semiconductor technology

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SEMICONDUCTOR TECHNOLOGY

• By
• Ashvani Shukla
• Manager(ci)
• Bgr energy

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Theory of Semiconductor

• The materials can be classified on the basis of
  energy gap between their valence band and
  conduction band. The valence band is the band
  consisting of free valence electron and the
  conduction band is empty band. Conduction takes
  place when an electron jumps from valence band to
  conduction band and the gap between these two
  bands is energy gap. Wider the gap between the
  bands, higher the energy it requires to shift the
  electron to conduction band.
• In case of conductors, this energy gap is absent
  or in other words conduction band and valence
  band overlap each other. Thus electron requires
  minimum energy to jump from valence band, e.g.
  Silver, Copper and Aluminum. In insulators, this
  gap is very large. Therefore, it requires large
  amount of energy to shift an electron from
  valence to conduction band. Thus insulators are
  poor conductors of electricity, e.g. mica,
  diamond. Semiconductors have energy gap in
  between conductors and insulators (1 eV) and
  thus require energy more than conductors but less
  than insulators. They dont conduct electricity
  at low temperature but as temperature increases
  conductivity increases e.g. silicon and
  germanium. This is the most basic theory of
  semiconductor. The materials that are neither
  conductor nor insulator with energy gap of about
  1 eV (electron volt) are called semiconductors.
  Most common type of materials that are used as
  semiconductors are germanium (Ge) and silicon
  (Si) because of their property to withstand high
  temperature. For Si and Ge energy gap is given
  as,

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• Eg 1.21 - 3.6 X 10-4T eV (for Si)
• Eg 0.785 - 2.23 X 10-4T eV (for Ge)
• Where, T absolute temperature in oK Assuming
  room temperature to be 300 oK, Eg 0.72eV for Ge
  and 1.1eV for Si.
• At room temperature resistivity of semiconductor
  is in between insulators and conductors.
  Semiconductors show negative temperature
  coefficient of resistivity i.e. its resistance
  decreases with increase in temperature. Both Si
  and Ge are elements of IV group i.e. both
  elements have 4 valence electrons. Both form
  covalent bond with neighboring atom. At absolute
  zero temperature both behave as insulator i.e.
  the valence band is full while conduction band is
  empty but as temperature is raised more and more
  covalent bonds break and electrons are set free
  and jump to conduction band.

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• Energy band diagram of a semiconductor. CB is the
  conduction band and VB is the valence band. At 0
  K, the VB is full with all the valence electrons.
• Intrinsic Semiconductors
• As per theory of semiconductor, semiconductor in
  its pure form is called as intrinsic
  semiconductor. In pure semiconductor number of
  electrons (n) is equal to number of holes (p) and
  thus conductivity is very low as valence
  electrons are covalent bonded. In this case we
  write n p ni, where ni is called the
  intrinsic concentration. It can be shown that ni
  can be written
• ni n0T3/2 exp(-VG /2VT)
• Where, n0 is a constant, T is the absolute
  temperature, VG is the semiconductor band gap
  voltage, and VT is the thermal voltage.
• The thermal voltage is related to the temperature
  by VT kT/q
• Where, k is the Boltzmann constant (k 1.381
  10 - 23 J/K).
• In intrinsic semiconductors conductivity (s) is
  determined by both electrons (se) and holes (sh)
  and depends on the carrier density. se neµe sh
  peh Conductivity, s se sh neµe peµh
  Ne (µe µh) Where n, p numbers of electrons
  and holes respectively. µh, µe mobility of free
  holes and electrons respectively N n p e
  charge on carrier

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• Extrinsic Semiconductors
• As per theory of semiconductor, impure
  semiconductors are called extrinsic
  semiconductors. Extrinsic semiconductor is formed
  by adding a small amount of impurity. Depending
  on the type of impurity added we have two types
  of semiconductors N - type and P-type
  semiconductors. In 100 million parts of
  semiconductor one part of impurity is added.
• N-type Semiconductor
• In this type of semiconductor majority carriers
  are electrons and minority carriers are holes. N
  - type semiconductor is formed by adding
  pentavalent ( five valence electrons) impurity in
  pure semiconductor crystal, e.g. P. As, Sb.

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• Four of the five valence electron of pentavalent
  impurity forms covalent bond with Si atom and the
  remaining electron is free to move anywhere
  within the crystal. Pentavalent impurity donates
  electron to Si thats why N- type impurity atoms
  are known as donor atoms. This enhances the
  conductivity of pure Si. Majority carriers are
  electrons therefore conductivitry is due to these
  electrons only and is given by, s neµe
• P-type Semiconductors
• In this type of semiconductor majority carriers
  are holes and minority carriers are electrons. P-
  type semiconductor is formed by adding trivalent
  ( three valence electrons) impurity in pure
  semiconductor crystal, e.g. B, Al Ba.

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• Three of the four valence electron of tetravalent
  impurity forms covalent bond with Si atom. This
  leaves an empty space which is referred to as
  hole. When temperature is raised electron from
  another covalent bond jumps to fill this empty
  space. This leaves a hole behind. In this way
  conduction takes place. P- type impurity accepts
  electron and is called acceptor atom. Majority
  carriers are holes and therefore conductivity is
  due to these holes only and is given by, s neµh