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Crystals All materials in this course are crystalline semiconductors Crystal - a structure that can fill all space based on the regular repetition of a particular unit cell Unit cell - unit to construct crystal by regular repetitionPrimitive-smallest cell which will replicate the crystalLarger Cubic-most semiconductors are diamond orzincblende for which the primitive unit cell is complex Compound Semiconductors For optoelectronics, most devices are fabricated of “compound semiconductors” particularly III-V materials made from ?Group III (Al, Ga, In) ?Group V (N, P, As, Sb) elements ?Sometimes Si and Ge (Group IV) are used as photodetectors ?Sometimes II-VI (e.g. ZnSe) and IV-VI materials (e.g., PbTe) Alloys of compound semiconductors used extensively to adjust the basic materials properties, e.g., lattice constant, bandgap, refractive index, optical emission or detection wavelength, etc. EXAMPLE - InxGa1- xAs (where x is the mole fraction of indium) InxGa1- xAs is not strictly crystalline because not every unit cell is identical (random III site location), but we treat such alloys as crystalline to a first approximation Conduction, valence bands and band gaps In the pure, perfect, semiconductor, there is an energy gap, Eg, between highest, filled, “valence” band(s), and lowest energy, empty, “conduction” band(s) In the valence bands, we use the concept of holes (positively charged pseudo-particle which is the absence of an electron) Valence bands in the pure semiconductor are empty of holes Hole energy increases downwards on the E - k diagram Gallium arsenide is a “direct gap” semiconductor lowest “minimum” in the conduction band is directly above the highest “maximum” in the valence band in E vs k (momentum) Silicon is an “indirect gap” semiconductor (Ge is also indirect) lowest CB min. is at zone edge, highest VB max. is at zone center Conduction, valence bandsand band gaps (cont) Direct gaps are important for most optoelectronic devices They have much stronger
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