.g., phosphorus, arsenic)
  • • 5 valence electrons -- 4 bond with Si, 1 is free
  • • Extra electrons are the majority carriers
  • • Dopant atom is a donor

p-Type (Positive)

  • • Group III element added (e.g., boron, gallium)
  • • 3 valence electrons -- creates a hole (missing electron)
  • • Holes are the majority carriers
  • • Dopant atom is an acceptor

Important Clarification

n-type is not negatively charged overall. The material is still electrically neutral -- it simply has more free electrons available as charge carriers. The extra electrons come from neutral donor atoms that become positive ions.

p-type is not positively charged overall. It is also neutral -- it simply has more "holes" (positive charge carriers). Holes are filled by neighbouring electrons, making holes appear to move through the lattice.

The p-n Junction and Diodes

When p-type and n-type semiconductors are joined, a p-n junction forms. At the junction, electrons from the n-side diffuse into the p-side and combine with holes, creating a depletion zone -- a region with no free charge carriers and an internal electric field that opposes further diffusion.

Diode Biasing

Forward Bias

Positive terminal connected to p-side, negative to n-side. Depletion zone narrows, current flows. The diode conducts.

Reverse Bias

Positive terminal connected to n-side, negative to p-side. Depletion zone widens, very little current flows. The diode blocks.

Applications of Semiconductor Devices

  1. 1. Diodes: Allow current to flow in one direction only. Used in rectifiers (AC to DC conversion).
  2. 2. LEDs: Light-emitting diodes produce light when electrons recombine with holes across the junction.
  3. 3. Solar cells: p-n junctions that convert light energy into electrical energy (photovoltaic effect).
  4. 4. Transistors: Two p-n junctions (npn or pnp) used as switches or amplifiers -- the basis of all modern computing.

Key Vocabulary

Band Gap

The energy difference between the top of the valence band and the bottom of the conduction band. Determines whether a material is a conductor, semiconductor, or insulator.

Doping

The intentional introduction of impurity atoms into a pure semiconductor crystal to modify its electrical conductivity by creating excess electrons (n-type) or holes (p-type).

Depletion Zone

The region at a p-n junction that is depleted of free charge carriers due to electron-hole recombination. It creates an internal electric field that opposes further diffusion.

Diode

A semiconductor device with a p-n junction that allows current to flow in one direction (forward bias) and blocks it in the other (reverse bias).

Worked Examples

1

Explain why the conductivity of a semiconductor increases with temperature, while that of a metal decreases.

Semiconductor: At higher temperatures, more electrons gain enough thermal energy to jump from the valence band across the small band gap to the conduction band. More charge carriers = higher conductivity.

Metal: Metals already have free electrons in the conduction band. Higher temperature increases lattice vibrations, which scatter electrons more, increasing resistance and decreasing conductivity.

2

Phosphorus (Group V) is added to pure silicon. Describe the type of semiconductor formed and identify the majority charge carrier.

Step 1: Silicon has 4 valence electrons. Phosphorus has 5 valence electrons.

Step 2: Four of phosphorus's electrons bond with neighbouring silicon atoms. The fifth electron is weakly bound and easily freed into the conduction band.

Answer: This creates an n-type semiconductor. The majority charge carriers are electrons. Phosphorus is the donor impurity.

3

Explain why a p-n junction diode allows current to flow in forward bias but not in reverse bias.

Forward bias: The external voltage pushes electrons in the n-region and holes in the p-region towards the junction. The depletion zone narrows and eventually allows charge carriers to cross, enabling current to flow.

Reverse bias: The external voltage pulls electrons and holes away from the junction. The depletion zone widens, increasing the barrier to charge flow. Only a tiny leakage current flows due to minority carriers. The diode effectively blocks current.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

A semiconductor differs from an insulator primarily because a semiconductor has:

Question 2

Adding boron (Group III) to silicon creates:

Question 3

In a p-n junction, the depletion zone is a region where:

Question 4

A diode is forward biased when:

Question 5

Solar cells convert light energy to electrical energy using:

Key Concepts Summary

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