EELE408 Photovoltaics Lecture 05: Semiconductor Basics 05... · EELE408 Photovoltaics Lecture 05:...
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1 EELE408 Photovoltaics Lecture 05: Semiconductor Basics Dr. Todd J. Kaiser [email protected] Department of Electrical and Computer Engineering Montana State University - Bozeman Bond Model Silicon Atom Si 14 electrons with 4 valence 3 Filled orbitals do not interact The four outer valence electrons interact Semicconductors 4 Elements around Silicon III IV V 5 Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si S Si Si Si Si Si Si S Si Si Si Si S Si Si S Si 6 Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si S Si Si Si Si Si Si Si S Si Si Si Si Si S Si Si Si S Si Covalent Bonds: Shared electrons to fill orbital
Transcript of EELE408 Photovoltaics Lecture 05: Semiconductor Basics 05... · EELE408 Photovoltaics Lecture 05:...
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EELE408 PhotovoltaicsLecture 05: Semiconductor Basics
Dr. Todd J. [email protected]
Department of Electrical and Computer EngineeringMontana State University - Bozeman
Bond Model
Silicon Atom
Si
14 electrons with 4 valence
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Filled orbitals do not interact The four outer
valence electrons interact
Semicconductors
4
Elements around Silicon
III IV V
5
Si
Si
SiSi Si
SiSi
Si
Si
Si
Si
Si
SiSi
Si
Si
Si
Si
Si
SiSi
Si
Si
Si Si
Si
Si
SSi
Si
Si
Si Si
Si
SSi
Si
Si Si
SSi
Si SSi
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Si Si
Si
Si SiSi
Si Si
Si
Si
Si
Si
Si
Si
Si
Si Si
Si
Si
SiSi
Si
Si
Si S
Si
Si
SiSi
Si
Si
Si
S
Si
SiSi
Si
Si
S
SiSi
Si
SSi
Covalent Bonds:
Shared electrons to fill orbital
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Intrinsic Silicon
Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
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Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
Poor conductor: No free electrons to carry currentNeed to engineer electrical properties (conduction)
Valence V: n-type doping
III IV V
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Extra Negative Electrons
Si Si Si Si Si Si Si
Si Si As Si Si Si Si
Si Si Si Si Si As Si
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Si Si Si Si Si As Si
Si As Si Si Si Si Si
Si Si Si Si Si Si Si
Each N‐type dopant brings an extra electron to the lattice
Valence III: p-type doping
III IV V
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Extra Positive Holes
Si Si Si Si Si Si Si
Si B Si Si B Si Si
Si Si Si Si Si Si Si
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Si Si Si Si Si Si Si
Si Si Si B Si Si Si
Si Si Si Si Si Si Si
Each P‐type dopant is short an electron, creating a hole in the lattice
Bond Model
• Electrons from broken bonds are free to move
• Electrons from neighboring bonds can also move into the ‘hole’ created by a broken bond, allowing the broken bond or ‘hole’ to propagate as if had a positive charge– Bubble in a liquid
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3
Band Model
Orbital Shells
• The positions of the electrons around the nucleus are quantized to specific energy levels or Shells
• The orbital determines the energy of the electron
r
qV
0
2
4
14
n = 1
n = 2
n = 3
0
Energy of Electrons
Energy
M4d
4f
N
5s5p
5d
5f
O
5g
2s2pLower Electron
Energy: More Tightly Bound to Nucleus
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n
1s
n=1
K
2s2p
L
n=2
3s3p
3d
n=3
4s4p
n=4 n=5 n=6
Higher Electron Energy: Less Bound to Nucleus
Orbitals fill from the bottom up.
Silicon Electron Configuration
[Ne]3s23p2
M
(n=3)
s
(l = 0)
p
(l = 1)
d
(l = 2)(m = ‐1) (m = 0) (m = 1)(m = ‐2) (m = 2)
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K
(n=1)
L
(n=2)
s
(l = 0)
s
(l = 0)
p
(l = 1)
Splitting of Energy Levels in a Crystalline Lattice
EnergyDiscreteEnergy States
Continuum ofEnergy States
Allowed
Forbidden
Si
Si
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Interatomic Distanced
Band gaps
Allowed
Allowed
Forbidden
Forbidden
Allowed and forbidden energy levels at atomic spacing
Energy
EnergyGap
ConductionBand
ValenceBand
Band Diagram
This corresponds to an electron jumping from the valence band to the conduction b d
Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
Position
Thermal energy causes an electron to break its bond to the silicon lattice
band
The electron is now free to move through the silicon lattice
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Energy
EnergyGap
ConductionBand
ValenceBand
Band Diagram
Additional current is caused by the movement of the “hole”
Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
Position
Other electrons can then move into the voids or holes in the lattice left by the released electron
Energy
EnergyGap
ConductionBand
Valence
Band Diagram
Position
Band
It is easier to think of a positive hole moving in the valence band
Energy
EnergyGap
ConductionBand
ValenceBand
Intrinsic Concentration
There is an intrinsic concentration of electrons that are able to move that is a function of
Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
Si Si Si Si Si Si Si
Position
At absolute zero there is insufficient thermal energy to break any bonds so no electrons are in the conduction band
temperature
As the temperature rises electrons escape the silicon lattice
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Energy
Energy
ConductionBand
Intrinsic Concentration
As the temperature rises more and more electrons are excited into the conduction band and the silicon becomes more conductive with an equal concentration of electrons in the valence band and holes in the conduction band
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Position
gyGap
ValenceBand
310
219
/101)300(
6884exp
3001038.9375275
cmKTn
T
TKKTn
i
i
Empirical equation for
near room temperature
Energy
Energy
ConductionBand
N-Type DopingDoping the silicon lattice with atoms with 5 valence electrons (V) create sites in the band diagram that require little energy to break the bond to the dopant atom and become free to move in the lattice or in other words move into the conduction band.
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Position
gyGap
ValenceBand
As As As As As As As As As As As As As
N type Material
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Majority Charge Carriers are Negative electrons
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+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Few holes
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Energy
Energy
ConductionBand
P-Type Doping
Doping the silicon lattice with atoms with 3 valence electrons (III) create sites in the band diagram that require little energy to trap an electron into the dopant atom. Holes are created in the valence band that are free to move.
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Position
gyGap
ValenceBand
B B B B B B B B B B B B B
P type Material
-------
Majority Charge Carriers are Positive holes
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-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Few electrons
Equilibrium Carrier Concentration
• Law of Mass Action– At equilibrium the product of the majority and minority carrier
concentration is a constant
– n0 is the equilibrium electron concentration
– p0 is the equilibrium hole concentration
200 inpn
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– ni is the intrinsic concentration
N-Type Carrier Concentration
• At room temperature in an N-type material, the electron concentration is approximately the donor doping concentration
D
i
D
N
np
Nn2
0
0
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– ND is the donor doping concentration
P-Type Carrier Concentration
• At room temperature in a P-type material, the hole concentration is approximately the acceptor doping concentration
A
i
A
N
nn
Np2
0
0
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– NA is the acceptor doping concentration
Energy
Energy
ConductionBand
Minority Concentration Dependence
Low Doping High Doping
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Position
gyGap
ValenceBand
Montana State University30
As the doping concentration increases the concentration of minority carriers decreases.
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Energy Levels of Elements in the Band Gap of Silicon (Traps)
Li Sb P As S Pt Mn Ag Hg
0.033 0.0390.044 0.049
0.18
0.37 0.370.33 0.33
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B Al Ga In Co Zn Cu Fe Au
0.0450.057
0.0650.16
0.390.31
0.55
0.24
0.37
0.52
0.40
0.55
0.53
0.35
0.54
0.340.36
Electrical conductivity in nonmetals
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Silicon
• Thermal energy breaks bond and creates a free electron and a hole
• The concentration of holes equals the concentration of free electrons
• An applied field causes
Electric Field
+
+
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An applied field causes electrons to contribute to electrical conduction
• Movement of electrons filling holes create net charge movement contributing to electrical conduction
+
+++
+ +
Conductivity of Semiconductor
• p: concentration of holes
• n: concentration of electrons
• h: hole mobility
• e: electron mobility
eh enep eh
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