Semicondutores Intrínsecos e Extrínsecos

Scientific concepts and phenomena presented

Intrinsic semiconductors

• Definition: Pure semiconductors whose behavior is determined by the electronic structure of the material.
• Examples: Silicon (Si), germanium (Ge), and compound semiconductors.
• At 0 K:
  • Valence band fully filled
  • Conduction band empty
  • A band gap (forbidden energy gap) prevents electrons from moving unless they gain sufficient energy.
• Band gap values (as stated):
  • Silicon: ~1.141 eV
  • Germanium: ~0.7 eV
• Bonding / stability model for Si (covalent network):
  • Silicon has atomic number 14 (used in the electron-configuration reasoning described).
  • Silicon forms four covalent bonds with neighboring Si atoms (sp³-like tetrahedral picture).
  • Bond angle: 109.5°
• Charge carriers created by excitation:
  • If an electron gains enough energy to reach the conduction band:
    • It becomes a free electron (negative charge carrier).
    • It leaves behind a hole (absence of electron; positive effective charge).
• Current mechanism in semiconductors:
  • Due to both electrons and holes.
• Conductivity in intrinsic semiconductors:
  • Total conductivity is the sum of electron and hole contributions:
    • σ ≈ σ(electrons) + σ(holes)
  • Using a Drude-model-like relationship:
    • Electron conductivity: carrier concentration × elementary charge × electron mobility
    • Hole conductivity: carrier concentration × elementary charge × hole mobility
  • Mobility is tied to scattering (qualitatively linked to mean free path/collision time):
    • Depends on effective mass and the average time/path between collisions.
  • Key qualitative results emphasized:
    • Mobility of holes < mobility of electrons
    • At room temperature, intrinsic semiconductors have low conductivity because the number of thermally excited carriers is small.

Extrinsic semiconductors (doping)

• Purpose: Increase conductivity near room temperature.
• Method: Increase carrier concentration by adding impurities → doping.

n-type extrinsic semiconductors (donor doping)

• Dopant type: Pentavalent impurities (five valence electrons)
  • Examples: phosphorus or antimony
• Mechanism:
  • Dopant replaces a silicon atom.
  • Four valence electrons form covalent bonds like silicon.
  • The extra (5th) electron is weakly bound:
    • Creates a donor level in the forbidden gap, ~0.01 eV below the conduction band (as stated).
  • At room temperature:
    • Thermal energy excites this electron into the conduction band.
    • Produces free electrons as majority carriers.
    • Produces few holes (holes are minority carriers).
• Conductivity outcome:
  • Conductivity is mainly from electrons, because the conduction-band electron concentration dominates over intrinsic excitation.

p-type extrinsic semiconductors (acceptor doping)

• Dopant type: Trivalent impurities (three valence electrons)
  • Examples: boron and aluminum
• Mechanism:
  • Dopant replaces a silicon atom.
  • Three electrons participate in bonding, leaving a missing electron for the fourth bond (a hole concept).
  • An acceptor/impurity level appears in the forbidden gap near the valence-band edge (requiring thermal activation).
  • Thermal excitation promotes an electron from the valence band into the acceptor level, leaving behind a hole that enables conduction.
• Conductivity outcome:
  • Holes are majority carriers, electrons are minority carriers.
  • Conductivity (roughly):
    • σ ≈ (hole concentration) × (elementary charge) × (hole mobility)
  • Because hole mobility is lower than electron mobility, p-type conductivity is generally lower than n-type for the same carrier concentration (as stated).

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Methodology / “process” outlined

Intrinsic → extrinsic transition

• If intrinsic semiconductors are too resistive at room temperature:
  • Increase carrier concentration by doping (adding impurities).

n-type doping steps

1. Add pentavalent impurities (e.g., P, Sb).
2. Form donor levels near the conduction band.
3. At room temperature, thermally release electrons → majority electrons.

p-type doping steps

1. Add trivalent impurities (e.g., B, Al).
2. Form acceptor levels near the valence band.
3. Thermal activation creates holes → majority holes.

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Researchers or sources featured

• Pauli’s principle (Pauli): referenced for electron-configuration reasoning.
• Drude model: referenced for mobility/conductivity relationships.
• No other specific researchers are explicitly named in the subtitles.