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chapter 19 baic semiconductor physicschapter 19 basic semiconductor physicschapter 19 basic semiconductor physicschapter 19 basic semiconductor physics.ppt

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Semiconductor Physics - * Copyright ? by John Wiley Sons 2003 Review of Basic Semiconductor Physics Current Flow and Conductivity ? Charge in volume A?x = ?Q = q n A ? x = q n A v?t ? Current density J = (?Q/?t)A-1 = q n v ? Metals - gold, platinum, silver, copper, etc. ? n = 1023 cm-3 ; ? = 107 mhos-cm ? Insulators - silicon dioxide, silicon nitride, aluminum oxide ? n 103 cm-3 ; ?? 10-10 mhos-cm ? Semiconductors - silicon, gallium arsenide, diamond, etc. ? 108 n 1019 cm-3 ; 10-10 ???? 104 mhos-cm Thermal Ionization ? Si atoms have thermal vibrations about equilibrium point. ? Small percentage of Si atoms have large enough vibrational energy to break covalent bond and liberate an electron. Electrons and Holes ? T3 T2 T1 ? Density of free electrons = n : Density of free holes = p ? p = n = ni(T) = intrinsic carrier density. ? ni2(T) = C exp(-qEg/(kT )) = 1020 cm-6 at 300 ?K ? T = temp in ?K ? k = 1.4x10-23 joules/ ?K ? Eg = energy gap = 1.1 eV in silicon ? q = 1.6x10-19 coulombs Doped Semiconductors ? Extrinsic (doped) semiconductors:p = po ≠ n = no ≠ ni ? Carrier density estimates: ? Law of mass action nopo = ni2(T) ? Charge neutrality Na + no = Nd + po ? P-type silicon with Na ni: po ≈ Na , no ≈ ni2/ Na ? N-type silicon with Nd ni: no ≈ Nd , po ≈ ni2/ Nd Nonequilibrium and Recombination ? Thermal Equilibrium - Carrier generation = Carrier recombination ? n = no and p = po ? Nonequilibrium - n no and p po ? n = no + ?n and p = no + ?n ; ?n = excess carrier density ? Excess holes and excess electrons created in equal numbers by breaking of covalent bonds ? Generation mechanisms -light (photoelectric effect), injection, impact ionization ? Recombination - removal of excess holes and electrons ? Mechanisms - free electron captured by empty covalent bond (hole) or trapped by impurity or crystal imperfection ? Rate equation: d(?n)/dt = - ?n????