University:
Massachusetts Institute of Technology
Course:
6.012 | Microelectronic Devices and Circuits
Academic year:

2023
Views:

301

Pages:

18
Author:

Nathan Ferguson

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 Key questions • How do semiconductors conduct electricity? • What is a ”hole”? • How many electrons and holes are there in a semiconductor in thermal equilibrium at a certain temperature? • How can one engineer the conductivity of semiconductors? 1. Silicon bond model: electrons and holes Si is in Column IV of periodic table: IIIA IVA 5 B Al IIB 30 31 VIA 6 C 13 VA N 14 Si O 16 15 P 32 8 7 S 33 34 Zn Ga Ge As Se 48 Cd 49 In 50 51 52 Sn Sb Te 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) Silicon crystal structure: ° 5.43 A ° 2.35A 3sp tetrahedral bond • Silicon is a crystalline material: – long range atomic arrangement • Diamond lattice: – atoms tetrahedrally bonded by sharing valence electrons (covalent bonding) • Each atom shares 8 electrons: – low energy and stable situation • Si atomic density: 5 × 1022 cm−3 Simple ”flattened” model of Si crystal: 4 valence electrons (– 4 q), contributed by each ion silicon ion (+ 4 q) two electrons in bond At 0K: • all bonds satisfied → all valence electrons engaged in bonding • no ”free” electrons At finite temperature: – + mobile electron incomplete bond (mobile hole) • finite thermal energy • some bonds are broken • ”free” electrons (mobile negative charge, −1.6×10−19 C) • ”free” holes (mobile positive charge, 1.6 × 10−19 C) ”Free” electrons and holes are called carriers: • mobile charged particles Beware: picture is misleading! • electrons and holes in semiconductors are ”fuzzier”: they span many atomic sites. A few definitions: • in 6.012, ”electron” means free electron • not concerned with bonding electrons or core electrons • define: n ≡ (free) electron concentration [cm−3 ] p ≡ hole concentration [cm−3 ] 2. Generation and Recombination Generation = break up of covalent bond to form electron and hole • requires energy from thermal or optical sources (or other external sources) • generation rate: G = Gth + Gopt + ... [cm−3 · s−1] • in general, atomic density n, p ⇒ G 6= f (n, p) – supply of breakable bonds virtually inexhaustible Recombination = formation of bond by bringing together electron and hole • releases energy in thermal or optical form • recombination rate: R [cm−3 · s−1 ] • a recombination event requires 1 electron + 1 hole ⇒ R ∝n·p Generation and recombination most likely at surfaces where periodic crystalline structure is broken. 3. Thermal equilibrium Thermal equilibrium = steady state + absence of external energy sources hυ δ<θ> =0 δt • Generation rate in thermal equilibrium: Go = f (T ) • Recombination rate in thermal equilibrium: Ro ∝ no ·po In thermal equilibrium: Go = Ro ⇒ no po = f (T ) ≡ n2i (T ) Important consequence: In thermal equilibrium and for a given semiconductor, np product is a constant that depends only on temperature! Electron-hole formation can be seen as chemical reaction: ) e− + h+ bond * similar to water decomposition reaction: ) H + + OH − H2 O * Law-of-mass action relates concentration of reactants and reaction products. For water: [H +][OH − ] K= [H2 O] Since: [H2O] [H +], [OH − ] Then: [H2 O] ' constant Hence: [H +][OH − ] ' constant 4. Intrinsic semiconductor Question: In a perfectly pure semiconductor in thermal equilibrium at finite temperature, how many electrons and holes are there? Since when a bond breaks, an electron and a hole are produced: no = po Also: nopo = n2i Then: no = po = ni ni ≡ intrinsic carrier concentration [cm−3 ] In Si at 300 K (”room temperature”): ni ' 1×1010 cm−3 ni very strong function of temperature: T ↑ → ni ↑ Note: an intrinsic semiconductor need not be perfectly pure [see next] 5. Doping: introduction of foreign atoms to engineer semiconductor electrical properties A. Donors: introduce electrons to the semiconductor (but not holes) • For Si, group-V atoms with 5 valence electrons (As, P, Sb) IIIA IVA 5 B Al IIB 30 31 VIA 6 C 13 VA N 14 Si O 16 15 P 32 8 7 S 33 34 Zn Ga Ge As Se 48 Cd 49 In 50 51 52 Sn Sb Te • 4 electrons of donor atom participate in bonding • 5th electron easy to release – at room temperature, each donor releases 1 electron that is available for conduction • donor site become positively charged (fixed charge) – As+ mobile electron immobile ionized donor Define: Nd ≡ donor concentration [cm−3 ] • If Nd ni, doping irrelevant (intrinsic semiconductor) → no = po = ni • If Nd ni, doping controls carrier concentrations (extrinsic semiconductor) → n2i po = no = Nd Nd Note: no po: n-type semiconductor Example: Nd = 1017 cm−3 → no = 1017 cm−3 , po = 103 cm−3 . In general: Nd ∼ 1015 − 1020 cm−3 log no log po no electrons= majority carriers ni po ni intrinsic holes= minority carriers log Nd extrinsic Chemical reaction analogy: dissolve a bit of KOH into water ⇒ [OH − ] ↑, [H +] ↓ B. Acceptors: introduce holes to the semiconductor (but not electrons) • For Si, group-III atoms with 3 valence electrons (B) IIIA IVA 5 B Al IIB 30 31 VIA 6 C 13 VA N 14 Si O 16 15 P 32 8 7 S 33 34 Zn Ga Ge As Se 48 Cd 49 In 50 51 52 Sn Sb Te • 3 electrons used in bonding to neighboring Si atoms • 1 bonding site ”unsatisfied”: – easy to ”accept” neighboring bonding electron to complete all bonds – at room temperature, each acceptor releases 1 hole that is available to conduction • acceptor site become negatively charged (fixed charge) B– + mobile hole and later trajectory immobile negatively ionized acceptor Define: Na ≡ acceptor concentration [cm−3 ] • If Na ni , doping irrelevant (intrinsic semiconductor) → no = po = ni • If Na ni , doping controls carrier concentrations (extrinsic semiconductor) → n2i no = po = Na Na Note: po no: p-type semiconductor Example: Na = 1016 cm−3 → po = 1016 cm−3 , no = 104 cm−3 . In general: Na ∼ 1015 − 1020 cm−3 log no log po po holes= majority carriers ni no ni intrinsic electrons= minority carriers log Na extrinsic Chemical reaction analogy: dissolve a bit of H2SO4 into water ⇒ [H +] ↑, [OH − ] ↓ Summary • In a semiconductor, there are two types of ”carriers”: electrons and holes • In thermal equilibrium and for a given semiconductor nopo is a constant that only depends on temperature: nopo = n2i • For Si at room temperature: ni ' 1010 cm−3 • Intrinsic semiconductor: ”pure” semiconductor. no = po = ni • Carrier concentrations can be engineered by addition of ”dopants” (selected foreign atoms): – n-type semiconductor: no = Nd, n2i po = Nd – p-type semiconductor: po = Na, n2i no = Na

Lecture 2 - Semiconductor Physics (I)

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