Download - An Alternative Semiconductor Definition!
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An Alternative Semiconductor Definition!
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What is a Semiconductor?B - Ch 1, Y - Ch 1, S - Ch 1
Conductivity/Resistivity Definition(σ = conductivity, ρ = resistivity)
Metals: Good Conductors!103 ≤ σ ≤ 108 (Ω-cm)-1; 10-8 ≤ ρ ≤ 10-3 Ω-cm
Semiconductors & Semimetals:10-8 ≤ σ ≤ 103 (Ω-cm)-1; 10-3 ≤ ρ ≤ 108 Ω-cm
NOTE THE HUGE RANGE!!Insulators:
σ ≤ 10-8 (Ω-cm)-1; ρ ≥ 108 Ω-cmActually, there are no rigid boundaries!
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Metals & Insulators: Measured resistivities range over more than 30 orders of magnitude!
Material Resistivity (Ωm) (295K)
Resistivity (Ωm) (4K)
10-12
“Pure” Metals Copper
10-5
Semiconductors Ge (pure) 5 102 1012
Insulators Diamond 1014
Polytetrafluoroethylene (P.T.F.E)
1020
1014
1020
Potassium
2 10-6 10-10
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Metals, Insulators & SemiconductorsAt low
temperatures, all materials are insulators or
metals.
Semiconductors:Resistivity decreases rapidly with increasing temperature. Semiconductors have resistivities intermediate between metals and insulators at room temperature.
Pure Metals:Resistivity increases rapidly with increasing temperature.
1020-
1010-
100 -
10-10-R
esis
tivity
(Ωm
)
100 200 3000Temperature (K)
Diamond
Germanium
Copper
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Semiconductors Conductivity/Resistivity Definition
Semimetals
Metals
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Semiconductors Conductivity/Resistivity Definition
Semimetals
Metals
Note the wide range of conductivity!
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Conductivity/Resistivity of Some Materials
Semiconductors!
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Conductivity of Some Materials
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One Way to Classify “Electronic Materials”
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Semiconductor ~ Small Band Gap Insulator• We’ll define bandgap Eg in detail later). Strictly speaking, it must
also be capable of being doped (we’ll define doping in detail later).
Typical BandgapsSemiconductors: 0 ~ ≤ Eg ≤ ~ 3 eV
Metals & Semimetals: Eg = 0 eVInsulators: Eg ≥ 3 eV
• Exception Diamond, with Eg = ~ 6 eV, is usually an insulator, but it can be doped & used as a semiconductor!
• Also, sometimes there is confusing terminology like GaAs: Eg = 1.5 eV is sometimes called semi-insulating!
Semiconductors: Band Gap Definition
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Some Semiconductor Characteristics• In pure materials (which are very rare):
The electrical conductivity σ exp(cT)T = Kelvin Temperature, c = constant
• In impure materials (most materials):– σ depends strongly on impurity (doping)
concentrations. “Doping” means to add impurities to change σ
– σ can be changed by light or electron radiation & by injection of electrons at contacts
– Transport of charge can occur by the motion of electrons or holes (defined later).
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The Best KnownSemiconductor is Silicon (Si)
• But, there are HUNDREDS (THOUSANDS!) of others!
• Elemental: Si, Ge, C (diamond)• Binary Compounds: GaAs, InP, .• Organic Compounds: (CH)n (polyacetyline)• Magnetic Semiconductors: CdxMn1-xTe, …• Ferroelectric Semiconductors: SbI, …• Superconducting Compounds (!!)
GeTe, SrTiO3, .. ( “High Tc materials!” )
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III IV V VI
IIII
Group IV Materials + III-V & II-VI Compounds
The Periodic Table:Relevant Parts for Elemental & Binary Semiconductors
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Some Elements & Compoundswhich can be Semiconductors (Purple!)
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Semiconductors (Main Constituents)
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The Periodic Table Cloth!
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Group IV Elements andIII-V & II-VI Compounds
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Group IV Elements III-V, II-VI, & IV-IV Compounds
Diamond
(α-Sn or gray tin)
Band gap (mostly) decreases & near neighbor distance (mostly) increases within a row going from IV elements to III-V compounds to II-VI compounds.
Diamond Lattice
Band gap (mostly) decreases & nearest neighbor distance (mostly) increases going down a column.
Band gap (mostly) decreases & near neighbor distance (mostly) increases going from IV elements to III-V to II-VI compounds.
Zincblende or Wurtzite Lattices
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Many Materials of Interest in This Course:Have Crystal Lattice Structures
Diamond or Zincblende(These will be discussed in detail again later!)
• In these structures, each atom is tetrahedrally coordinated with four (4) nearest-neighbors.
• The bonding between neighbors is (mostly) sp3 hybrid bonding (strongly covalent).
There are 2 atoms/unit cell(repeated to form an infinite solid).
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The Zincblende (ZnS) Lattice
Zincblende Lattice:A Tetrahedral
Bonding Configuration
Zincblende Lattice:The Cubic Unit Cell. If all atoms are the same, itbecomes theDiamond Lattice!
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Zincblende & Diamond Lattices
Diamond LatticeThe Cubic Unit Cell
Zincblende LatticeThe Cubic Unit Cell
Semiconductor Physicists & Engineersneed to know the geometry of these structures!
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Diamond LatticeThe Cubic Unit C`ell.
Semiconductor Physicists & Engineersneed to know these structures!
Diamond Lattice
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Zincblende (ZnS) Lattice
Zincblende LatticeThe Cubic Unit Cell.
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Some Materials of Interest in This CourseHave Crystal Lattice Structures
Wurtzite Structure(This will be discussed in detail again later!)
• This is similar to the Zincblende structure, but it has hexagonal symmetry instead of cubic.
• In these structures, each atom is tetrahedrally coordinated with four (4) nearest-neighbors.
• The bonding between neighbors is (mostly) sp3 hybrid bonding (strongly covalent).
There are 2 atoms/unit cell (repeated to form an infinite solid).
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Wurtzite Lattice
Semiconductor Physicists & Engineers
need to know these structures!
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Room Temperature Properties of SomeSemiconductor Materials
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Room Temperature Properties of Some Semiconductors
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Lattice Constants of Some Semiconductors
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Room Temperature Properties of Si, Ge, & GaAs