Overview: Semiconductor Electronics

• Conductors have many free electrons, so electric current flows easily; insulators have almost no free electrons, so current does not flow easily.
• In conductors, resistance increases with temperature, whereas in semiconductors it decreases.
• Semiconductors have properties between conductors and insulators, and at absolute zero, they behave like insulators.

A material having a partially filled valence band (or overlapping valence and conduction bands), allowing electrons to move easily and conduct electricity.

A material in which the valence band is completely filled and the conduction band is empty, separated by a large energy gap (a few eV), so electrons cannot move freely.

A material with a small energy gap (about 1 eV) between valence and conduction bands, allowing limited conduction at room temperature.

An empty or partially filled band above the valence band in which electrons can move freely and conduct current.

OR

Conduction band is the wide range of energies possessed by the conduction band electrons. It is the lowest unfilled band, for insulators. But it is partially filled for conductors. Current conduction is due to the electrons in this band. 

The highest occupied energy band containing valence electrons.

OR

Valence band is the wide range of energies possessed by the valence electrons. Valence band is the highest energy band, occupied by the valence electrons. It is completely filled for inert gases, but partially filled for other materials. 

An energy band is the wide range of energies possessed by an electron in a solid.

• Semiconductors have a small energy gap (about 1 eV) between the valence band and conduction band.
• At absolute zero, they act like insulators because the valence band is full and the conduction band is empty.
• At room temperature, some electrons move into the conduction band, leaving holes, and both contribute to conduction; conductivity increases with temperature.

• An intrinsic semiconductor is a pure semiconductor (like silicon or germanium) without impurities.
• At low temperatures, it behaves like an insulator because all electrons are bound in covalent bonds.
• At room temperature, some bonds break and create electron–hole pairs.
• In an intrinsic semiconductor, the number of electrons equals the number of holes (ne = nh = ni).
• Its conductivity increases with temperature because more electron–hole pairs are produced.

• A p–n junction is formed by joining p-type and n-type semiconductors.

• Electrons diffuse from n → p.

• Holes diffuse from p → n.

• Recombination occurs near the junction.

• Immobile ions are left behind.

• A depletion region is formed (no free charge carriers).

• An electric field develops across the junction.

• A barrier potential is established.

• At equilibrium:
  Diffusion current = Drift current
  Net current = 0

• Applied voltage reduces barrier potential.

• Depletion region width decreases.

• The majority of carriers cross the junction.

• Current increases rapidly after the threshold voltage.

• Current is in the mA range.

• Barrier potential increases.

• Depletion region widens.

• Diffusion current decreases.

• Small reverse current flows (minority carriers).

• The reverse current is almost independent of voltage (until breakdown).

• At breakdown → current increases sharply.