MSE Lecture 13 - SUMMARY FROM LAST CLASS Conductivity n is...

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Dr. P. Lucas U of A MSE 110 Conductivity SUMMARY FROM LAST CLASS n is the number of charge carriers (electrons) per unit volume (m -3 ). e is the charge of the electron (1.6x10 -19 C). μ is the mobility of the electron. Metals have high n but low μ , semiconductors have high μ but low n and insulators have n =0. The mobility μ of an electron moving in a solid is finite due to collision with structural defects and vibrating atoms. Vibrations amplitude increases with temperature hence the conductivity of metals decrease with increasing temperature. µ σ ne = = kT E N n g c 2 exp In semiconductors, the number of charge carrier n increase with increasing temperature according to: hence the conductivity of semiconductors increase with increasing temperature . In semiconductors both the electron and holes contribute to the conductivity: σ =n h h +n e e
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Dr. P. Lucas U of A MSE 110 Semiconductors SEMICONDUCTOR DEVICES Pure semiconductors are elements from group IV: Ge, Si with 4 electrons on the outer shell Si:[Ne]2s 2 2p 2 . They form tetrahedral sp 3 hybrid orbitals each populated with one electron. Following the octet rule each sp 3 will share an electron with an adjacent atom and form 4 covalent bonds in tetrahedral configuration. Si and Ge semiconductor crystals therefore adopt the tetrahedral structure of diamond. This covalent network generates a full bonding band (valence) and a empty antibonding band (conduction) separated by a small energy gap (E g ) which produce semiconducting properties.
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Dr. P. Lucas U of A MSE 110 Semiconductivity Some electrons get thermally excited across the band gap into the conduction band. The population of conducting electrons therefore increase with temperature. = kT
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MSE Lecture 13 - SUMMARY FROM LAST CLASS Conductivity n is...

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