Transcript Part I
Introduction to Semiconductors Information from Kittel’s book (Ch. 8) + many outside sources. Most lectures on semiconductors will be based on those prepared for Physics 5335 Semiconductor Physics. That course was taught last in the Fall of 2010. It is scheduled to be taught next in the Fall of 2012!! As discussed at the start of the semester, Phys. 5335 clearly has overlap with this Solid State course, but the 2 courses are complementary & are NOT the same. I encourage you to take Phys. 5335! More information (last update, Dec., 2010!!) about Phys. 5335 is on the course webpage: 5335 Homepage: http://www.phys.ttu.edu/~cmyles/Phys5335/5335.html. 5335 Lecture Page: http://www.phys.ttu.edu/~cmyles/Phys5335/lectures.html An Alternate Semiconductor Definition! What is a Semiconductor? Kittel Ch. 8 & many other sources Classification of Solids by their Conductivity/Resistivity (σ = conductivity, ρ = resistivity) Metals: Good Conductors! 103 ≤ σ ≤ 108 (Ω-cm)-1 & 10-8 ≤ ρ ≤ 10-3 Ω-cm Insulators: Poor Conductors! σ ≤ 10-8 (Ω-cm)-1; ρ ≥ 108 Ω-cm Semiconductors/Semimetals: 10-8 ≤ σ ≤ 103 (Ω-cm)-1; 10-3 ≤ ρ ≤ 108 Ω-cm Note the HUGE range!! Note also that there are no rigid boundaries! Semiconductors Conductivity/Resistivity Definition Metals Semimetals Semiconductors Bandgap Definition • Semiconductors are ~ Small Bandgap Insulators (we defined bandgap Eg earlier). Strictly speaking, a semicondcutor must also be capable of being doped (we’ll define doping later). Typical Bandgaps Semiconductors: 0 ~ ≤ Eg ≤ ~ 3 eV Metals & Semimetals: Eg = 0 eV Insulators: Eg ≥ 3 eV • An Exception is Diamond: Eg = ~ 6 eV. Diamond 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! More Semiconductor Characteristics • In pure materials (very rare): The electrical conductivity σ exp(cT) T = Kelvin Temperature, c = constant • Impure materials (most): – 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”) The Periodic Table The Relevant Parts for Elemental & Binary Semiconductors III IV V VI II II Group IV Materials & III-V & II-VI Compounds The Periodic Table Cloth! Group IV Elements & III-V & II-VI Compounds Diamond Lattice Group IV Elements, III-V & II-VI Compounds Zincblende or Wurtzite Lattices Diamond→ (α-Sn or gray tin) Band gap (mostly) decreases, n-n distance (mostly) increases within a row going from IV elements to III-V compounds to II-VI compounds. Band gap (mostly) decreases, n-n distance (mostly) increases going from IV elements to III-V compounds to II-VI compounds. Band gap (mostly) decreases, n-n distance (mostly) increases going down a column. Many Materials of Interest: Have crystal lattice structures = Diamond or Zincblende (discussed in detail earlier!): • Each atom is tetrahedrally coordinated with four (4) nearest-neighbors. • The bonding is (mostly) sp3 hybrid bonding (strongly covalent). • Two atoms/unit cell (repeated to form an infinite solid). 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 Zincblende/Diamond Lattices Diamond Lattice Zincblende Lattice The Cubic Unit Cell The Cubic Unit Cell Semiconductor Physicists & Engineers need to know these structures! Diamond Lattice Diamond Lattice The Cubic Unit Cell Semiconductor Physicists & Engineers need to know these structures! Zincblende (ZnS) Lattice Zincblende Lattice The Cubic Unit Cell. Some Materials of Interest: Have crystal lattice structures = Wurtzite Structure (discussed in detail earlier!): • Similar to Zincblende, but has hexagonal symmetry instead of a cubic. • Each atom is tetrahedrally coordinated with four (4) nearest-neighbors. • The bonding is (mostly) sp3 hybrid bonding (strongly covalent). • Two atoms/unit cell (repeated to form an infinite solid). Wurtzite Lattice Semiconductor Physicists & Engineers need to know these structures!