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 & 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!

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

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

Semiconductors Conductivity/Resistivity Definition

Semimetals

Metals

Semiconductors Conductivity/Resistivity Definition

Semimetals

Metals

Note the wide range of conductivity!

Conductivity/Resistivity of Some Materials

Semiconductors!

Conductivity of Some Materials

One Way to Classify “Electronic Materials”

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

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).

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!” )

III IV V VI

IIII

Group IV Materials + III-V & II-VI Compounds

The Periodic Table:Relevant Parts for Elemental & Binary Semiconductors

Some Elements & Compoundswhich can be Semiconductors (Purple!)

Semiconductors (Main Constituents)

The Periodic Table Cloth!

Group IV Elements andIII-V & 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, itbecomes theDiamond Lattice!

Zincblende & Diamond Lattices

Diamond LatticeThe Cubic Unit Cell

Zincblende LatticeThe Cubic Unit Cell

Semiconductor Physicists & Engineersneed to know the geometry of these structures!

Diamond LatticeThe Cubic Unit C`ell.

Semiconductor Physicists & Engineersneed to know these structures!

Diamond Lattice

Zincblende (ZnS) Lattice

Zincblende LatticeThe Cubic Unit Cell.

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).

Wurtzite Lattice

Semiconductor Physicists & Engineers

need to know these structures!

Room Temperature Properties of SomeSemiconductor Materials

Room Temperature Properties of Some Semiconductors

Lattice Constants of Some Semiconductors

Room Temperature Properties of Si, Ge, & GaAs