Introduction to Semiconductors Information from Kittel’s book (Ch. 8) + many outside sources.

5335 Homepage: http://www.phys.ttu.edu/~cmyles/Phys5335/5335.html. 5335 Lecture Page: http://www.phys.ttu.edu/~cmyles/Phys5335/lectures.html

Most lectures on semiconductors will be based on those prepared for

Physics 5335 Semiconductor Physics.That course was taught last in the Fallof 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 & areNOT the same. I encourage you to take Phys. 5335! More information(last update, Dec., 2010!!) about Phys. 5335 is on the course webpage:

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 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!

Semiconductors - Bandgap Definition

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

II

III IV V VI

II

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

The Periodic Table Cloth!

Group IV Elements &III-V & II-VI Compounds

Group IV Elements, III-V & II-VI Compounds

Diamond Lattice

Band gap (mostly) decreases, n-n distance (mostly) increases within a row going from IV elements to III-V compounds to II-VI compounds.

Diamond→

(α-Sn or gray tin)

Band gap (mostly) decreases, n-n distance (mostly) increases going down a column.

Band gap (mostly) decreases, n-n distance (mostly) increases going from IV elements to III-V compounds to II-VI compounds.

Zincblende or Wurtzite Lattices

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 LatticeThe Cubic Unit Cell.

If all atoms are the same,it becomes the

Diamond Lattice!

Zincblende LatticeA Tetrahedral Bonding

Configuration

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: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).

Semiconductor Physicists & Engineers need to know

these structures!

Wurtzite Lattice