At absolute zero temperature (0 K) conduction band of semiconductor is completely empty and the semiconductor behaves as an insulator.
When temperature increases the valence electrons acquires thermal energy to jump to the conduction band. If they move to C.B. they leave behind the deficiency of electrons in the valence band. This deficiency of electron is called as cotter or hole. A hole is taken as a seat of positive charge, having value of charge same to that of an electron.
(1) Holes acts as virtual charge, although there is no charge on holes.
(2) Effective volume of hole is more than electron.
(3) Mobility of hole is less than electron.
Types of Semiconductors
Intrinsic semiconductor: A pure semiconductor is called intrinsic semiconductor. It has thermally created current carriers
(i) They have four electrons in the outer part of orbit of atom and atoms are held together by covalent bond
(ii) Free electrons and holes both are charge carriers and (in C.B.) (in V.B.)
(iii) The drift speed of electrons is bigger than that of holes
(iv) For them Fermi energy level lies at the centre of the C.B. and V.B.
(v) In ideal semiconductor, impurity has to be less than 1 in parts of semiconductor.
(vi) In intrinsic semiconductor ; where ne = 
Electron density in conduction band, nh= Hole density in V.B., Density of intrinsic carriers.
(vii) Due to less number of charge carriers at room temperature, intrinsic semiconductors have low conductivity so they have no practical use.
Net current and coundictivity;
When some potential energy difference is operated across a piece of intrinsic semiconductor current flows in it due to both electron and holes
i.e. i = ie + ih => 
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Hence conductivity of semiconductor 
where ve = drift velocity of electron, vh = drift velocity of holes,
E = Applied electric field μe = ve/E mobility of e- and mobility of holes
μh = vh/E
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Extrinsic semiconductor:
(i) It is also called impure semiconductor.
(ii) The phenomena of adding impurity is called Doping.
(iii) Impurities are of two types :
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Pentavalent impurity
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Trivalent impurity
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The elements whose atom has five valance impurities e.g. Sb, P, As etc. These are also known as donor impurities. These impurities are also known as donor impurities because they donates extra free electron.
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The elements whose each atom has three valance electrons are called trivalent impurities e.g. Al, B, In, Ga, etc. These impurities are also called acceptor impurities as they accept electron.
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(iv) The quantity of molecules of impurity element is about 1 in atoms of the semiconductor.
(v) ne ≠ nh
(vi) In these Fermi level moves towards conduction or valence energy bands.
(vii) Their conductivity is very high and they are naturally used.
types of exterinsic semiconductor
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N-type semiconductor
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P-type semiconductor
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(i)
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(ii) Majority charge carriers - electrons
Minority charge carriers - holes
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Majority charge carriers - holes
Minority charge carriers - electrons
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(iii) ne >> nh; ie >> ih
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nh >> ne; ih >> ie
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(iv) Conductivity s » ne me e
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Conductivity s » nh mh e
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(iv) N-type semiconductor is electrically neither positive nor negative
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P-type semiconductor is also electrically neutral (not positively charged)
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(v) Impurity is called Donar impurity because one impurity atom generate one e- .
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Impurity is called Acceptor impurity.
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(vi) Donor energy level has just below the conduction band.
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Acceptor energy level has just above the valence band.
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