Lecture 3 6pp - World of Electronics Electrical Properties...

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1 World of Electronics 2 Electrical Properties Units and Definitions: • Ohm’s Law • Current density • Electric field • Electrical conductivity • Resistance and conductance V = IR J = I/A E = V/d, d = distance J = " E g = 1/R V = voltage I = current R = resistance 3 Classical Model of Electronic Conduction in Metals n conduction electrons/volume • Apply electric field E to give electron accel. of eE/m , where m = electron mass • Assume ! = average time between collisions v d = eE m " J = nev d = ne 2 m E Drift velocity Current density 4 Classical Model of Electronic Conduction in Metals = ne 2 # m = ne e m $ % & ( ) = ne μ • For most metals, the conductivity decreases about 0.4% per K. • This property is used for temperature measurements. μ = mobility 5 Energy Band Model for Electronic Conduction - Metals N 3s electrons occupied empty Closely spaced levels 6 Energy Band Model for Electronic Conduction Conduction band Valence band Conduction electron Hole Thermal excitation Bandgap Hole = absence of electron in valence band
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7 Intrinsic Semiconductors Material Bandgap at 300 K (eV) C (diamond) 6.0 GaN 3.37 SiC (4H) 3.25 SiC (6H) 3.0 AlAs 2.15 GaAs 1.4 Si 1.1 InSb 0.17 Ge 0.67 Sn (gray Sn) 0.0 Band gap related to bond strength 8 Intrinsic Semiconductors J = ( ne μ n + pe p ) E • Since electrons and holes are present in a semiconductor, both contribute to electrical conduction • Excitation of electrons to form electrons and holes can be treated as an equilibrium chemical reaction: VB # electrons + holes 9 Intrinsic Semiconductors Since n = p in intrinsic semiconductors, we can write: [ electrons ][ holes ] [ VB ] = C exp " E g k B T # $ % & ( np = C 'exp " E g k B T # $ % & ( n i " exp # E g 2 k B T $ % & ( ) 10 Comparing Conductivity of Semiconductors and Metals ln " 1/T ionization-controlled saturation mobility-controlled temperature- controlled metals / 2k B Increasing temperature Two ways to think about increase of conduction electrons with increasing temperature: (1)Thermally assisted excitation across the bandgap (2)Thermally assisted excitation from the covalent bond 11 Utilizing Bandgaps in Semiconductors • Thermistors • Photodetectors • Light emitters Detecting temperature change Detecting photons Efficient light source - 60 to 80%
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Lecture 3 6pp - World of Electronics Electrical Properties...

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