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Prof. Junqiao Wu Department of Materials Science and Engineering, U.C. Berkeley

Summer Lectures on Semiconductor Physics for Engineers. Prof. Junqiao Wu Department of Materials Science and Engineering, U.C. Berkeley wuj@berkeley.edu , 322 HMMB, 510-642-4391. Contents: Semiconductor basis (2 lectures, 6/2, 6/4)

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Prof. Junqiao Wu Department of Materials Science and Engineering, U.C. Berkeley

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  1. Summer Lectures onSemiconductor Physics for Engineers Prof. Junqiao Wu Department of Materials Science and Engineering, U.C. Berkeley wuj@berkeley.edu, 322 HMMB, 510-642-4391 • Contents: • Semiconductor basis (2 lectures, 6/2, 6/4) • General trends, k-space, band structure, density of states and Fermi distribution • kp theory, defects and effective mass approximation • Quantum confinement and nanostructures • Electrostatics (2 lectures, 6/8, 6/11) • Maxwell equations and Poisson’s equation • Band bending and carrier equilibration • Electrodynamics (2 lectures, 6/16, 6/18) • Drude’s model and classical dielectric theory • Boltzmann transport theory • Thermal effects (2 lectures, 6/23, 6/25) • Phonons and thermal physics • Thermoelectrics • Optical effects (1 lecture, 6/30) • Light-semiconductor interactions • Device physics of light emitting diodes and solar cells • Prerequisites: • calculus, vectors, ordinary and partial differential equations, linear algebra, electromagnetism, optics basics, solid state physics basics, quantum mechanics basics • Time and location: • Tuesday and Thursdays 5:00-6:30PM, June 2009. • Rm350, HMMB. • Website: http://www.mse.berkeley.edu/~jwu/courses/semi.html

  2. s2p2 Most common semiconductors: group IV (Si), III-V (GaAs), II-VI (CdSe), and their alloys Insulators: group I-VII (NaCl) cation andanion

  3. covalent ionic General trends in semiconductors

  4. Conduction bands bandgap Valence bands simple cubic face-center cubic zincblende Brillouin zone (for fcc structure)

  5. You won’t regret visiting http://www.ioffe.ru/SVA/

  6. Si n-doped Fermi p-doped

  7. Defects in semiconductors

  8. Band offsets

  9. after contact before contact vacuum level qVi=q(M-S) q qS q qM qM qS qVi ECB qB ECB - EFS EFM EFS - EFM EVB EVB W - - - - - - - - + + + + + + + + Schottky Contact: example of M/nS with M>S

  10. Ohmic contact Work function

  11. Si @ 300K net charge distribution air S D nanowire oxide gate Nano Lett.; 7, 2778 (2007).

  12. n-type p-type ionized donor + ND NA - - - ionized acceptor - - + + + + + + free electron - - - + - + EFn + - - - - - - - + free hole + - - ED + + + + + EA Eg - qV0 + + + - - - - - - + EFp - + + + + + W EF - - - - - - - - + + + + + + + + p-n junction and depletion width for ND=NA diffusion current + - + - - - - + + + + - - + + + + - - + - - + drift current - Total net current = 0 (unless externally biased)

  13. I n p n p Vf 0 V reverse forward n p Vr Biasing a diode

  14. resistivity Hot carriers mobility

  15. 1D diatomic chain 2xTO 1xLO Si 1xLA 2xTA Wavevector q Phonon spectrum

  16. Phys. Rev. 98, 940 (1955) Bulk Si Seebeck coefficient

  17. Majumdar, Science (2004) enriched Si (28Si=99.9%) Natural Si (28Si=92% 29Si=5% 30Si=3%) Thermal conductivity and thermoelectric figure of merit

  18. n1 n2 transmitted scattered absorbed reflected 3 1’ 2 1 incident emitted d Optical process in semiconductors • Reflected • Same energy (h) and wavelength () • Specular: 1’ = 1 • Refracted then Absorbed • Snell’s law: n1sin1=n2sin2 • Frequency: 1 = 2 =  = /2 • Speed of light: n1c1 = n2c2 • Wavelength: n1 1=n2 2 • Intensity: I  exp(-d) • Absorption: (h)  (h-Eg)1/2 • Transmitted • 3 = 1 • 3 = 1, 3 = 1 • Scattered • Rayleigh scattering, h = h1 • Raman and Brillouin scattering, h = h1  ħ • not directional • Emitted • not directional • h  Eg

  19. i r ħ 0 E0 E1 R E2  R GaAs •  (E-E0)1/2 for direct bandgap semiconductors ħ 0 E1 E2 E0 Dielectric Functions of Semiconductors

  20. Wavelength (m) 103 102 101 100 10-1 106 local impurity vibration core levels phonons (lattice vibration) impurity electronic 104 deeper-bands cyclotron, spin, magnon CB - Absorption coefficient (cm-1) - exciton 102 - + bandgap free carrier + VB 100 10-3 10-2 10-1 100 101 Photon energy (eV) Absorptions in Semiconductors

  21. 2 6 3 4 illumination 5 usable qV 1 2 Fundamentals of photovoltaics • “Red” loss • Thermalization loss (“blue loss”) • Junction loss • Contact loss • Recombination loss • Reflection loss

  22. AlN GaN GaAs GaPN InGaN AlGaP GaAsP AlGaN AlGaAs AlGaInP ZnSe, LED materials

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