lec2 - 6.012 Microelectronic Devices and Circuits Fall 2005...

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6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 2-1 Lecture 2 - Semiconductor Physics (I) September 13, 2005 Contents : 1. Silicon bond model: electrons and holes 2. Generation and recombination 3. Thermal equilibrium 4. Intrinsic semiconductor 5. Doping; extrinsic semiconductor Reading assignment: Howe and Sodini, Ch. 2, §§ 2.1-2.3
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6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 2-2 Key questions How do semiconductors conduct electricity? What is a ”hole”? How many electrons and holes are there in a semicon- ductor in thermal equilibrium at a certain tempera- ture? How can one engineer the conductivity of semicon- ductors?
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6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 2-3 1. Silicon bond model: electrons and holes Si is in Column IV of periodic table: IIIA IVA VA VIA IIB B C N O Al Si P S Zn Ga Ge As Se Cd In Sn Sb Te 5 6 7 8 13 14 15 30 31 33 34 48 49 50 51 16 32 52 Electronic structure of Si atom: 10 core electrons (tightly bound) 4 valence electrons (loosely bound, responsible for most chemical properties) Other semiconductors: Ge, C (diamond form), SiGe GaAs, InP, InGaAs, InGaAsP, ZnSe, CdTe (on average, 4 valence electrons per atom)
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6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 2-4 Silicon crystal structure: 5.43 A ° A ° 3sp tetrahedral bond 2.35 Silicon is a crystalline material: long range atomic arrangement Diamond lattice: atoms tetrahedrally bonded by sharing valence elec- trons ( covalent bonding ) Each atom shares 8 electrons: low energy and stable situation Si atomic density: 5 × 10 22 cm - 3
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6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 2-5 Simple ”flattened” model of Si crystal: two electrons in bond 4 valence electrons (–4 q ), contributed by each ion silicon ion ( + 4 q ) At 0K: all bonds satisfied all valence electrons engaged in bonding no ”free” electrons
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6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 2-6 At finite temperature: + incomplete bond (mobile hole)
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