ssp_20 - 6-16. SemiconductorsSemiconductor: Egsmallenough...

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Unformatted text preview: 6-16. SemiconductorsSemiconductor: Egsmallenough for thermal excitationof electrons into conductionband.Fig. 6.1 Energy level diagram of asemiconductor.Doping: very small fraction of impurities changes electricalconductivity by orders of magnitude.⇒Solid state electronics.Formation of a band gap in Si (Ge from 4s2, 4p2)Fig.6.2 Foundation of a band gaps in Si by splitting of atomic levels into bonding andantibonding levels (cf. Chp. 1).Diamond (2s2,2p2): Eg= 5.4 eV ⇒Isolator6-2Temperature dependence of the energygapTable 6.1 Temperature dependence of the energy gapTemperature0K300KSiEg=1.17 eVEg=1.12 eVGeEg=0.75 eVEg=0.67 eVAt 300K linear T-dependenceReason for T dependence:expansion⇓reduced overlap of wave functions⇓reduced energy splitting6-3Band structuresIndirect semiconductor:Maximum of valence bandat Γ, i.e.k= (0,0,0) ≠minimum of conductionband:Si: kalong [100]Ge: kalong [111]Spin-orbit splitting takeninto account in calculationfor Ge.Fig. 6.2 Calculated bandstructures of silicon andgermanium. For germanium the spin-orbitsplitting is also taken into account. Bothsemiconductors are so-called indirectsemiconductors, i.e. the maximum of thevalence band and the minimum of theconduction band are at different positions in theBrillouin zone. The minimum of the conductionband of silicon lies along the ΓΧ= [100]direction and that of germanium along the ΓL= [111] direction....
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This note was uploaded on 01/19/2010 for the course MATERIALS M504 taught by Professor Adelung during the Spring '02 term at Uni Kiel.

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ssp_20 - 6-16. SemiconductorsSemiconductor: Egsmallenough...

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