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Extrinsic Semiconductor

An impure semiconductor is called extrinsic semiconductor. When pure semiconductor material is mixed with small amounts of certain specific impurities with valency different from that of the parent material, the number of mobile electrons/holes drastically changes. The process of addition of impurity is called doping (Fig. 3).
Pentavalent impurities The elements whose atoms has five valance electrons are called pentavalent impurities, e.g., As, P, Sb, etc. These impurities are also called donor impurities because they donate extra free electron.
Fig. 3
Trivalent impurities The elements whose atoms have three valance electrons are called trivalent impurities, e.g., In, Ga, Al, B, etc. These impurities are also called acceptor impurities as they accept electron.
  • The compounds of trivalent and pentavalent elements also behave like semiconductors, e.g., GaAs, InSb, InP, GaP, etc.
  • The number of atoms of impurity element is about 1 in 108 atoms of the semiconductor.
  • In extrinsic semiconductors nenh
  • In extrinsic semiconductors, fermi level shifts toward valence or conduction energy bands.
  • Their conductivity is high and they are used for practical purposes.
  • In a doped extrinsic semiconductor, the number density of e of the conduction band (ne) and the number density of holes in the valence band (nh) differs from that in a pure semiconductor. If ni is the number density of electron in conduction band or the number density of holes in valence band in a pure semiconductor, then nenh = ni2 (mass action law)
  • Extrinsic semiconductors are of two types: (a) N-type semiconductor and (b) P-type semiconductor

N-type semiconductor

N-type semiconductors are obtained by adding a small amount of pentavalent impurity to a pure sample of semiconductor (Ge) (Fig. 4).
  • Majority charge carriers: electrons
    Minority charge carriers: holes
  • ne >> nh; ie >> ih
  • Conductivity σ neµee
  • N-type semiconductor is electrically neutral (not negatively charged).
  • Impurity is called donar impurity because one impurity atom generate is one electron.
  • Donor energy level lies just below the conduction band (Fig. 5).
Fig. 4
Fig. 5

P-type semiconductor

P-type semiconductors are obtained by adding a small amount of trivalent impurity to a pure sample of semiconductor (Ge) (Fig. 6).
Fig. 6
  • Majority charge carriers: holes
    Minority charge carriers: electrons
  • nh >> ne; ih >> ie
  • Conductivity, σ nhµhe
  • P-type semiconductor is also electrically neutral (not positively charged).
  • Impurity is called acceptor impurity.
  • Acceptor energy level lies just above the valence band (Fig. 7).
Fig. 7

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