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ECE 351 – Electronic Devices

ECE 371

Semiconductor Physics

1


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


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


Semiconductors

• Silicon and Germanium are group 4 elements – they have 4 electrons in their valence shell.

4

Si

Valence Electron


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


Silicon6

Si SiSi

Si

Si


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


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.


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


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

+

-


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.


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


Electron Volt

14

( )( )( )

joulescoulomb

joulecoul

voltcouleV

19

19

19

10602.1

110602.1

110602.11

−

−

−

×=

×=

×=


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


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


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



• 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



• 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


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

-



• 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



• Adding impurities is called doping.• A semiconductor doped with donor

impurities has excess free electron and is called an n-type semiconductor.

22



• 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


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



• 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


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

+



• 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)

herniter

Text Box

(775.611m,49.981m)

herniter

Text Box

VD = 0.776 V

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


7


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


The following figure shows this limit as a function of the band-gap of the semiconductor

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THE P-N JUNCTION DIODE

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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

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GENERIC PV CELL

12

GENERIC PV CELL

Electrical contacts are attached to the top and bottom of the cell for the electrons to flow

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SILICON PV CELL

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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


I = ISC – Id

18


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


At a temperature of 25oC

)1(ln0257.0

)1( 9.38

+=

−−=

o

SCOC

VoSC

II

V

eIII

20


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


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


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


The following figure shows modules in parallel

31


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


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 STC

35

Simulink Model

36

Simulink PV Model

37

Simulink Model

38


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


40


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


43


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


46

Variation of MPP with Temp

47


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


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


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


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(

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