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An Alternative Semiconductor Definition!
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 and 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-cm
Actually, there are no rigid boundaries!
Semiconductors Conductivity/Resistivity Definition
Semimetals
Metals
Semiconductor ~ A small bandgap 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: Bandgap Definition
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):– The electrical conductivity σ depends strongly on
impurity concentrations.• “Doping” means to add impurities to change σ
– The electrical conductivity 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).
The Best Known Semiconductor is Silicon (Si)
• However, there are HUNDREDS (maybe 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!” )
III IV V VI
IIII
Group IV Materials & III-V & II-VI Compounds
The Periodic Table: The Relevant Parts for Elemental & Binary Semiconductors
The Periodic Table Cloth!
Group IV Elements andIII-V and II-VI Compounds
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
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).
The Zincblende (ZnS) Lattice
Zincblende Lattice:A Tetrahedral
Bonding Configuration
Zincblende Lattice:The Cubic Unit Cell.
If all atoms are the same,it becomes the
Diamond Lattice!
Zincblende & Diamond Lattices
Diamond LatticeThe Cubic Unit Cell
Zincblende LatticeThe Cubic Unit Cell
Semiconductor Physicists & Engineersneed to know these structures!
Diamond LatticeThe Cubic Unit Cell.
Semiconductor Physicists & Engineersneed to know these structures!
Diamond Lattice
Zincblende (ZnS) Lattice
Zincblende LatticeThe Cubic Unit Cell.
Some Materials of Interest in This Course
have 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).
Wurtzite Lattice
Semiconductor Physicists & Engineersneed to know these structures!
Room Temperature Properties of Some Important Semiconductor Materials