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ECE 351 – Electronic Devices
ECE 371
Semiconductor Physics
1
ECE 351 – Electronic Devices
Electronic Devices• Most electronic devices are made out of
semiconductors, insulators, and conductors.• Semiconductors
– Old Days – Germanium (Ge)– Now – Silicon (Si)– Now – Gallium Arsenide (GaAs) used for high speed
and optical devices and solar panels.– New – Silicon Carbide (SiC) – High voltage Schottky
diodes and switching devices.– New – Silicon Nitride – High speed power electronics– Solar panels (CdTe, CuInSe2, CuGaSe2)
2
ECE 351 – Electronic Devices
Elements
• Elements in the periodic table are grouped by the number of electrons in their valence shell (most outer shell).– Conductors – Valence shell is mostly empty (1
electron)– Insulators – Valence shell is mostly full– Semiconductors – Valence shell is half full
3
ECE 351 – Electronic Devices
Semiconductors
• Silicon and Germanium are group 4 elements – they have 4 electrons in their valence shell.
4
Si
Valence Electron
ECE 351 – Electronic Devices
Silicon
• When two silicon atoms are placed close to one another, the valence electrons are shared between the two atoms, forming a covalent bond.
5
Si
Covalent bond
Si
ECE 351 – Electronic Devices
Silicon6
Si SiSi
Si
Si
ECE 351 – Electronic Devices
Silicon
7
Si SiSi
Si
Si
•An important property of the 5-atom silicon lattice structure is that valence electrons are available on the outer edge of the silicon crystal so that other silicon atoms can be added to form a large single silicon crystal.
ECE 351 – Electronic Devices 8
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 Si Si Si
ECE 351 – Electronic Devices 9
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 Si Si Si
•At 0 ºK, each electron is in its lowest energy state so each covalent bond position is filled.•If a small electric field is applied to the material, no electrons will move because they are bound to their individual atoms.=> At 0 ºK, silicon is an insulator.
ECE 351 – Electronic Devices
Silicon
• As temperature increases, the valence electrons gain thermal energy.
• If a valence electron gains enough energy, it may break its covalent bond and move away from its original position.
• This electron is free to move within the crystal.
10
ECE 351 – Electronic Devices 11
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 Si Si Si
+
-
ECE 351 – Electronic Devices 12
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 Si Si Si
+
-
Since the net charge of a crystal is zero, if a negatively (-) charged electron breaks its bond and moves away from its original position, a positively charged “empty state” is left in its original position.
ECE 351 – Electronic Devices
Semiconductors• As temperature increases, more bonds are
broken creating more negative free electrons and more positively charged empty states.
• To break a covalent bond, a valence electron must gain a minimum energy Eg, called the energy band gap. (Number of free electrons is a function of Eg.)
13
ECE 351 – Electronic Devices
Electron Volt
14
( )( )( )
joulescoulomb
joulecoul
voltcouleV
19
19
19
10602.1
110602.1
110602.11
−
−
−
×=
×=
×=
ECE 351 – Electronic Devices
Semiconductors• Bandgap energy of various semiconductors
– Silicon = 1.12 eV– GaAs = 1.43 eV– CdTe = 1.49 eV– CuInSe2 = 1.04 eV– CuGaSe2 = 1.67 eV
15
ECE 351 – Electronic Devices
Intrinsic Semioconductor
• Definition – An intrinsic semiconductor is a single crystal semiconductor with no other types of atoms in the crystal. – Pure silicon– Pure germanium– Pure gallium arsenide.
16
ECE 351 – Electronic Devices
Extrinsic Semiconductors
• Since the concentrations of free electrons and holes is small in an intrinsic semiconductor, only small currents are possible.
• Impurities can be added to the semiconductor to increase the concentration of free electrons and holes.
17
ECE 351 – Electronic Devices
Extrinsic Semiconductors
• An impurity would have one less or one more electron in the valance shell than silicon.
• Impurities for group 4 type atoms (silicon) would come from group 3 or group 5 elements.
18
ECE 351 – Electronic Devices
Extrinsic Semiconductors
• The most common group 5 elements are phosphorous and arsenic.
• Group 5 elements have 5 electrons in the valence shell.
• Four of the electrons fill the covalent bonds in the silicon crystal structure.
• The 5th electron is loosely bound to the impurity atom and is a free electron at room temperature.
19
ECE 351 – Electronic Devices 20
Si Si Si Si Si Si
Si Si Si Si Si Si
Si Si P Si Si Si
Si Si Si Si Si Si
Si Si Si Si Si Si
-
ECE 351 – Electronic Devices
Extrinsic Semiconductors
• The group 5 atom is called a donorimpurity since it donates a free electron.
• The group 5 atom has a net positive charge that is fixed in the crystal lattice and cannot move.
• With a donor impurity, free electrons are created without adding holes.
21
ECE 351 – Electronic Devices
Extrinsic Semiconductors
• Adding impurities is called doping.• A semiconductor doped with donor
impurities has excess free electron and is called an n-type semiconductor.
22
ECE 351 – Electronic Devices
Extrinsic Semiconductors
• The most common group 3 impurity is boron which has 3 valence electrons.
• Since boron has only 3 valence electrons, the boron atom can only bond with three of its neighbors leaving one open bond position.
23
ECE 351 – Electronic Devices 24
Si Si Si Si Si Si
Si Si Si Si Si Si
Si Si B Si Si Si
Si Si Si Si Si Si
Si Si Si Si Si Si
ECE 351 – Electronic Devices
Extrinsic Semiconductors
• At room temperature, silicon has free electrons that will fill the open bond position, creating a hole in the silicon atom whence it came.
• The boron atom has a net negative charge because of the extra electron, but the boron atom cannot move.
25
ECE 351 – Electronic Devices 26
Si Si Si Si Si Si
Si Si Si Si Si Si
Si Si B Si Si Si
Si Si Si Si Si Si
Si Si Si Si Si Si
+
ECE 351 – Electronic Devices
Extrinsic Semiconductors
• Since boron accepts a valence electron, it is called an acceptor impurity.
• Acceptor impurities create excess holes but do not create free electrons.
• A semiconductor doped with an acceptor impurity has extra holes and is called a p-type semiconductor.
27
Vd
0
D1D1N4004D1D1N4004
+
-
V1DC = 15
+
-
V1DC = 15
+
R1
100+
R1
100
Date/Time run: 03/15/04 ** Profile: "SCHEMATIC1-IV PLot" [ C:\Website\Rose_Classes\ECE250\notes\OrCAD Simulations\diode i-v char...
Temperature: 27.0
Date: March 15, 2004 Page 1 Time: 16:35:04
(A) IV PLot.dat (active)
V(Vd)
-16V -14V -12V -10V -8V -6V -4V -2V 0V 2VI(D1)
0A
50mA
100mA
150mA
(775.611m,49.981m)
BAND-GAP ENERGY
The energy that an electron must acquire to jump across the forbidden band is called the band-gap energy Eg
1
BAND-GAP ENERGY
The band-gap energy Eg for silicon is 1.12 eV
For PV the energy is coming from sun photons
When a photon with more than 1.12 eV is absorbed by a solar cell, a single electron jumps into conduction band
2
BAND-GAP ENERGY
Photons are characterized by their wavelengthor their frequency as well as their energy
The three are related by
sJconstantsPlanckhJphotonaofEnergyE
mWavelengthHzFrequencyv
smlightofSpeedc
chvhE
vc
−×==
===
×==
==
=
−34
8
10626.6'
/103
λ
λ
λ
3
BAND-GAP ENERGY
For PVs, photons with wavelengths greater than 1.1 µm have an energy less than 1.12 eV
4
BAND-GAP ENERGY
The band-gaps for other important PV materials are shown below
5
BAND-GAP IMPACT ON EFFICIENCY
For air mass ratio = 1.5, the incoming solar energy is
2% in ultraviolet (UV) range
54% in visible range
44% in infrared (IR) range
6
BAND-GAP IMPACT ON EFFICIENCY
7
BAND-GAP IMPACT ON EFFICIENCY
Maximum possible fraction of the sun’s energy that could be collected with a silicon cell is 49.6%
There are other losses
Black-body radiation losses are 7%
Recombination related to slow-moving holes making it difficult for electrons to pass through without falling into a whole leading to a loss of 10%
8
BAND-GAP IMPACT ON EFFICIENCY
The following figure shows this limit as a function of the band-gap of the semiconductor
9
THE P-N JUNCTION DIODE
10
GENERIC PV CELL
A p-n junction is exposed to sunlight and it creates electrons and holes due to photon absorption
When these charged carrier reach the vicinity of the junction, the electric field of the depletion region will push the holes into the p-side and electrons into the n-side
11
GENERIC PV CELL
12
GENERIC PV CELL
Electrical contacts are attached to the top and bottom of the cell for the electrons to flow
13
SILICON PV CELL
14
GENERIC PV CELL
Remember that a P-N junction is also a diode. The electron-Hole pairs produce a current, but
some of the current is lost in the diode
15
VCC
ISC
VCC
I
V+
-
PV Equivalent Circuit Our First Simple PV Model is the Following. ISC is dependent on light exposure.
16
VCC
+PV -
VCC VCC
VCC
ISC
I
V
+
-
I
V
+
-
SIMPLEST EQUIVALENT CIRCUIT FOR PV CELL
A simple equivalent circuit model for PV cell is
17
SIMPLEST EQUIVALENT CIRCUIT FOR PV CELL
I = ISC – Id
18
SIMPLEST EQUIVALENT CIRCUIT FOR PV CELL
The equation for the current-voltage relationship is
When the leads from the PV cell is left unconnected, then I = 0, solve for V:
)1( / −−= TkqVoSC eIII
)1(ln +=o
SCOC I
IqTkV
19
SIMPLEST EQUIVALENT CIRCUIT FOR PV CELL
At a temperature of 25oC
)1(ln0257.0
)1( 9.38
+=
−−=
o
SCOC
VoSC
II
V
eIII
20
SIMPLEST EQUIVALENT CIRCUIT FOR PV CELL
Short circuit current is directly proportional to insolation
21
MORE ACCURATE EQUIVALENT CIRCUIT FOR PV CELL
Shading can cause major problems
22
IMPROVED CIRCUIT FOR PV CELL All real current sources have a parallel resistance.
23
More Improved Model All real sources have a series resistance:
24
General Model for a PV Cell
25
CELLS TO MODULES TO ARRAYS
An individual cell only produces about 0.5-0.6 V
Therefore, the basic building block for PV applications is a module consisting of a number of pre-wired series cells
All modules are encased in a tough and weather-resistant packages
26
CELLS TO MODULES TO ARRAYS
A typical module has 36 cells in series and is designed as a 12 V module, although they are capable of delivering higher voltages
Large 72-cell modules are also available with a rating of 24 V
Multiple modules can be wired in series to increase the voltage and in parallel to increase the current
These are called arrays27
CELLS TO MODULES TO ARRAYS
28
FROM CELLS TO MODULES
PV cells are connected in series to form a module
)( Sdmodule RIVnV −=
)( Sdmodule RIVnV −=29
FROM MODULES TO ARRAYS
The following figure shows modules in series
Modules are connected in series and parallel to form arrays
30
FROM MODULES TO ARRAYS
The following figure shows modules in parallel
31
FROM MODULES TO ARRAYS
The following figure shows series/parallel connection of modules
32
PV CURVES UNDER STANDARD TEST CONDITIONS (STC)
A PV module can be operated in three possible ways
Module sitting in the sun with no load connected V = VOC
I = 0 P = 0
Module sitting in the sun with terminals shorted V = 0 I = ISC
P=033
PV CURVES UNDER STANDARD TEST CONDITIONS (STC)
Module sitting in the sun with a load connected V = 0 I = 0 P = 0
The following figure is a generic I-V curve for a PV module, identifying VOC
ISC
P = VI
34
PV CURVES UNDER STANDARD TEST CONDITIONS (STC)
PV curves under STC
35
Simulink Model
36
Simulink PV Model
37
Simulink Model
38
PV CURVES UNDER STANDARD TEST CONDITIONS (STC)
The maximum power point (MPP) is the spot near the knee of I-V curve
The voltage and current at MPP are the rated voltage and current
Another way to find MPP is trying to find the largest possible rectangle that will fit beneath the I-V curve Rectangle area => V x I = P
39
PV CURVES UNDER STANDARD TEST CONDITIONS (STC)
40
PV CURVES UNDER STANDARD TEST CONDITIONS (STC)
A quantity that is often used to characterize module performance is the fill factor (FF) Measure of the quality of PV cell
FF is the ratio of maximum power to the product of open-circuit voltage and short-circuit current
FF for crystalline silicon is 70-75% (Range: 0.5-0.82)
SCOC
RR
SCOC IVIV
IVpointpowermaximumatPowerFFFactorFill ==)(
41
Variation of MPP with Insolation
42
Since PV I-V curve shifts all around as the amount of insolation changes and as the cell temperature changes, then a standard test condition (STC) has been established
STC Solar irradiance of 1kW/m2 (1 sun) Air-mass-ratio (AM) of 1.5 Cell temperature of 25oC
Manufacturers provide performance data for STC
PV CURVES UNDER STANDARD TEST CONDITIONS (STC)
43
PV CURVES UNDER STANDARD TEST CONDITIONS (STC)
44
IMPACTS OF TEMPERATURE & INSOLATION ON I-V CURVE
Manufacturers provide I-V curves that show how the curve shift as insolation and cell temperature changes
As insolation drops, the short circuit currents drops in proportion If insolation drops in half, ISC drops in half If insolation drops, VOC drops in a logarithmic way
45
IMPACTS OF TEMPERATURE & INSOLATION ON I-V CURVE
46
Variation of MPP with Temp
47
IMPACTS OF TEMPERATURE & INSOLATION ON I-V CURVE
As cell temperature increases, VOC decreases significantly while ISC increases slightly
Therefore, when the cell heats up the MPP shifts to the left and upward
Maximum power decreases by about 0.5% per 1oC (above cell temperature of 25oC)
48
IMPACTS OF TEMPERATURE & INSOLATION ON I-V CURVE
Two factors impact the cell temperature
Ambient temperature
Insolation
Only a small fraction of the insolation is converted to electricity and most of it is absorbed and converted to heat
49
IMPACTS OF TEMPERATURE & INSOLATION ON I-V CURVE
To help system designers to account for changes in cell performance with temperature, they provide an indicator called nominal operating cell temperature (NOCT) for an open-circuited cell
The standard NOCT is the cell temperature when Ambient is 20oC Solar irradiation is 0.8 kW/m2
Wind speed is 1 m/s50
IMPACTS OF TEMPERATURE & INSOLATION ON I-V CURVE
To account for other ambient conditions, the following is used
Where Tcell is cell temperature in oC Tamb is the ambient temperature in oC S is the solar irradiation in kW/m2
SNOCTTT ambcell ⋅−
+= )8.0
20(
51