872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 15

Electronic Materials

1 Electrical Properties

a Basic Relations

b Microscopic Behavior

d Atomistic Behaviore Resistivity of Metals

2 Semiconductors

a Intrinsic

b Extrinsic n-type

d Extrinsic p-type

e Compound Semiconductors

f Applications

3 Electrical Properties of Ceramics

1 Electrical Properties

V I

R

a Basic relations

VIe-

Ao(crosssectarea)

L

- Ohmrsquos law

Where I ndash current [A = Cs] C - Coulomb

V ndash potential [V = JC]

R ndash resistance = VA ndash Ohm

o

LR

A

oR A

L

thus

Note that

[ m] - resistivity

Alternative measure ndash conductivity

1

o

L

R A

[ m-1]

b Microscopic behavior

i Current density J

Current

Area o o o

V I V J E

A A R A

L

R L

- electric field

strength

Note that2 2

[ ] andA C V V

J E m m s L m

vd

V

Drift velocity

Charge carrier (+ or -)

Alternative relation for current density

3

No of carrierscharge per carrier drift velocity

volume

m[C]

sd

J

J n m q v

thus

Drift velocity The drift velocity of a free electron is the average electron

velocity in the direction of the force imposed by an electric field

Comparing relations for current density gives

or d d E

E n q n q

- charge carrier

mobility 2m

V s

thus

Generalizing for different types of carriers

p p p n n nn q n q

where n ndash negative charges ( of free charge per unit volume or density)

ndash ositive char es

ii Conductivity

qp = qn = |e| =016middot10-18 C ndash charge of an electron

Mobilityμ

bull When electric field is applied free charge (electrons or holes) move but

are scattered by imperfections in crystal lattice (impurities vacancies

thermal vibrations etc)

bull Scattering causes e- to lose kinetic energy and change their directions

of motion There is net e- motion toward the + electrodes

c Atomistic Behavior and Energy Band Structure

Valence Band Highest occupied energy band containing

valence electrons

Conduction Band The lowest unoccupied electron energy band

Conducting or free electrons Electrons that have been

promoted from filled to empty state

bull Metals-- Thermal energy puts

many electrons into

a higher energy state

+-

net e- flow

bull Energy States-- both cases show

Band Gap Structure Metals

Energyempty

Energy

that for metals

nearby energy

states are

accessible by

thermal

fluctuations

filledband

partlyfilledvalenceband

GAP

filled states

filledband

filledvalenceband

emptyband

filled states

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 25

bull Insulators--Higher energy states not

accessible due to large gap

bull Semiconductors--Higher energy states

separated by a smaller gapEnergy

emptyband

Energy

empty

Band Gap Structure Insulators and

Semiconductors

7

filledband

filledvalenceband

filled states

GAP

filledband

filledvalenceband

filled states

GAPGap small enough

to be overcome by

- Electrical potential

- Thermal activation

- Energy from some

light sources

d Conductivity and Resistivity of Metals

Conductivity

Where n ndash density of free electrons (Cu ~ 1029 1m3)

me ndash electron mobility (Cu ~ 0003 m2Vs)

Note that 0

0

e

e

d

dT

d

- Result of decreasing electron

ldquofreerdquo path or μ

defectsdN

For metals

bull Since 1) valence band is partially filled 2) the valence band and

conduction band overlap almost all e- are free e- Therefore n is near

constant and is large

bull What lowers the conductivity of metal

bull What lowers the mobility of e- impurities defects anything that

scatters e-

d Conductivity and Resistivity of Metals

Resistivity

- Temperature component t

t o+ a T

o a ndash material constants

- Impurity component i

i =A ci (1 ndash ci)

A ndash constant ci ndash atomic frac of impurity

- Deformation component d

t i d

2 Semiconductors

Types

bull Intrinsic pure elemental solids and compounds

bull Extrinsic doped elemental solids and compounds

with properties controlled by impurities

Doping process of adding impurities usually by diffusion

to create extrinsic semiconductors

a Intrinsic

TemperatureLight

Electric field

Valence 4+

electron(-)

hole(+)

Empty conduction band

Full valence band

Egasymp 11eV

Conductivity

Where p ndash hole density

h ndash hole mobility

Since

Temperature effectsi Charge carriers

In this case activation energy for creating charge

carriers is

thus change of number of charge carriers with temp

will be equal to

k ndash Boltzmanrsquos constant

= 13810-23 Jmol K

= 862 10-5 eVatom K

ii Conductivity

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 35

bull Intrinsic

bull Extrinsic

--

--occurs when impurities are added with a different

valence electrons than the host (eg Si atoms)

Intrinsic vs Extrinsic Conduction

electronsm3 electron mobility

holesm 3

hole mobilityh e e p e n

bull n -type Extrinsic

no applied electric field

5+

4 + 4 + 4 + 4 +

4 +

4 +4 +4 +4 +

4 + 4 +

Phosphorus atom

valence

electron

Si atom

conduction

electron

hole

bull p -type Extrinsic

no applied electric field

Boron atom

3 +

4 + 4 + 4 + 4 +

4 +

4 +4 +4 +4 +

4 + 4 +

b Extrinsic n ndash type

(impurities are added with an extra valence electron)

P ndash donor atom

For n-type extrinsic conductivity n gtgt p

thus

and since

eg d E E

kT o

n n

ln

1T

Effect of decreasing e

with T

Exhaustion no of extrinsic electrons is equal to no of impurity atoms

c Extrinsic p ndash type

(impurities are added with one less valence electron)

B ndash donor atom

Empty conduction

band

Full valence band

E g

hole(+)

In this case of p-type extrinsic conductivity p gtgt n thusand since

then

d Compound Semiconductors

Def Semiconductors with ldquoon- averagerdquo four

valence electrons

Examples III - V or II ndash VI type

III - V

II - VI

Table 182 Band Gap Energies Electron and Hole Mobilitiesand Intrinsic Electrical Conductivities at Room

Temperature for Semiconducting Materials

Note Compound can be made extrinsic by adding dopants or

if the elements are not in equal amounts

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 45

e Applications

i Light Detector Photoconductivity

Empty conductionband

Full valence band

Eg

Energy of photon

where h ndash Planckrsquos constant = 66310-34 Js = 41310-15 eVs

v ndash frequency of light [1s]

ndash light wavelength

h cE h

hole(+)

electron(-)

When

ELight gt Eg - photoconductivity

bull Upper bound

bull Lower bound

Thus for photoconductivity

Materials used for light detectors CdS Ge InP InGaAs

c ndash spee o g = m s

18red

E eV

15 8

6

413 10 3 10 31

04 10violet

violet

h c eVs m sE eV

m

31gE eV

bull Allows flow of electrons in one direction only (eg useful

to convert alternating current to direct currentbull Processing dif fuse P into one side of a B-doped crystal

bull Results

--No applied potential

no net current fl ow

--Reverse bias carrier

++ +

++

--

--

-

p-type n-type

- -

ii p-n rectifying junctionlight emitting diode (LED)

--Forward bias carrier flow

through p-type and n-type

regions

bull holes and electrons

recombine at p-n junction

bull current flows + possible

light

flow away from p-n junction

carrier conc greatly reduced

at junction little current flow

++

++

+

---

--

p-type n-type+ -

++

++

+

---

--- +

Recombination

Empty conductionband

E ghole(+)

electron(-)

- Photon wavelength

- If wavelength of emitted light

is in visible spectrum the

obtained light is

d a

h c

E E

E g

Full valence bandPhoton

bull one color monochromatic

bull non-directional and out of

phase incoherent 31 eV

18 eV

bull Electrical conductivity and resistivity are--material parameters

--geometry independent

bull Electrical resistance is--a geometry and material dependent parameter

bull Conductors semiconductors and insulators

--different in whether there are accessible energystates for conductance electrons

SUMMARY

bull or me a s con uc v y s ncrease y--reducing deformation

--reducing imperfections

--decreasing temperature

bull For pure semiconductors conductivity is increased by--increasing temperature

--doping (eg adding B to Si (p-type) or P to Si (n-type))

Electrical Properties of Ceramics

Topics

bull Dielectricsbull Ferroelectrics

bull Piezoelectric

a Dielectrics

Def Nonmetallic insulators exhibiting an electric dipole

structure

Dipole Pair of equal but opposite sign electricalcharges separated by a small distance

Characteristicsbull Dielectric constant

bull Dielectric strength

i Dielectric constant r

Parallel-plate capacitorStored charge Q [C]

Q = CmiddotV

Where C ndash capacitance

[C ] = [CV = F (Farad)]

bull Capacitor with vacuum

where o =885middot10-12 Fm ndash permittivity of

vacuumbull Capacitor with dielectric

where ndash permittivity of dielectric

-dielectric constant

r

gt 1 - reason Polarization

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 55

bull Polarization P

Def Increase in surface charge density due to the

presence of dielectric

Surface charge density or dielectric

displacement D [Cm2

] is equal to- for vacuum capacitor

- for dielectric capacitor

E ndash electric field strength

Sources of Polarization

bull Electronic Pe displacement of

the center of electron cloud

bull Ionic Pi displacement of ions in

respond to electrical field

bull Orientation Po reorientation of

permanent dipoles

P = Pe+ Pi +Po

ii Dielectric strength

Def Electrical potential gradient at which dielectric

ldquobreaks-downrdquo and becomes conductive

Units Vmil or kVmm (1 mil = 0001 in)

b Ferroelectrics

Def Dielectrics exhibiting spontaneous polarization

Example Barium titanite BaTiO3 (Perovskite structure)

Characteristics

Above 120oC ndash cubic structure

- no polarization

o ndash

Below 120oC - Tetragonal structure

- ferroelectric behavior

Application capacitors

c Piezoelectrics

Def Dielectrics which exhibit

bull Polarization induced by external pressure

bull Change of dimensions induced by electric field

Examples

- Barium titanite

- PZT Pb (ZrTi)O3 with Curie temp ~ 200oC

-Quartz SiO3

Application

- microphones

- strain gauges

- ultrasonic detectorsgenerators

- sensors and actuators in ldquosmartrdquo materials

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 25

bull Insulators--Higher energy states not

accessible due to large gap

bull Semiconductors--Higher energy states

separated by a smaller gapEnergy

emptyband

Energy

empty

Band Gap Structure Insulators and

Semiconductors

7

filledband

filledvalenceband

filled states

GAP

filledband

filledvalenceband

filled states

GAPGap small enough

to be overcome by

- Electrical potential

- Thermal activation

- Energy from some

light sources

d Conductivity and Resistivity of Metals

Conductivity

Where n ndash density of free electrons (Cu ~ 1029 1m3)

me ndash electron mobility (Cu ~ 0003 m2Vs)

Note that 0

0

e

e

d

dT

d

- Result of decreasing electron

ldquofreerdquo path or μ

defectsdN

For metals

bull Since 1) valence band is partially filled 2) the valence band and

conduction band overlap almost all e- are free e- Therefore n is near

constant and is large

bull What lowers the conductivity of metal

bull What lowers the mobility of e- impurities defects anything that

scatters e-

d Conductivity and Resistivity of Metals

Resistivity

- Temperature component t

t o+ a T

o a ndash material constants

- Impurity component i

i =A ci (1 ndash ci)

A ndash constant ci ndash atomic frac of impurity

- Deformation component d

t i d

2 Semiconductors

Types

bull Intrinsic pure elemental solids and compounds

bull Extrinsic doped elemental solids and compounds

with properties controlled by impurities

Doping process of adding impurities usually by diffusion

to create extrinsic semiconductors

a Intrinsic

TemperatureLight

Electric field

Valence 4+

electron(-)

hole(+)

Empty conduction band

Full valence band

Egasymp 11eV

Conductivity

Where p ndash hole density

h ndash hole mobility

Since

Temperature effectsi Charge carriers

In this case activation energy for creating charge

carriers is

thus change of number of charge carriers with temp

will be equal to

k ndash Boltzmanrsquos constant

= 13810-23 Jmol K

= 862 10-5 eVatom K

ii Conductivity

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 35

bull Intrinsic

bull Extrinsic

--

--occurs when impurities are added with a different

valence electrons than the host (eg Si atoms)

Intrinsic vs Extrinsic Conduction

electronsm3 electron mobility

holesm 3

hole mobilityh e e p e n

bull n -type Extrinsic

no applied electric field

5+

4 + 4 + 4 + 4 +

4 +

4 +4 +4 +4 +

4 + 4 +

Phosphorus atom

valence

electron

Si atom

conduction

electron

hole

bull p -type Extrinsic

no applied electric field

Boron atom

3 +

4 + 4 + 4 + 4 +

4 +

4 +4 +4 +4 +

4 + 4 +

b Extrinsic n ndash type

(impurities are added with an extra valence electron)

P ndash donor atom

For n-type extrinsic conductivity n gtgt p

thus

and since

eg d E E

kT o

n n

ln

1T

Effect of decreasing e

with T

Exhaustion no of extrinsic electrons is equal to no of impurity atoms

c Extrinsic p ndash type

(impurities are added with one less valence electron)

B ndash donor atom

Empty conduction

band

Full valence band

E g

hole(+)

In this case of p-type extrinsic conductivity p gtgt n thusand since

then

d Compound Semiconductors

Def Semiconductors with ldquoon- averagerdquo four

valence electrons

Examples III - V or II ndash VI type

III - V

II - VI

Table 182 Band Gap Energies Electron and Hole Mobilitiesand Intrinsic Electrical Conductivities at Room

Temperature for Semiconducting Materials

Note Compound can be made extrinsic by adding dopants or

if the elements are not in equal amounts

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 45

e Applications

i Light Detector Photoconductivity

Empty conductionband

Full valence band

Eg

Energy of photon

where h ndash Planckrsquos constant = 66310-34 Js = 41310-15 eVs

v ndash frequency of light [1s]

ndash light wavelength

h cE h

hole(+)

electron(-)

When

ELight gt Eg - photoconductivity

bull Upper bound

bull Lower bound

Thus for photoconductivity

Materials used for light detectors CdS Ge InP InGaAs

c ndash spee o g = m s

18red

E eV

15 8

6

413 10 3 10 31

04 10violet

violet

h c eVs m sE eV

m

31gE eV

bull Allows flow of electrons in one direction only (eg useful

to convert alternating current to direct currentbull Processing dif fuse P into one side of a B-doped crystal

bull Results

--No applied potential

no net current fl ow

--Reverse bias carrier

++ +

++

--

--

-

p-type n-type

- -

ii p-n rectifying junctionlight emitting diode (LED)

--Forward bias carrier flow

through p-type and n-type

regions

bull holes and electrons

recombine at p-n junction

bull current flows + possible

light

flow away from p-n junction

carrier conc greatly reduced

at junction little current flow

++

++

+

---

--

p-type n-type+ -

++

++

+

---

--- +

Recombination

Empty conductionband

E ghole(+)

electron(-)

- Photon wavelength

- If wavelength of emitted light

is in visible spectrum the

obtained light is

d a

h c

E E

E g

Full valence bandPhoton

bull one color monochromatic

bull non-directional and out of

phase incoherent 31 eV

18 eV

bull Electrical conductivity and resistivity are--material parameters

--geometry independent

bull Electrical resistance is--a geometry and material dependent parameter

bull Conductors semiconductors and insulators

--different in whether there are accessible energystates for conductance electrons

SUMMARY

bull or me a s con uc v y s ncrease y--reducing deformation

--reducing imperfections

--decreasing temperature

bull For pure semiconductors conductivity is increased by--increasing temperature

--doping (eg adding B to Si (p-type) or P to Si (n-type))

Electrical Properties of Ceramics

Topics

bull Dielectricsbull Ferroelectrics

bull Piezoelectric

a Dielectrics

Def Nonmetallic insulators exhibiting an electric dipole

structure

Dipole Pair of equal but opposite sign electricalcharges separated by a small distance

Characteristicsbull Dielectric constant

bull Dielectric strength

i Dielectric constant r

Parallel-plate capacitorStored charge Q [C]

Q = CmiddotV

Where C ndash capacitance

[C ] = [CV = F (Farad)]

bull Capacitor with vacuum

where o =885middot10-12 Fm ndash permittivity of

vacuumbull Capacitor with dielectric

where ndash permittivity of dielectric

-dielectric constant

r

gt 1 - reason Polarization

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 55

bull Polarization P

Def Increase in surface charge density due to the

presence of dielectric

Surface charge density or dielectric

displacement D [Cm2

] is equal to- for vacuum capacitor

- for dielectric capacitor

E ndash electric field strength

Sources of Polarization

bull Electronic Pe displacement of

the center of electron cloud

bull Ionic Pi displacement of ions in

respond to electrical field

bull Orientation Po reorientation of

permanent dipoles

P = Pe+ Pi +Po

ii Dielectric strength

Def Electrical potential gradient at which dielectric

ldquobreaks-downrdquo and becomes conductive

Units Vmil or kVmm (1 mil = 0001 in)

b Ferroelectrics

Def Dielectrics exhibiting spontaneous polarization

Example Barium titanite BaTiO3 (Perovskite structure)

Characteristics

Above 120oC ndash cubic structure

- no polarization

o ndash

Below 120oC - Tetragonal structure

- ferroelectric behavior

Application capacitors

c Piezoelectrics

Def Dielectrics which exhibit

bull Polarization induced by external pressure

bull Change of dimensions induced by electric field

Examples

- Barium titanite

- PZT Pb (ZrTi)O3 with Curie temp ~ 200oC

-Quartz SiO3

Application

- microphones

- strain gauges

- ultrasonic detectorsgenerators

- sensors and actuators in ldquosmartrdquo materials

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 35

bull Intrinsic

bull Extrinsic

--

--occurs when impurities are added with a different

valence electrons than the host (eg Si atoms)

Intrinsic vs Extrinsic Conduction

electronsm3 electron mobility

holesm 3

hole mobilityh e e p e n

bull n -type Extrinsic

no applied electric field

5+

4 + 4 + 4 + 4 +

4 +

4 +4 +4 +4 +

4 + 4 +

Phosphorus atom

valence

electron

Si atom

conduction

electron

hole

bull p -type Extrinsic

no applied electric field

Boron atom

3 +

4 + 4 + 4 + 4 +

4 +

4 +4 +4 +4 +

4 + 4 +

b Extrinsic n ndash type

(impurities are added with an extra valence electron)

P ndash donor atom

For n-type extrinsic conductivity n gtgt p

thus

and since

eg d E E

kT o

n n

ln

1T

Effect of decreasing e

with T

Exhaustion no of extrinsic electrons is equal to no of impurity atoms

c Extrinsic p ndash type

(impurities are added with one less valence electron)

B ndash donor atom

Empty conduction

band

Full valence band

E g

hole(+)

In this case of p-type extrinsic conductivity p gtgt n thusand since

then

d Compound Semiconductors

Def Semiconductors with ldquoon- averagerdquo four

valence electrons

Examples III - V or II ndash VI type

III - V

II - VI

Table 182 Band Gap Energies Electron and Hole Mobilitiesand Intrinsic Electrical Conductivities at Room

Temperature for Semiconducting Materials

Note Compound can be made extrinsic by adding dopants or

if the elements are not in equal amounts

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 45

e Applications

i Light Detector Photoconductivity

Empty conductionband

Full valence band

Eg

Energy of photon

where h ndash Planckrsquos constant = 66310-34 Js = 41310-15 eVs

v ndash frequency of light [1s]

ndash light wavelength

h cE h

hole(+)

electron(-)

When

ELight gt Eg - photoconductivity

bull Upper bound

bull Lower bound

Thus for photoconductivity

Materials used for light detectors CdS Ge InP InGaAs

c ndash spee o g = m s

18red

E eV

15 8

6

413 10 3 10 31

04 10violet

violet

h c eVs m sE eV

m

31gE eV

bull Allows flow of electrons in one direction only (eg useful

to convert alternating current to direct currentbull Processing dif fuse P into one side of a B-doped crystal

bull Results

--No applied potential

no net current fl ow

--Reverse bias carrier

++ +

++

--

--

-

p-type n-type

- -

ii p-n rectifying junctionlight emitting diode (LED)

--Forward bias carrier flow

through p-type and n-type

regions

bull holes and electrons

recombine at p-n junction

bull current flows + possible

light

flow away from p-n junction

carrier conc greatly reduced

at junction little current flow

++

++

+

---

--

p-type n-type+ -

++

++

+

---

--- +

Recombination

Empty conductionband

E ghole(+)

electron(-)

- Photon wavelength

- If wavelength of emitted light

is in visible spectrum the

obtained light is

d a

h c

E E

E g

Full valence bandPhoton

bull one color monochromatic

bull non-directional and out of

phase incoherent 31 eV

18 eV

bull Electrical conductivity and resistivity are--material parameters

--geometry independent

bull Electrical resistance is--a geometry and material dependent parameter

bull Conductors semiconductors and insulators

--different in whether there are accessible energystates for conductance electrons

SUMMARY

bull or me a s con uc v y s ncrease y--reducing deformation

--reducing imperfections

--decreasing temperature

bull For pure semiconductors conductivity is increased by--increasing temperature

--doping (eg adding B to Si (p-type) or P to Si (n-type))

Electrical Properties of Ceramics

Topics

bull Dielectricsbull Ferroelectrics

bull Piezoelectric

a Dielectrics

Def Nonmetallic insulators exhibiting an electric dipole

structure

Dipole Pair of equal but opposite sign electricalcharges separated by a small distance

Characteristicsbull Dielectric constant

bull Dielectric strength

i Dielectric constant r

Parallel-plate capacitorStored charge Q [C]

Q = CmiddotV

Where C ndash capacitance

[C ] = [CV = F (Farad)]

bull Capacitor with vacuum

where o =885middot10-12 Fm ndash permittivity of

vacuumbull Capacitor with dielectric

where ndash permittivity of dielectric

-dielectric constant

r

gt 1 - reason Polarization

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 55

bull Polarization P

Def Increase in surface charge density due to the

presence of dielectric

Surface charge density or dielectric

displacement D [Cm2

] is equal to- for vacuum capacitor

- for dielectric capacitor

E ndash electric field strength

Sources of Polarization

bull Electronic Pe displacement of

the center of electron cloud

bull Ionic Pi displacement of ions in

respond to electrical field

bull Orientation Po reorientation of

permanent dipoles

P = Pe+ Pi +Po

ii Dielectric strength

Def Electrical potential gradient at which dielectric

ldquobreaks-downrdquo and becomes conductive

Units Vmil or kVmm (1 mil = 0001 in)

b Ferroelectrics

Def Dielectrics exhibiting spontaneous polarization

Example Barium titanite BaTiO3 (Perovskite structure)

Characteristics

Above 120oC ndash cubic structure

- no polarization

o ndash

Below 120oC - Tetragonal structure

- ferroelectric behavior

Application capacitors

c Piezoelectrics

Def Dielectrics which exhibit

bull Polarization induced by external pressure

bull Change of dimensions induced by electric field

Examples

- Barium titanite

- PZT Pb (ZrTi)O3 with Curie temp ~ 200oC

-Quartz SiO3

Application

- microphones

- strain gauges

- ultrasonic detectorsgenerators

- sensors and actuators in ldquosmartrdquo materials

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 45

e Applications

i Light Detector Photoconductivity

Empty conductionband

Full valence band

Eg

Energy of photon

where h ndash Planckrsquos constant = 66310-34 Js = 41310-15 eVs

v ndash frequency of light [1s]

ndash light wavelength

h cE h

hole(+)

electron(-)

When

ELight gt Eg - photoconductivity

bull Upper bound

bull Lower bound

Thus for photoconductivity

Materials used for light detectors CdS Ge InP InGaAs

c ndash spee o g = m s

18red

E eV

15 8

6

413 10 3 10 31

04 10violet

violet

h c eVs m sE eV

m

31gE eV

bull Allows flow of electrons in one direction only (eg useful

to convert alternating current to direct currentbull Processing dif fuse P into one side of a B-doped crystal

bull Results

--No applied potential

no net current fl ow

--Reverse bias carrier

++ +

++

--

--

-

p-type n-type

- -

ii p-n rectifying junctionlight emitting diode (LED)

--Forward bias carrier flow

through p-type and n-type

regions

bull holes and electrons

recombine at p-n junction

bull current flows + possible

light

flow away from p-n junction

carrier conc greatly reduced

at junction little current flow

++

++

+

---

--

p-type n-type+ -

++

++

+

---

--- +

Recombination

Empty conductionband

E ghole(+)

electron(-)

- Photon wavelength

- If wavelength of emitted light

is in visible spectrum the

obtained light is

d a

h c

E E

E g

Full valence bandPhoton

bull one color monochromatic

bull non-directional and out of

phase incoherent 31 eV

18 eV

bull Electrical conductivity and resistivity are--material parameters

--geometry independent

bull Electrical resistance is--a geometry and material dependent parameter

bull Conductors semiconductors and insulators

--different in whether there are accessible energystates for conductance electrons

SUMMARY

bull or me a s con uc v y s ncrease y--reducing deformation

--reducing imperfections

--decreasing temperature

bull For pure semiconductors conductivity is increased by--increasing temperature

--doping (eg adding B to Si (p-type) or P to Si (n-type))

Electrical Properties of Ceramics

Topics

bull Dielectricsbull Ferroelectrics

bull Piezoelectric

a Dielectrics

Def Nonmetallic insulators exhibiting an electric dipole

structure

Dipole Pair of equal but opposite sign electricalcharges separated by a small distance

Characteristicsbull Dielectric constant

bull Dielectric strength

i Dielectric constant r

Parallel-plate capacitorStored charge Q [C]

Q = CmiddotV

Where C ndash capacitance

[C ] = [CV = F (Farad)]

bull Capacitor with vacuum

where o =885middot10-12 Fm ndash permittivity of

vacuumbull Capacitor with dielectric

where ndash permittivity of dielectric

-dielectric constant

r

gt 1 - reason Polarization

872019 Electronicmaterials-handout

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bull Polarization P

Def Increase in surface charge density due to the

presence of dielectric

Surface charge density or dielectric

displacement D [Cm2

] is equal to- for vacuum capacitor

- for dielectric capacitor

E ndash electric field strength

Sources of Polarization

bull Electronic Pe displacement of

the center of electron cloud

bull Ionic Pi displacement of ions in

respond to electrical field

bull Orientation Po reorientation of

permanent dipoles

P = Pe+ Pi +Po

ii Dielectric strength

Def Electrical potential gradient at which dielectric

ldquobreaks-downrdquo and becomes conductive

Units Vmil or kVmm (1 mil = 0001 in)

b Ferroelectrics

Def Dielectrics exhibiting spontaneous polarization

Example Barium titanite BaTiO3 (Perovskite structure)

Characteristics

Above 120oC ndash cubic structure

- no polarization

o ndash

Below 120oC - Tetragonal structure

- ferroelectric behavior

Application capacitors

c Piezoelectrics

Def Dielectrics which exhibit

bull Polarization induced by external pressure

bull Change of dimensions induced by electric field

Examples

- Barium titanite

- PZT Pb (ZrTi)O3 with Curie temp ~ 200oC

-Quartz SiO3

Application

- microphones

- strain gauges

- ultrasonic detectorsgenerators

- sensors and actuators in ldquosmartrdquo materials

872019 Electronicmaterials-handout

httpslidepdfcomreaderfullelectronicmaterials-handout 55

bull Polarization P

Def Increase in surface charge density due to the

presence of dielectric

Surface charge density or dielectric

displacement D [Cm2

] is equal to- for vacuum capacitor

- for dielectric capacitor

E ndash electric field strength

Sources of Polarization

bull Electronic Pe displacement of

the center of electron cloud

bull Ionic Pi displacement of ions in

respond to electrical field

bull Orientation Po reorientation of

permanent dipoles

P = Pe+ Pi +Po

ii Dielectric strength

Def Electrical potential gradient at which dielectric

ldquobreaks-downrdquo and becomes conductive

Units Vmil or kVmm (1 mil = 0001 in)

b Ferroelectrics

Def Dielectrics exhibiting spontaneous polarization

Example Barium titanite BaTiO3 (Perovskite structure)

Characteristics

Above 120oC ndash cubic structure

- no polarization

o ndash

Below 120oC - Tetragonal structure

- ferroelectric behavior

Application capacitors

c Piezoelectrics

Def Dielectrics which exhibit

bull Polarization induced by external pressure

bull Change of dimensions induced by electric field

Examples

- Barium titanite

- PZT Pb (ZrTi)O3 with Curie temp ~ 200oC

-Quartz SiO3

Application

- microphones

- strain gauges

- ultrasonic detectorsgenerators

- sensors and actuators in ldquosmartrdquo materials