Semiconductors

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Semiconductors

• CEC

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Contents

• Definitions.
• Energy Band Diagrams.
• Intrinsic Semiconductors.
• Extrinsic Semiconductors.
• Comparison.
• PN Junction.
• PN Junction Applications.

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Definitions

• Energy Band The range of energies possessed by
  an electron in a solid.
• Valence Electron Electrons in the outermost
  orbit of an atom.
• Valence Band Range of energies possessed by
  valence band electrons.
• Conduction Band Range of energies possessed by
  free electrons.

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Definitions

• Fermi Level
• - highest energy level that an electron can
• occupy at the absolute zero temperature.
• - lies between valence and conduction
• bands at absolute zero temperature.
• - tends to change as solids are warmed.
• - value of Fermi level at absolute zero
• temperature known as Fermi energy.

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Definitions

• Forbidden Energy Gap
• - gap between valence band and
• conduction band.
• - electron can be moved from valence
• band to conduction band by applying
• energy more than forbidden energy
• gap.
• Hole absence of an electron, ve charge.

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Energy Band Concept

6 eV
1.1 eV/0.72 eV
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Fermi Level in Solids
Top of the collection of electron energy levels
at absolute zero temperature.

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Semiconductors

• Resistivity 10-5 to 106 Om.
• Conductivity 105 to 10-6 mho/m.
• Temperature coefficient of resistance Negative.
• Current Flow Due to electrons and holes.
• Materials Silicon, Germanium, Gallium Arsenide.

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Semiconductor Types

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Comparison
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Doping
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Doped Semiconductors

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Intrinsic Semiconductors

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Intrinsic Semiconductors
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Fermi Level

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Compound Semiconductor- Gallium Arsenide

Face Centered Cubic Structure (FCC)
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Compound Semiconductors

Gallium Arsenide Crystal Structure
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PN Junction
Holes
Electrons

Concentration gradient causes carriers to move to
the other side of the junction.
Junction
Immobile ions
Potential Barrier opposes carrier movement.
Depletion Region Forward bias reduces the width,
reverse bias increases the width.
VB 0.7 V for Si. 0.3 V for Ge.
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Biasing a PN Junction

• Forward Bias Positive terminal of the battery
  connected to the P - type material, negative
  terminal of the battery connected to N - type
  material.
• Current flow during forward bias.
• Reverse Bias Positive terminal of the battery
  connected to the N - type material, negative
  terminal of the battery connected to P - type
  material.
• No current flow during reverse bias.

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Biased PN Junctions

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Comparison

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Applications of PN Junction

• Elementary building blocks of active
  (semiconductor) electronic components or devices.
• Semiconductor Diode One PN Junction.
• Bipolar Junction Transistor Two PN Junctions.
• Thyristors Multiple PN Junctions.
• Schottky Junction metal semiconductor
  junction, metal replaces an extrinsic
  semiconductor material.

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Applications of PN Junction

• Light Sources Light Emitting Diodes (LEDs) and
  Laser Diodes.
• Photodetectors PN Junction Photodiode, PIN
  Photodiode, Avalanche Photodiode etc.

LED
Photodiodes
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Transistor

Two PN Junctions
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Silicon Controlled Rectifier

Three PN Junctions
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Light Emitting Diode

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Photodiodes

PN Junction Photodiode
PIN Photodiode
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Schottky Junction

Metal Semiconductor Junction
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