Ga and P Atoms to Covalent Solid GaP
Band Gaps in Binary Group III-V Semiconductors
Mixed Semiconductors
• Affect of replacing some of the As with P in GaAs
Band Gap (eV)
(nm)
GaAs
1.35
919 (IR)
GaP
2.24
554 (green)
Band Gaps in Binary Group II-VI Semiconductors
Group Variations
Affect in going from Group IV, Group III-V to Group II-VI
Atomic Radius (pm)
Ge (Group IV)
122
Ga (Group III)
122
As (Group V)
121
Zn (Group II)
126
Se (Group VI)
117
All nearly the same size so must consider difference in electronegativity to
explain the band gap variation
Band Gap (eV)
(nm)
Ge
0.67
1850 (IR)
GaAs
1.35
919 (IR)
ZnSe
2.58
481 (blue-green)
Band Gap in Nitride Binary Semiconductors
Material
Band Gap (eV)
AlN
6.3
GaN
3.4
InN
1.9
Band Gap Energies
Group IV Band Gaps (eV)
C
5.5
Si
1.11
Ge
0.67
Group III-V Band Gaps (eV)
III\V
N
P
As
Sb
Al
6.3
2.45
2.16
1.6
Ga
3.4
2.26
1.43
0.7
In
1.9
1.35
0.36
0.17
Group II-VI Band Gaps (eV)
II\VI
S
Se
Te
Zn
3.6
2.7
2.25
Cd
2.42
1.73
1.49
Pb
0.37
0.27
0.29
Band-Gap Energy
• The band-gap energy increases as the splitting
between the bonding and antibonding orbitals
increases
– Greater overlap of atomic orbitals
• Smaller atoms lend to larger band-gaps
– As the electronegativity difference between the
atoms increases.
• Electrons confined to smaller space
Band-Gap Energy Example
• Which will have the larger band gap and why?
– Si or Ge
– GaAs or GaP
– GaP0.4As0.60 or GaP0.65As0.35
– GaAs or ZnS
Semiconductors
•
A substance in which some electrons can cross
the band gap
• Electrical conductivity of semiconductors can be
modified via doping.
– Doping - adding trace amounts of an element to a
substance to modify its properties.
• n-type: prepared by doping with a valence electron rich
element; “negative”
• p-type: prepared by doping with a valence electron deficient
element; “positive”
•
Dopant concentration controls the conductivity
Semiconductors
• n-type semiconductors
– The extra valence electron fills the donor level, which is
just below the conduction band.
– Little energy is required to promote the donor level
electron to the conduction band.
• p-type semiconductors
– The lack of valence electrons creates an acceptor level,
just above the valence band.
– Little energy is required to promote a valence band
electron to the acceptor level.
– The vacancies in the valence band are referred to as
holes.
Semiconductors.
(a) A silicon crystal
doped with arsenic,
which has one more
valence electron than
silicon.
(b) A silicon crystal
doped with boron,
which has one less
electron than silicon
n-Type Semiconductors
• Formation of n-type doped silicon.
– Doping with phosphorus introduces an extra valence electron.
– The extra electron fills the donor level, which lies close to the conduction
band.
p-Type Semiconductors
• Formation of a p-type doped silicon.
– The dopant has fewer than 4 valence electrons.
– An acceptor level slightly higher than the valence band is created.
– Aluminum is a common p-type dopant.
Semiconductor Example Problem
• Which kind of material (n- or p-type) would result
if pure germanium was doped with:
– Indium (In)
– Phosphorus (P)
– Sulfur (S)
Semiconductors
• A p-n junction can be constructed from p-type and
n-type material.
– The flow of electrons across the junction is easily
regulated by applying voltage.
– Current flows across the junction when the negative
pole of a battery is connected to the n-type material.
– Current does not flow across the junction when the
negative pole is connected to the p-type material.
– Important in solid state electronics.
p-n Junction
•
The p-n junction
involves the contact of a
p-type and an n-type
semiconductor.
•
Important building
block for electronic
circuits
•
Electrical conduction
only occurs readily in
one direction across the
junction
No current
Current Flows
The Range of Resistivity of Materials Varies Widely