{"id":2360,"date":"2022-01-02T18:43:10","date_gmt":"2022-01-02T18:43:10","guid":{"rendered":"https:\/\/physics.educour.in\/isc\/?p=2360"},"modified":"2022-01-06T09:12:43","modified_gmt":"2022-01-06T09:12:43","slug":"extrinsic-semiconductor","status":"publish","type":"post","link":"https:\/\/physics.educour.in\/isc-physics\/extrinsic-semiconductor\/","title":{"rendered":"Extrinsic Semiconductor"},"content":{"rendered":"\t\t
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Doping \u2013<\/strong><\/span> \u201cThe deliberate addition of impurity atom (3rd<\/sup> group or 5th<\/sup> group) element atoms into an otherwise pure Si or Ge crystal, to <\/em>\u00a0<\/em>increase its conductivity, is called doping\u201d. <\/em><\/span><\/p>

The impurity atoms are called dopants<\/em>. The material that is formed after doping is called a doped semiconductor<\/em> or an extrinsic semiconductor<\/em>.<\/span>
[The dopant has to be such that it does not distort the original pure semiconductor lattice. It occupies only a very few of the original semiconductor atom sites in the crystal. A necessary condition to attain this is that the sizes of the dopant and the semiconductor atoms should be nearly the same.]<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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n \u2013 type extrinsic semiconductor: <\/strong><\/span><\/h3>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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\u00a0When 5th<\/sup> group element atoms [like Arsenic (As), Antimony (Sb), Phosphorous <\/span>
(P), etc.] are introduced (dropped) into an otherwise pure semiconductor, the crystal thus formed is called n-type semiconductor.<\/span><\/p>

When an antimony (or arsenic) atom is introduced in an otherwise pure semiconducting crystal, the antimony atom replaces one of the Si atom from its lattice point. The antimony atom shares four of its valence electrons with four neighboring Si atom. Due to this the fifth valence electron of the Sb atom becomes very weakly bound with its parent atom.<\/span><\/p>

The semiconductor\u2019s energy band structure is affected by doping. In the case of n \u2013type extrinsic semiconductors, additional energy states due to donor impurities (E<\/em>D<\/sub><\/em>) , called \u2018donor level\u2019, is formed at slightly below the bottom E<\/em>C <\/sub>of the conduction band and electrons from this level can move into the conduction band with very small supply of energy (about 0.01eV energy is required).<\/span><\/p>

\u00a0<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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\"\"<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t

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\"\"<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t

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\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0When such a crystal is brought to the room temperature all these fifth electrons of Sb atoms, after getting a small energy (0.01 electrons volt), leaves the atom and becomes free. Due to this the antimony atoms become positively charged immobile atoms (ions). In the energy band diagram this process in shown by extrinsic transition between the donor level and the conduction band. So the conduction band will have most electrons coming from the donor impurities.<\/span><\/p>

\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0Besides this there are a few electrons, originally present in the bond, which after getting sufficient energy breaks the bonds & causes generation of free electron \u2013 hole pair. This is shown by the intrinsic transition between the valence band & the conduction band.<\/span><\/p>

So at room temperature in a n-type semiconductor we get<\/span><\/p>

  • A large number of free electrons – (majority charge carriers)<\/span><\/li>
  • A few holes \u2013 (minority charge carriers)<\/span><\/li>
  • Negatively charge immobile doped atoms fixed at their lattice points.<\/span><\/li><\/ul>

    \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0So at any instant in a n-type semiconductor the free electron density is much higher than hole density i.e.\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0ne<\/sub> >> nh<\/sub><\/span><\/strong><\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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    P- type semiconductor<\/span><\/h3>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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    \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 When the third group element atom (Indium, Boron, Al) are doped in an otherwise pure Si or Ge crystal, the crystal thus formed is known as p-type semiconductor.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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    \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0When an Aluminum (Al) atom is introduced in Si crystal, it replaces one of the Si atoms and gets its position in the lattice. The Al atom shares all its three valance electrons with three neighboring Si atoms & there by formed three covalent bonds. But it doesn\u2019t satisfy the Al atom and also one neighboring Si atom. These two atoms together form a positively charged region, which attracts the electrons from the neighboring band. Due to this the electrons present in the neighboring atoms become weekly bound.<\/span><\/p>

    \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0In the case of p-type extrinsic semiconductors, additional energy state due to acceptor impurities (E<\/em>A<\/em>), called \u2018acceptor level\u2019 is formed. The acceptor energy level E<\/em>A <\/em>is slightly above the top E<\/em>V <\/em>of the valence band as shown in Fig. With very small supply of energy an electron from the valence band can jump to the level E<\/em>A \u00a0<\/em>and thereby ionise the acceptor atom negatively.<\/span><\/p>

    \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 When such a crystal is brought to the room temperature every Al atom present in it receives one electron from some neighboring bonds. Due to this transfer of electrons a hole is created in the bond. In this process Al atom becomes negatively charged immobile atom. In energy band diagram this process is shown by extrinsic transition between the valence band and the acceptor level, which is present just above the valence band at an energy difference of about 0.01 \u2013 0.05 eV.<\/span><\/p>

    \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Besides this there are a few electrons originally present in the bond, after receiving sufficient energy breaks the bonds and generate electrons -hole pair.(Intrinsic transition). Thus at room temperature in a p-type semiconductor we get<\/span><\/p>

    • A large number of holes \u2013 (majority charge carriers.)<\/span><\/li>
    • A few free electrons \u2013 (minority charge carriers)<\/span><\/li>
    • Negatively charged immobile Al atoms.<\/span><\/li><\/ul>

      \u00a0\u00a0 <\/strong>So, for p-type semiconductors\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 nh<\/sub> >> ne<\/sub><\/span><\/strong><\/span><\/p>

      Note that the crystal maintains an overall charge neutrality as the charge of additional charge carriers is just equal and opposite to that of the ionised cores in the lattice.<\/span><\/em><\/span><\/strong><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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