Semiconductor Fundamentals

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Semiconductor Fundamentals
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• Objectives
• After completing this unit, the student should be
  able to
• Identify materials that act as semiconductors.
• Define covalent bonding.
• Describe the doping process for creating N- and
  P-type semiconductor materials.

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• Objectives (Cont.)
• Explain how doping supports current flow in a
  semiconductor material.
• Identify the advantages of semiconductors.
• Identify the disadvantages of semiconductors.

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• Semiconductor materials
• Characteristics fall between those of insulators
  and conductors.
• There are three pure semiconductor elements
• Carbon (C).
• Germanium (Ge).
• Silicon (Si).

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• Germanium
• Brittle, grayish element.
• Discovered in 1886.
• Recovered from the ashes of certain types of
  coal.
• Reduced to solid formpure germanium.

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• Silicon
• Discovered in 1823.
• Found in the earths crust as silicon dioxide.
• White or sometimes colorless.
• Abundantly found in sand, quartz, agate, and
  flint.
• Chemically reduced to pure silicon in solid form.
• Most commonly used semiconductor material.

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• Covalent bonding
• The process of sharing valence electrons,
  resulting in the formation of crystals.

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• Negative temperature coefficient
• As the temperature increases, its resistance
  decreases.
• For silicon, resistance is cut in half for every
  6 degrees Celsius of rise in temperature.
• For germanium, resistance is cut in half for
  every 10 degrees Celsius of rise in temperature.

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• Silicon has 1000 times more resistance than
  germanium at room temperature, thus making it
  more stable.
• Germanium is used where heat-sensitive
  applications are necessary.
• Today, silicon is used for most solid-state
  applications.

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• Conduction in pure germanium and silicon
• Electrical activity is highly dependent on
  temperature.
• Germanium and silicon crystals function as
  insulators at low temperatures.
• As the temperature rises, they begin to acquire
  the characteristics of a conductor.

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• Hole
• The absence of an electron.
• Represents the loss of a negative charge.
• Therefore, it has the characteristic of a
  positively charged particle.
• Each corresponding electron and hole are referred
  to as an electron-hole pair.

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• Holes constantly drift toward the negative
  terminal of the voltage source.
• Electrons flow toward the positive terminal.
• Current flow in a semiconductor consists of the
  movement of both electrons and holes.

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• The amount of current flow is determined by the
  number of electron-hole pairs.
• The ability to support current flow increases
  with the temperature of the material.

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• To increase conductivity of semiconductors, a
  process called doping is used.
• Doping is the process of adding impurities to a
  semiconductor material.
• Pentavalent is made of atoms with five valence
  electrons.
• Arsenic (As).
• Antimony (Sb).
• Trivalent is made of atoms with three valence
  atoms.
• Indium (In).
• Gallium (Ga).

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• N-type material
• Has more electrons than holes.
• Negative charge is the majority carrier.
• Free electrons flow toward the positive terminal.

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• P-type material
• Has more holes than electrons.
• Positive charge is the majority carrier.
• The holes move toward the negative terminal.

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• In Summary
• Semiconductor materials
• Materials with characteristics that fall between
  those of insulators and conductors.
• Pure semiconductor materials
• Germanium (Ge).
• Silicon (Si).
• Carbon (C).

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• Silicon is used for most semiconductor devices.
• Valence indicates an atoms ability to gain or
  lose electrons.
• Semiconductor materials have valence shells that
  are half full.
• Covalent bonding occurs when atoms share their
  valence electrons.

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• Heat creates problems by allowing electrons to
  break their covalent bonds.
• A hole is the absence of an electron in the
  valence shell.
• Current flow consists of both electron flow and
  hole movement.
• Doping adds impurities to a semiconductor
  material.

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• Trivalent materials
• Have atoms with three valence electrons.
• Are used to make P-type material.
• Holes are the majority carrier.
• Electrons are the minority carrier.

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• Pentavalent materials
• Have atoms with five valence electrons.
• Are used to make N-type material.
• Electrons are the majority carrier.
• Holes are the minority carrier.
• N-and P-type semiconductor materials have a
  higher conductivity than pure semiconductor
  material.