Hence it is transmi5ed through the sample n n no t

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Unformatted text preview: impurity concentra`on FEB 17 FEB 19 Invariance of Fermi level at Op`cal absorp`on and equilibrium luminescence FEB 21 Genera`on and recombina`on **Subject to Change** 3 Today’s Discussion •  •  •  •  •  Op`cal Absorp`on Genera`on Electron- Hole Recombina`on Assignments Topics for Next Lecture 4 Op`cal Absorp`on of Photons and Excess Carriers •  A photon with energy hν<Eg will not be absorbed. Hence, it is transmi5ed through the sample. n = δ n + no (T ) Ÿ༉ gop g(T) Ÿ༉ Ÿ༉ p = δ p + po (T ) Ÿ༉ r(T) –  Insulators are frequently transparent due to their large bandgap •  A photon with energy hν>Eg can be absorbed to excite an electron from the valance band to the conduc`on band. •  Thermal genera`on of carriers provides the equilibrium carrier concentra`ons, no(T) and po(T) •  Op`cal genera`on of excess carriers ( δn and δp) can contribute to the conduc`vity 6 Terminology •  Excess Carriers: Concentra`on of carriers above the equilibrium concentra`on created by some perturbing event (op`cal injec`on, current injec`on, etc.) –  For op`cal absorp`on, excess carriers are created in pairs (1 electron + 1 hole) •  Absorp0on Coefficient “α”: A propor`onality constant rela`ng the change in photon flux to the instantaneous flux at a given point –  A measure for how rapidly photons are absorbed 7 Total Absorp`on I 0 : incident photon flux I t : transmitted photon flux I r : reflected photon flux I a : photons absorbed per unit area per unit time I ( x ) : photon flux at "x" photons All have units of cm 2 • s I0 It Ir Incident photons are either absorbed, transmitted, or reflected: Io = Ia + It + Ir Note: Streetman assumes I r ~ 0 8 What Happens Inside the Sample? •  •  •  We assume that the pr...
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