2.2 Extrinsic semiconductor

  Semi-conductor Devices and Electronics

    1.0 Introduction
    2.0 Types of semiconductor
    3.0 Mass action law
    4.0 Electrical conductivity in semiconductor
    5.0 $p-n$ junction
    6.0 Breakdown voltage
    7.0 $I-V$ characteristics of a $p-n$ junction
    8.0 Rectifier
    9.0 Light emitting diode (LED)
    10.0 Zener diode
    11.0 Transistor
    12.0 Boolean identities
    13.0 Logic gates
    14.0 De Morgan's theorem

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2.2 Extrinsic semiconductorA doped semiconductor is known as an extrinsic semiconductor.

Doping: It is a process of deliberate addition of a desirable impurity to a pure semiconductor in order to increase its conductivity. The impurity atoms added are known as dopants.

An extrinsic semiconductor is of two types,

• $n$-type semiconductor
• $p$-type semiconductor

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2.2.1 $n$-type semiconductor

When a pure tetravalent semiconductor of Silicon $(Si)$ or Germanium $(Ge)$ is doped with a group $V$ pentavalent impurities like Arsenic $(As)$, Antimony $(Sb)$, Phosphorus $(P)$ etc, we obtain a $n$-type semiconductor.

• The pentavalent impurity atoms are known as donor atoms.
• It is called as $n$-type semiconductor because the conduction of electricity in such semiconductor is due to the motion of electrons i.e. negative charges.
• It is called donor type semiconductor, because the doped impurity atom donate one free electron to semiconductor for conduction.
• In $n$-type semiconductor electrons are majority carriers and holes are minority carriers.
• $n$-type semiconductor is neutral
• In $n$-type semiconductor, $${n_e} \approx {N_d} \gg {n_h}$$ where

$n_e$: Density of electron in conduction band
$n_h$: Density of holes in valence band
$N_d$: Density of donor atoms

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2.2.2 $p$-type semiconductor

When a pure tetravalent semiconductor of Silicon $(Si)$ or Germanium $(Ge)$ is doped with a group $III$ trivalent impurities like Aluminium $(Al)$, Boron $(B)$, Indium $(In)$ etc, we obtain a $p$-type semiconductor.

• The trivalent impurity atoms are known as acceptor atoms.

• It is called $p$-type because the conduction of electricity in such semiconductor is due to the motion of holes i.e. positive charges.

• It is called as acceptor type semiconductor because, the doped impurity atom creates a hole in the semiconductor which accepts the electron, resulting conduction in $p$-type semiconductor.

• In $p$-type semiconductor, holes are majority carriers and electrons are minority carriers.

• $p$-type semiconductor is neutral.

• In $p$-type semiconductor, $${n_h} \approx {N_a} \gg {n_e}$$ where
  

$n_e$: Density of electron in conduction band
$n_h$: Density of holes in valence band
$N_a$: Density of acceptor atoms

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