An Alternative Semiconductor Definition!

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# An Alternative Semiconductor Definition! - PowerPoint PPT Presentation

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! 10 3 ≤ σ ≤ 10 8 ( Ω - cm) -1 ; 10 -8 ≤ ρ ≤ 10 -3 Ω - cm

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## An Alternative Semiconductor Definition!

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Presentation Transcript
1. An Alternative Semiconductor Definition!

2. 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!

3. Semiconductors Conductivity/Resistivity Definition  Metals  Semimetals 

4. Semiconductors:Bandgap Definition 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 Bandgaps Semiconductors: 0 ~ ≤ Eg ≤ ~ 3 eV Metals & Semimetals:Eg = 0 eV Insulators: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!

5. 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 σdepends 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).

6. The Best KnownSemiconductor is Silicon (Si) • However, there are HUNDREDS (maybeTHOUSANDS) 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!” )

7. The Periodic Table: The Relevant Parts for Elemental & Binary Semiconductors III IV V VI II II Group IV Materials & III-V & II-VI Compounds

8. The Periodic Table Cloth!

9. Group IVElements andIII-V and II-VI Compounds

10. Diamond Lattice  Group IVElements III-V, II-VI, & IV-IV Compounds   Zincblende or Wurtzite Lattices  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. Band gap (mostly) decreases & near neighbor distance (mostly) increases going from IV elements to III-V to II-VI compounds. Band gap (mostly) decreases & nearest neighbor distance (mostly) increases going down a column.

11. 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).

12. The Zincblende (ZnS) Lattice Zincblende Lattice: The Cubic Unit Cell. If all atoms are the same, it becomes the Diamond Lattice! Zincblende Lattice: A Tetrahedral Bonding Configuration

13. Zincblende & Diamond Lattices Diamond Lattice The Cubic Unit Cell Zincblende Lattice The Cubic Unit Cell Semiconductor Physicists & Engineers need to know these structures!

14. Diamond Lattice Diamond Lattice The Cubic Unit Cell. Semiconductor Physicists & Engineers need to know these structures!

15. Zincblende (ZnS) Lattice Zincblende Lattice The Cubic Unit Cell.

16. 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).

17. Wurtzite Lattice Semiconductor Physicists & Engineers need to know these structures!