lect18 - SEMICONDUCTOR LIGHT SOURCES Basic parameters of...

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Unformatted text preview: SEMICONDUCTOR LIGHT SOURCES Basic parameters of semiconductor light sources Light emitting diodes Semiconductor injection lasers Material considerations and fabrication SEMICONDUCTORS Energy bands in Si and GaAs Book: 16.1 SEMICONDUCTORS Electrons in the conduction band and holes in the valence band at T > 0° K SEMICONDUCTORS free space Lattice: approximation Si GaAs Direct bandgap GaAs, CdS Indirect bandgap Si, Ge DIRECT AND INDIRECT BANDGAP Crystalline solid – not isotropic electron propagation directions are not equivalent Wave vector k represents propagation direction Elemental semiconductors , Si, Ge, C etc. Binary semiconductors , 2 atomic species III-V SC, GaAs, AlAs, GaN, for example II-VI SC, CdS, CdSe, ZnS, ZnSe, for example Ternary SC , 3 atomic species Mixture of two III-V or two II-VI SC GaAlAs, ZnCdSe, for example Quaternary SC , 4 atomic species Mixture of three or four III-V or II-VI SC GaInPAs, ZnMgSSe, for example Energy bandgap E g Index of refraction n SEMICONDUCTORS SEMICONDUCTORS Elemental semiconductors , Si, Ge, C, etc. Binary SC III-V SC, GaAs, AlAs, GaN, II-VI SC, CdS, ZnS, ZnSe, for example Si Si Ga As Type 3 Type 5 Type 4 SEMICONDUCTOR TYPES: INTRINSIC Si, Ge and C (elemental SC) Type 5 atoms, P, As, Sb, Bi n-type Donor impurities (excess e) - Electrons Type 3 atoms, B, Al, Ga, In, Tl p-type Acceptor impurities (excess h) - Holes Si P n-type Si B ? p-type SEMICONDUCTOR TYPES: EXTRINSIC GaAs and III-V SC (binary SC) Type 6 atoms, Te, Se, S n-type Type 2 atoms, Zn, Cd, Mn p-type Type 4 atoms, Si, Ge, C n- or p-type pn junctions semiconductor diodes pnp or npn transistors p n p n p EXTRINSIC SEMICONDUCTORS DENSITY OF STATES Cross section of the E-k diagram - Allowed energy levels (at all)- Density of states near the edges of the conduction and valence bands At T = 0 ° K All states with E < E F are occupied. All states with E > E F are empty. For T > 0 ° K Probability of occupation of states with E<E F is greater than 50%. Probability of occupation of states with E>E F is less than 50%. kT / ) E E ( FD F e 1 1 ) E ( f FERMI LEVEL The Fermi function f: probability that an energy level E is filled with an electron OCCUPATION In the valence band: 1 - f (E) - probability that energy level E is occupied by a hole CONCENTRATION OF E AND HOLES Intrinsic semiconductor CONCENTRATION OF E AND HOLES n-type semiconductor JUNCTIONS JUNCTIONS only fixed charges Prevent further movements INTERACTIONS WITH PHOTONS Absorption and emission of photons: Band-to-band transitions in GaAs Valence-band to acceptor-level transition in Hg-doped Ge (Ge:Hg) Free-carrier transitions within the conduction band of Ge Book: 16.2 ABSORPTION IN SC Direct bandgap GaAs, CdS Indirect bandgap Si, Ge DIRECT AND INDIRECT BANDGAP...
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This note was uploaded on 02/12/2012 for the course ECE 414 taught by Professor Alenxendra during the Spring '11 term at Purdue University.

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lect18 - SEMICONDUCTOR LIGHT SOURCES Basic parameters of...

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