Physics PY4118 Physics of Semiconductor Devices 5. Band Filling ColΓ‘iste na hOllscoile Corcaigh, Γire University College Cork, Ireland ROINN NA FISICE Department of Physics 5.1
Physics PY4118
Physics of Semiconductor Devices
5. Band Filling
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.1
PY4118 Physics of
Semiconductor Devices
Semiconductor Bands
The lowest filled bands are filled with electrons that are bound near the atomic nucleus.
The electrons in the highest bands are used in covalent bonds. They are called valence electrons. Their bands are valence bands.
The next band, unpopulated at 0πΎ, is called the conduction band.
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.2
PY4118 Physics of
Semiconductor Devices
Fermi Level and Energy
βΌ Pauli exclusion leads to electrons populating higher states rather than all sitting in the ground state
βΌ Fermi statistics govern how electrons move into even higher states due to temperature
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.3
PY4118 Physics of
Semiconductor Devices
Fermi Level in Metals
Metal
Energy level at the bottom of the partially filled band
Highest occupied energy level at π = 0πΎ
partially
filled
band
Pauliβs Exclusion Principle at Work.ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.4
πΈ = πΈπΉ
πΈ = 0
PY4118 Physics of
Semiconductor Devices
Fermi Level in Metals
Fermi Velocity:
Copper:
RT: 1000Β°C:
The amount of thermal energy is much less that the Fermi Energy!Even at high Temperature.
Metals conduct very well even at very low TColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.5
1
2ππ£πΉ
2 = πΈπΉ
πΈπΉ = 7ππ π£πΉ = 1.6 Γ 106π
π
ππ΅π β 0.025ππ ππ΅π β 0.11ππ
PY4118 Physics of
Semiconductor Devices
The Occupation Probability (1)
βΌ Or: βWhat is the probability that an available level will be populated?β
βΌ A group of probability distribution functions have been derived using Statistical Mechanics.
βΌ Other names are βEnergy Distribution Functionsβ
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.6
PY4118 Physics of
Semiconductor Devices
The Occupation Probability (2)
βΌ Maxwell - Boltzmann β Identical β no Pauli
β distinguishable
βΌ Bose-Einsteinβ Identical β no Pauli
β indistinguishable
βΌ Fermi-Diracβ Identical - Pauli
β indistinguishable
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.7
π πΈ = πβπΈβπΈπΉππ΅π
π πΈ =1
ππΈβπΈπΉππ΅π β 1
π πΈ = π πΈ =1
ππΈβπΈπΉππ΅π + 1
PY4118 Physics of
Semiconductor Devices
The Occupation Probability (3)
If: or:
Remember, at RT:
Fermi - Dirac Maxwell - Boltzmann
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.8
ππΈβπΈπΉππ΅π β« 1 πΈ β πΈπΉ > ππ΅π
ππ΅π β 1
40ππ
π πΈ =1
ππΈβπΈπΉππ΅π + 1
π πΈ = πβπΈβπΈπΉππ΅π
PY4118 Physics of
Semiconductor Devices
π(π¬)
π
ππ¬π¬π
Β½
If π = 0πΎ, π(πΈ) is a simple step function with an edge at πΈπΉ, i.e., all the states with energies below the fermi energy πΈπΉ, are completely occupied, and all the states with energies above πΈπΉ are completely vacant.
The Fermi Function (1)
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.9
π πΈ < πΈπΉ , 0 =1
πββ + 1=1
1β 1
π πΈ > πΈπΉ , 0 =1
πβ + 1=1
ββ 0
π πΈ = πΈπΉ , 0 =1
π0 + 1=1
2
PY4118 Physics of
Semiconductor Devices
The Fermi Function (2)
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10 12
Energy (eV)
P(E
)
Maxwell - Boltzmann
Fermi - Dirac
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.10
PY4118 Physics of
Semiconductor Devices
The Fermi Function (3)
0
0.2
0.4
0.6
0.8
1
1.2
6.5 6.7 6.9 7.1 7.3 7.5
Energy (eV)
P(E
)
Maxwell - Boltzmann
Fermi - Dirac
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.11
PY4118 Physics of
Semiconductor Devices
The Fermi Function (4)
0
0.2
0.4
0.6
0.8
1
1.2
6.5 6.6 6.7 6.8 6.9 7 7.1 7.2 7.3 7.4 7.5
Energy (eV)
P(E
)
300K
500K
1000K
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.12
PY4118 Physics of
Semiconductor Devices
The Fermi Function (5)
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.13
π πΈ =1
ππΈβπΈπΉππ΅π + 1
ππΈ
πΈ β πΈπΉ(ππ)
PY4118 Physics of
Semiconductor Devices
βΌ At π» = π, energy levels below π¬π are filled with electrons, while all levels above π¬π are empty.
βΌ Electrons are free to move into βemptyβ states of conduction band with only a small electric field πΈ, leading to high electrical conductivity!
βΌ At π» > π, electrons have a probability to be thermally βexcitedβ from below the Fermi energy to above it.
Band Diagram: Metal
πΈπΉ πΈπΉ
Fermi βfillingβ function
Energy band to be
βfilledβ
Moderate Tπ = 0 πΎ
βFillβ the energy band
with electrons.
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.14
πΈπΆ,π πΈπΆ,π
PY4118 Physics of
Semiconductor Devices
Band Diagram: Insulator
βΌ At π» = π, lower valence band is filled with electrons and upper conduction band is empty, leading to zero conductivity.β Fermi energy πΈπΉ is at midpoint of large energy gap (2 β 10 ππ) between conduction
and valence bands.
βΌ At π» > π, electrons are NOT thermally βexcitedβ from valence to conduction band, leading to zero conductivity.
πΈπΉ
πΈπΆ
πΈπ
Conduction band(Empty)
Valence band(Filled)
πΈπππ
π > 0
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.15
PY4118 Physics of
Semiconductor Devices
Band Diagram: Semiconductor
βΌ At π» = π, valence band is filled with electrons and conduction band is empty, leading to zero conductivity.
βΌ At π» > π, electrons thermally βexcitedβ from valence to conduction band, leading to partially empty valence and partially filled conduction bands.
πΈπΉπΈπΆ
πΈπ
Conduction band(Partially Filled)
Valence band(Partially Empty)
π > 0
Thus: SemiconductorColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.16
PY4118 Physics of
Semiconductor Devices
Re-examine the Semiconductor
πΈπΉπΈπΆ
πΈπ
Conduction band(Partially Filled)
Valence band(Partially Empty)
π > 0
It is the absence of an electron that makes a hole
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.17
π πΈ
1 β π πΈ
PY4118 Physics of
Semiconductor Devices
Symmetry of f(E)
The function is symmetric around the Fermi energy.That is: the distribution of electrons above πΈπΉ equals the distribution of holes below πΈπΉ
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.18
π πΈ β πΈπΉ =1
ππΈβπΈπΉππ΅π + 1
=1
πΞπΈππ΅π + 1
1 β π πΈ β πΈπΉ = 1 β1
πΞπΈππ΅π + 1
=πΞπΈππ΅π
πΞπΈππ΅π + 1
=1
1 + πβΞπΈππ΅π
= π πΈπΉ β πΈ
PY4118 Physics of
Semiconductor Devices
Examples
πΈβπΈπΉ(ππ)
0.1
π(πΈ) π = 290πΆ
2 Γ 10β2
π(πΈ) π = 800πΆ
2 Γ 10β1
0.5 2 Γ 10β9 7 Γ 10β4
1 4 Γ 10β18 5 Γ 10β7
1.5 9 Γ 10β27 4 Γ 10β10
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.19
5 1 Γ 10β87 3 Γ 10β32
IR
Red
Blue
π πΈ =ππ₯π0
=1
ππΈβπΈπΉππ΅π + 1
PY4118 Physics of
Semiconductor Devices
Focus
βΌ The Pauli exclusion principle leads to high energy electron states being filled even at low temperature.
βΌ Fermi Dirac statistics provide the probability that available electron states will be populated.
βΌ But how many electrons, and how many electron states are there?
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.20
PY4118 Physics of
Semiconductor Devices
Conduction Electrons in Metals?How many are there?
Number of
Conduction Electrons
In Sample
Number of
Atoms
In Sample
Number of
Valence electrons
In Sample
Sample
Volume
Number of
Conduction Electrons
In Sample
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.21
= Γ
π = Γ·
PY4118 Physics of
Semiconductor Devices
Conduction Electrons?How many are there?
Number of
Atoms
In Sample=
Sample
Volume
Material
Density
Molar Mass
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.22
Γ
/ππ΄ππ΄ = Avogadro's Number
= 6.022 Γ 1023/πππ
PY4118 Physics of
Semiconductor Devices
Number Density
Material
Density
Molar Mass
Number of
Valence electrons
In Sample
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.23
Γπ =
Γ ππ΄
PY4118 Physics of
Semiconductor Devices
Number Density
8.96 g/cm3
63.54 g
1
Copper
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.24
Γ Γ ππ΄π =
= 9 Γ 1022ππβ3 = 9 Γ 1028πβ3
PY4118 Physics of
Semiconductor Devices
Number Density
2.65 g/cm3
28.08 g
0
Silicon
This calculation was for metals.For semiconductors we need Fermi statistics
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.25
Γ Γ ππ΄π =
= 0 πβ3
PY4118 Physics of
Semiconductor Devices
Focus
βΌ We know the probability that a state is filled.
βΌ We know the number of electrons available.
βΌ How many electron states are there?
Density of States
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.26
PY4118 Physics of
Semiconductor Devices
Infinite barrier 3D box (1)
Divide by:
3x 1D solutions
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.27
π2π
ππ₯2+π2π
ππ¦2+π2π
ππ§2+2ππΈ
β2π = 0 π2 =
2ππΈ
β2
π π₯, π¦, π§ = ππ₯ π₯ ππ¦ π¦ ππ§ π§
1
ππ₯
π2ππ₯ππ₯2
+1
ππ¦
π2ππ¦
ππ¦2+
1
ππ§
π2ππ§ππ§2
+ π2 = 0
π2 = ππ₯2 + ππ¦
2 + ππ§2
1
ππ
π2ππ
ππ₯2+ ππ
2 = 0, π = π₯, π¦, π§
PY4118 Physics of
Semiconductor Devices
Infinite barrier 3D box (2)
With a box of dimensions: π΄, π΅, πΆ with a corner at (0,0,0)
We are interested in the number of states with an energy less than the Fermi Energy:
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.28
π π₯, π¦, π§ = π· sin ππ₯π₯ sin ππ¦π¦ sin ππ§π§
ππ₯ =ππ₯π
π΄, ππ¦ =
ππ¦π
π΅, ππ§ =
ππ§π
πΆ
πΈ =β2π2
2π=β2π2
2π
ππ₯π΄
2
+ππ¦
π΅
2
+ππ§πΆ
2
πΈ =β2ππΉ
2
2πβ ππΉ
2 =2π
β2πΈπΉ
PY4118 Physics of
Semiconductor Devices
Density of States (1)
The volume (in k-space) of one state is:
The volume (in k-space) of the Fermi sphere is:
BUT, we are only interested in the positive quadrant:
AND, there are 2 spin states
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.29
ππ₯
ππ¦
ππ§
ππ =π
π΄
π
π΅
π
πΆ=π3
π
ππΉ =4
3πππΉ
3
ππ > 0
PY4118 Physics of
Semiconductor Devices
π = 2 Γ1
2
3ππΉππ
= 2 Γ1
8
43πππΉ
3
π3
π
=1
3
ππΉ3π
π2
Density of States (2)
So, the number of filled states is:spin
Thus:
And:
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.30
3D
ππΉ =3π2π
π
13 π =
π
π
πΈπΉ =β2ππΉ
2
2π=
β2
2π
3π2π
π
23
=β2 3π2π
23
2π
PY4118 Physics of
Semiconductor Devices
Copper:
Fermi Energy Revisited β Metal
β¦as we had assumed
Note, that this calculation was only based on theproperties of the material. Our previous assumptionwas not used.
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.31
πΈπΉ =β2 3π2π
23
2π
π = 8.5 Γ 1028πβ3
πΈπΉ = 7ππ
PY4118 Physics of
Semiconductor Devices
Silicon:
Fermi Energy Semiconductor
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.32
πΈπΉ =β2 3π2π
23
2π
π = 0 β 5 Γ 1021πβ3
πΈπΉ = 0 β 1.1ππ
PY4118 Physics of
Semiconductor Devices
Density of States (3)
Inverting we get the number density:
And can then calculate the density of states:
For a semiconductor:
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.33
π =1
3π22ππΈ
β2
32
π πΈ =ππ
ππΈ
π πΈ =ππ
ππΈ=3
2
1
3π22ππΈ
β2
12 2π
β2=
1
2π22π
β2
32
πΈ
π πΈ =1
2π22πβ
β2
32
πΈ β πΈπΆ
PY4118 Physics of
Semiconductor Devices
Aside
We now have the tools we need to talk about semiconductors:
βΌ Bands: limit the allowable π, πΈ values
βΌ Fermi statistics: provide the proportion of filled states
βΌ Density of states provides the number of available states
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.34
PY4118 Physics of
Semiconductor Devices
Population of Bands
To actually predict the distribution of carriers in the band we need both Fermi statistics and the density of states:
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.35
PY4118 Physics of
Semiconductor Devices
Density of Occupied States
Copper @ 8ππ:
Above the Fermi Level the number of occupied states
Decreases exponentially.
Fermi-Dirac
Statistics
The number of carriers within
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.36
ππ πΈ ΞπΈ = π πΈ π πΈ ΞπΈ =ππ πΈ
ππΈπ πΈ ΞπΈ
ΞπΈ
π πΈ = 5 Γ 10β18
π πΈ = 1.9 Γ 1028
ππ πΈ = 9.7 Γ 1010
PY4118 Physics of
Semiconductor Devices
Density of Occupied States
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
0 2 4 6 8 10
Energy (eV)
De
ns
ity
of
Oc
cu
pie
d S
tate
s (
x1
028)
eV
-1m
-3
0K
1000K
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.37
PY4118 Physics of
Semiconductor Devices
Density of Semiconductor States
Intrinsic Semiconductor
ColΓ‘iste na hOllscoile Corcaigh, Γire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 5.38
π πΈ π πΈ π πΈ
π πΈ