EE216.W2010.Lecture2

EE216.W2010.Lecture2 - Lecture 2. Semiconductors in...

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1 Lecture 2. Semiconductors in Equilibrium Solids and crystal structure The bond model: electrons and holes, dopants Energy bands density of states; probability of occupancy Mass action law, intrinsic carrier concentration EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Range of Resistivity for Solids. Insulators, Semiconductors, and Metals
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2 . Amorphous Grain Grain boundary medium range order; many small regions called grains, each having a crystalline structure, joined at "grain boundaries" which are full of defects. Examples: polycrystalline silicon gate electrode in a MOSFET, aluminum contacts and metalization. Crystalline - three dimensional long range order of atoms; repeating "unit cell". Examples: Si wafer, diamond Amorphous no well defined order. Example: gate oxide in a MOSFET Polycrystalline Solids can be classified as : EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Unit Cell The positions of atoms within the lattice and their bonding to neighbors determine the electrical, physical, metallurgical and chemical properties of a solid. Particularly important are the outermost or valence electrons. For this course we will deal primarily with crystalline materials with cubic symmetry. A crystal with cubic symmetry can have a (Bravais) lattice that is either ‘simple’ (aka ‘primitive’), body-centered cubic (bcc), or face-centered cubic (fcc). The atoms do not have to be at lattice points. The unit cell is a repeat unit that, when repeated in 3-D, completely fills the volume with no overlap.
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3 Unit Cell of a Si Crystal Si has a diamond structure, which is two face centered cubic (FCC) unit cells offset by 1/4 diagonal. Each Si atom has 4 nearest neighboring atoms. a=0.545 nm How many atoms/unit cell? How many atoms/cc? The atomic weight of Si is 28. What is its density? EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe Covalent Bonding For example, in Si (F.C.C.), each Si atom (with 4 valence electrons) is surrounded by 4 other Si atoms. Each atom shares its valence electrons with the four neighboring atoms such that the outermost shells are completed (as in the case of ionic bondings). No free electrons are available for conduction under normal condition. Under excitation (e.g., thermal or light) some electrons can break loose and become free for conduction. Si is thus a semi conductor (and is transparent to infra-red but not to visible light). EE 216 Principles and Models of Semiconductor Devices (Winter 2010) K. C. Saraswat and R. T. Howe
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4 Electronic Properties of Silicon • Silicon is in column 14 (group IVA) – Electronic structure: 1s 2 2s 2 2p 6 3(sp) 4 – Diamond lattice, with 0.235 nm bond length Silicon is a very poor conductor at temperature: why? 3(sp)
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EE216.W2010.Lecture2 - Lecture 2. Semiconductors in...

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