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