Intrinsic and Extrinsic semiconductors

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• ANIL NEERUKONDA INSTITUTE OF TECHNOLOGY
  SCIENCES(A)Department of Electronics and
  Communication Engineering
• ECE 125 Basic Electronics Engineering

• Academic year 2022-23
• Class Section 1/4 ECE-A
• Name of the Faculty Mr.D.Anil Prasad

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UNIT-I(Semiconductor Diodes)

• Fermi level in Intrinsic Extrinsic
  semiconductors. Mass-Action law. Mobility and
  conductivity, Hall effect, Generation and
  recombination of charges, Drift and diffusion
  current, Band structure of open-circuit p-n
  junction, V-I characteristics, transition and
  diffusion capacitance, reverse recovery time,
  Avalanche and zener breakdown, zener diodes,
  Light Emitting Diodes.

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Contents

• Intrinsic Semiconductors
• Hole as carrier transport in semiconductor
• Extrinsic Semiconductors
• n-type
• p-type
• Compensated semiconductor

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Intrinsic Semiconductor

• A pure semiconductor is called Intrinsic
  Semiconductor.
• At T00K no charge carriers are present in
  Conduction band(CB). Valence band(VB) is filled
  with electron which are tightly bound to nucleus.
  Hence at T00K all semiconductor acts as
  insulator.
• At T3000K due to thermal energy the atoms in the
  crystal are constantly vibrating, therefore they
  deform inter-atomic bonds. Thus in some regions
  at some moments bonds may be overstretched and
  bond energy may be smaller than thermal energy.

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Intrinsic Semiconductor

• Covalent bonds are broken and EHP are generated.
• Intrinsic semiconductor
• no of electrons no of holes

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Hole as a carrier transport in semiconductor

• Hole is the absence of electron in covalent bond
  is called hole. The current in the semiconductor
  is not only due to free electron in CB but also
  due to valence electron in VB
• Bound electrons The valence electrons taking
  part in the bonding process is called bound
  electrons.
• The electrons already in the bonding process can
  participate in conduction because of the presence
  of vacancies or holes.
• The bound electrons tend to move through
  vacancies and contribute to current.

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Hole as a carrier transport in semiconductor

• A valence electron in an adjacent atom can
  relatively easily break out from its covalent
  relationship when a bond is imperfect and a hole
  results.
• A hole remains in its original location when an
  electron leaves a bond to fill one. As a result,
  the hole really goes in the opposite direction
  from the electron.
• This hole may now be filled with an electron from
  a different covalent bond, in which case the hole
  will move one more step in the opposite direction
  of the motion of the electron.

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Hole as a carrier transport in semiconductor

• The charge of a Hole is positive.
• The movement of bound electrons is different from
  the movement of free electrons in response of
  electric field, E.

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Hole as a carrier transport in semiconductor

• Free electrons tend to move faster as compared to
  bound electrons.
• The force is same on both free electrons and
  bound electrons i.e FqE but the rate of movement
  is different.
• This difference can be captured by assigning
  particle effective mass. mnltmp

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Extrinsic Semiconductor

• Doping It is the Process of adding impurities
  into an intrinsic semiconductor .
• Extrinsic semiconductor Semiconductor in which
  impurity atoms were added to intrinsic
  semiconductor to create charge carriers in
  addition to intrinsic carriers generated such
  that the conductivity can be made due to either
  electrons or holes predominantly are called
  Extrinsic semiconductors.

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Extrinsic Semiconductor

• n-type semiconductor When pentavalent impurity
  atoms are added to intrinsic semiconductor then
  n-type semiconductor is formed.
• p-type semiconductor When trivalent impurity
  atoms are added to intrinsic semiconductor then
  p-type semiconductor is formed.

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n-type Semiconductor

• By doping process add pentavalent atoms such as
  P,As etc. The added P atoms are far away form
  each other because of lightly doped. As there is
  no interaction among the phosphorous atoms all
  the valence electrons are at the same energy
  level.

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n-type Semiconductor

• When a pentavalent atom replaces Si atom then it
  provides 4 electrons to make covalent bond with
  surrounding Si atoms and its 5th valence electron
  will occupy a new energy level just below the
  conduction band.
• At 1000K these impurity atoms starts donating
  their 5th electron to the conduction band of Si
  and are called donar atoms.

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n-type Semiconductor

• At 3000K all these impurity atoms becomes
  positive charged ions after donating their 5th
  valence electrons called impurity ionization
  process
• At 00K no impurity atom is ionized hence no free
  electrons are available in conduction band, no
  EHP generation takes place hence doped
  semiconductor also acts as insulator at T00K

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n-type Semiconductor

• In n-type semiconductor electrons are majority
  charge carriers and holes are minority charge
  carriers.
• ngtgt(ni,p)

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p-type Semiconductor

• By doping process add trivalent atoms such as B,
  Al, Ga etc. The added B atoms are far away form
  each other because of lightly doped. As there is
  no interaction among the Boron atoms all the
  valence electrons are at the same energy level.
• When boron, a trivalent element, replaces the
  silicon atom, only three of the covalent bonds
  can be filled, and the empty space in the fourth
  bond is referred to as a hole.

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p-type Semiconductor

• When acceptor impurities are added to intrinsic
  semiconductor they produce an allowable discrete
  energy level which is just above VB. The impurity
  level is called acceptor energy level.
• These impurities make available carriers which
  are holes that can accept electrons at T1000K.
  These impurities are known as acceptor
  impurities.

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p-type Semiconductor

• At 3000K all these impurity atoms becomes
  negative charged ions after accepting electron
  form silicon atom called impurity ionization
  process
• At 00K no impurity atom is ionized hence no free
  holes are available in valence band, no EHP
  generation takes place hence doped semiconductor
  also acts as insulator at T00K

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p-type Semiconductor

• In p-type semiconductor holes are majority charge
  carriers and electrons are minority charge
  carriers.
• pgtgt(ni,n)

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Compensated Semiconductor

• It contains both donar and acceptor impurity
  atoms in the same region.
• It is called compensated semiconductor because in
  a p-type region by adding ND atoms it is
  compensated to behave as intrinsic when NDNA and
  is compensated by adding NDgtNA to behave as
  n-type.

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References

• Robert L Boylestad, Electronic Devices And
  Circuit Theory, Prentice Hall, seventh
  edition,2021
• Jacob Millman and Christos Halkias, Electronics
  Devices and Circuits, Black edition, October,2017

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Thank you

• for listening