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ELECTRONIC MATERIALS
MATERIALS FOR ELECTRONICS
Applied Electronics Department
Technical University of Cluj-Napoca
Cluj-Napoca, Cluj, 400027, Romania
Phone: +40-264-401412, E-mail: [email protected]
Lecture 04
ELECTRONIC MATERIALS
Lecture 04
When atoms combine to form a solid it is the attraction between the positive ion
cores and the valence electrons which holds the material together. While the ion
cores occupy fixed positions (either in an amorphous or a crystalline structure) the
valence electrons whiz around between them, forming a kind of electrostatic glue.
In some materials this "glue" is piled up into distinct bonds between particular ion
cores (so-called covalent bonding), but in others the electrons are more evenly
distributed in the space between the ion cores (known as metallic bonding). A
third form of bonding occurs when some of the valence electrons from one atomic
species are donated wholesale to another atomic species. Atoms of the species
which donates electrons become positive ions and atoms of the species which
accepts the electrons become negative ions. This leads to a direct electrostatic
attraction between the ions and is known as ionic bonding.
ELECTRONIC MATERIALS
Lecture 04
These different forms of bonding are largely responsible for the different thermal
and electrical properties of conductors and insulators. Electrical current is
transmitted through these solids by the motion of electrons. In a metallically
bonded material the electrons can drift easily between the ion cores, but in a
covalently bonded material they have to "hop" from one bond to the next in order
to move. In an ionically bonded material the valence electrons are tightly bound to
ion-cores which are themselves "tied" to fixed ionic sites in the crystal structure.
Thus ionic solids are generally poor conductors, covalent solids may be slightly
better, and metals are the best of all.
ELECTRONIC MATERIALS
Lecture 04
CLASSIFICATION OF MATERIALS
CONDUCTORS, INSULATORS, SEMICONDUCTORS
Materials/substances may be classified according to their capacity to carry or
conduct electric charge:
Conductors are materials capable of carrying electric current, i.e. material which
has “mobile charge carriers” (e.g. electrons, ions,..)
(Copper, aluminum, gold and silver are good conductors. Metallic materials are in
general good conductors.)
Insulators are materials with no or very few free charge carriers.
(Glass, quartz, most covalent and ionic solids, plastics are insulators.)
Semiconductors are materials with electrical properties between that of
conductors and insulators.
(Germanium Ge, silicon Si, GaAs, GaP, InP are semiconductors used widely in the
fabrication of electronic devices.)
ELECTRONIC MATERIALS
Lecture 04
Electrical Conductivity
σConductors=106-108[S/m]
σSemiconductors=10-6-105[S/m]
σInsulators=10-7-10-18[S/m]
"Solid State Chemistry: An introduction" L. Smart and E. Moore, (Chapman and Hall,1995)
ELECTRONIC MATERIALS
Lecture 04
CLASSIFICATION OF MATERIALS
CONDUCTORS, INSULATORS, SEMICONDUCTORS
Some representative resistivities ():
• R = L/A, R = resistance, L = length, A = cross section area;  = resistivity
at 20o C
resistivity [m]
resistance [] (L=1m, diam =1mm)
• aluminum
2.8x10-8
3.6x10-2
• brass
8x10-8
10.1x10-2
• copper
1.7x10-8
2.2x10-2
• platinum
10x10-8
12.7x10-2
• silver
1.6x10-8
2.1x10-2
• carbon
3.5x10-5
44.5
• germanium
0.45
5.7x105
• silicon
 640
 6x108
• porcelain
1010 - 1012
1016 - 1018
• teflon
1014
1020
• blood
1.5
1.9x106
• fat
24
3x107
ELECTRONIC MATERIALS
Lecture 04
CLASSIFICATION OF MATERIALS
Band Theory of Solids
A useful way to visualize the difference between conductors, insulators and
semiconductors is to plot the available energies for electrons in the materials.
Instead of having discrete energies as in the case of free atoms, the available
energy states form bands. Crucial to the conduction process is whether or not
there are electrons in the conduction band. In insulators there is a large energy
gap between the valence and the conduction bands. In conductors, like metals, the
valence band overlaps the conduction band, and in semiconductors there is a
small energy gap between the valence and conduction bands. With such a small
gap, the presence of a small percentage of a doping material can increase
conductivity dramatically.
An important parameter in the band theory is the Fermi level, the top of the
available electron energy levels at low temperatures. The position of the Fermi
level with the relation to the conduction band is a crucial factor in determining
electrical properties.
ELECTRONIC MATERIALS
Lecture 04
Band Theory of Solids
Schematic of band structures for (a) insulators, (b) semiconductors, and
(c) conductors. (Temperature is 0K.)
ELECTRONIC MATERIALS
Lecture 04
Insulator Energy Bands
wI>3eV
Most solid substances are insulators, and in terms
of the band theory of solids this implies that there
is a large forbidden gap between the energies of
the valence electrons and the energy at which the
electrons can move freely through the material
(the conduction band).
wF
While the doping of insulators can dramatically
change their electrical properties. However, the
doping of semiconductors has a much more
dramatic effect on their electrical conductivity and
is the basis for solid state electronics.
ELECTRONIC MATERIALS
Lecture 04
Semiconductor Energy Bands
wF
wI<3eV
For intrinsic semiconductors like silicon and
germanium, the Fermi level is essentially halfway
between the valence and conduction bands.
Although no conduction occurs at 0 K, at higher
temperatures a finite number of electrons can
reach the conduction band and provide some
current. In doped semiconductors, extra energy
levels are added.
ELECTRONIC MATERIALS
Lecture 04
Semiconductor Energy Bands
Intrinsic Semiconductor
A silicon crystal is different from an insulator because at any temperature above
absolute zero temperature, there is a finite probability that an electron in the lattice
will be knocked loose from its position, leaving behind an electron deficiency called
a hole.
If a voltage is applied, then both the electron and the hole can contribute to a small
current flow.
The conductivity of a semiconductor can be modeled in terms of the band theory of
solids. The band model of a semiconductor suggests that at ordinary temperatures
there is a finite possibility that electrons can reach the conduction band and
contribute to electrical conduction. The term intrinsic here distinguishes between
the properties of pure "intrinsic" silicon and the dramatically different properties of
doped n-type or p-type semiconductors.
ELECTRONIC MATERIALS
Lecture 04
Semiconductor Energy Bands
Doped Semiconductors
The application of band theory to n-type and p-type semiconductors shows that
extra levels have been added by the impurities. In n-type material there are
electron energy levels near the top of the band gap so that they can be easily
excited into the conduction band. In p-type material, extra holes in the band gap
allow excitation of valence band electrons, leaving mobile holes in the valence
band.
ELECTRONIC MATERIALS
DIELECTRIC MATERIALS
Applied Electronics Department
Technical University of Cluj-Napoca
Cluj-Napoca, Cluj, 400027, Romania
Phone: +40-264-401412, E-mail: [email protected]
Lecture 04
ELECTRONIC MATERIALS
Lecture 04
Dielectric means a non-conductor or poor conductor of electricity.
Dielectric means a material that presents electric polarization.
The dielectric is an insulating material or a very poor conductor of electric current.
When dielectrics are placed in an electric field, practically no current flows in them
because, unlike metals, they have no loosely bound, or free, electrons that may
drift through the material. Instead, electric polarization occurs.
The positive charges within the dielectric are displaced minutely in the direction of
the electric field, and the negative charges are displaced minutely in the direction
opposite to the electric field. This slight separation of charge, or polarization,
reduces the electric field within the dielectric.
The resistivity of insulators is:
  10 7  1018   m
ELECTRONIC MATERIALS
Lecture 04
If a material contains polar molecules, they will generally be in random orientations
when no electric field is applied. An applied electric field will polarize the material
by orienting the dipole moments of polar molecules. This decreases the effective
electric field between the plates and will increase the capacitance of the parallel
plate structure. The dielectric must be a good electric insulator so as to minimize
any DC leakage current through a capacitor.
ELECTRONIC MATERIALS
Lecture 04
Applications of dielectric materials:
•Dielectrics for capacitors
•Insulators
•Piezoelectric Transducer
•Electrooptic transducer
•Temperature transducer
•Electret
The electret (formed of elektr- from "electricity" and -et from "magnet") is a
dielectric material that has a quasi-permanent electric charge or dipole
polarisation. An electret generates internal and external electric fields, and is the
electrostatic equivalent of a permanent magnet.
ELECTRONIC MATERIALS
Lecture 04
The electric properties of dielectrics
When a dielectric is introduced in an electric field, two phenomena can be
observed: a conduction phenomenon and a polarization phenomenon.
The conduction phenomenon consists of the ordered move with respect to the
body of some charge carriers. This phenomenon appears because the electric
conductivity of the material is different from zero (there are no perfect insulators).
The polarization phenomenon consists in the limited movement, under the effect
of the electric field, of the electrons and ions; it also consists in the orientation of
the polar molecules in the direction of the electric field applied to the polar
dielectrics. In the non-homogenous dielectrics it appears an agglomeration of
charges on the separation surfaces of their homogenous parts. All these limited
movements of charges create the polarization current.
The conduction currents and the polarization currents are producing some energy
losses in the material. In the DC state, the polarization currents disappear in a very
short time after the voltage is applied, and the energy loss is only due to the
circulation of the conduction currents. In AC state, there are the polarization
currents until the voltage is removed, and the energy loss is due to the circulations
of both type of currents (conduction and polarization currents).
ELECTRONIC MATERIALS
Lecture 04
The electric properties of dielectrics are related to the phenomena mentioned
before, and are:
a) The relative dielectric constant
εr
is related to the polarization phenomenon.
The relative static permittivity is represented as εr or sometimes κ or K or Dk.
It is defined as being the ratio between the capacitance of a capacitor having as
dielectric the studied material and the capacitance of the same capacitor having
vacuum as dielectric.
C
r 
C0
It is also called relative permittivity.
ELECTRONIC MATERIALS
Lecture 04
Measurement:
The relative dielectric constant εr can be measured for static electric fields as
follows: first the capacitance of a test capacitor C0 is measured with vacuum
between its plates. Then, using the same capacitor and distance between its
plates the capacitance CX with a dielectric between the plates is measured. The
relative dielectric constant can be then calculated as:
For time-varying electromagnetic fields, the dielectric constant of materials
becomes frequency dependent and is generally called permittivity.
ELECTRONIC MATERIALS
Lecture 04
The relative permittivity is a nondimensional number and it has the value:
•approximately 1 for gases
•1 – 3 for solids and nonpolar liquids
•3 – 15 for polar solids
•tens for polar liquids
•hundreds or thousands for ferroelectrics
The permittivity of a material is usually given relative to that of vacuum, as a
relative permittivity εr (also called dielectric constant). The actual permittivity is then
calculated by multiplying the relative permittivity by ε0:
[F/m]
where
χe
is the electric susceptibility of the material and
O 
1
12
F / m

8
,
854

10
9
4  9  10
ELECTRONIC MATERIALS
Lecture 04
A low-K dielectric is one with a small dielectric constant. In digital circuits,
insulating dielectrics separate the conducting parts (wire interconnects and
transistors) from one another. To make higher-speed chips, the transistors must be
placed closer and closer together, and thus the insulating layer becomes thinner.
This leads to charge build up and crosstalk, adversely affecting the maximum
operating speed and performance of the chip.
Low-K dielectrics have very low dielectric constants, reducing parasitic
capacitance and enabling faster switching speeds and lower heat dissipation.
The K refers to the dielectric constant. For example the dielectric constant of SiO2,
the insulating material used in silicon chips, is 3.9. By doping it with fluorine to
produce fluorinated silica glass, this is lowered to 3.5. Another approach is to make
a porous dielectric. The pores lead to a smaller average dielectric constant, since
air has a dielectric constant of roughly 1.0005.
ELECTRONIC MATERIALS
Lecture 04
The term high-κ dielectric refers to materials with a high dielectric constant (κ)
which may be used in next generation semiconductor components to replace the
silicon dioxide (SiO2) gate dielectric, especially for the low standby power (LSTP)
applications. With the continued scaling of the gate oxide to below 2 nm, leakage
currents due to tunneling are very high, so the thickness must be increased without
reducing the associated capacitance.
ELECTRONIC MATERIALS
Lecture 04
From an electrical standpoint, the MOS structure is equivalent to a parallel plate
capacitor. When a voltage is applied between the gate and source terminals, the
resulting electric field penetrates through the oxide, creating a so-called "inversion
channel" within the channel underneath.
The inversion channel is of the same
type — P-type or N-type — as the
source and drain of the transistor,
providing a conduit through which
current can pass. Ignoring quantum
mechanical and depletion effects from
the Si substrate and gate, the
capacitance C of this parallel plate
capacitor is given by
Where
A is the capacitor area
κ is the relative dielectric constant of the
material (3.9 for silicon dioxide)
ε0 is the permittivity of free space
d is the distance between the gate and body
ELECTRONIC MATERIALS
Lecture 05
b) The volume resistivity ρV and the surface resistivity ρS are related to the
conduction phenomenon.
The volume resistivity is the resistance to leakage current through the body of an
insulating material. In SI, volume resistivity is numerically equal to the directcurrent resistance between opposite faces of a one-meter cube of the material
(Ohm-m).
It takes values between 106 and 1018[Ωm] for insulating materials.
U
a
R V   V  2
IV
a
 V  R V  a
If a  1m   V  R V  1mm
In practice it is used [Ω cm] (1 Ω cm=10-2 Ω m).
ELECTRONIC MATERIALS
Lecture 05
The surface resistivity is determined by the ratio of DC voltage U drop per unit
length L to the surface current IS per unit width D. Surface resistivity is a property
of a material. Surface resistivity is measured in Ω.
U
S  L
IS
D
If D  L

U
 S   RS 
IS
The current doesn't pass through the material, only through the surface of the
material.
The thickness of the layer (and ρS) depends on the chemical composition, on the
structure of the material, on the impurities, on the processing degree of the
material, on the humidity, etc.
For insulating materials ρs = 108 ÷1018[Ω].
ELECTRONIC MATERIALS
Lecture 05
c) The Dielectric Loss Factor (or dielectric dissipation factor) – tan δ – is
related both to the conduction phenomena and to the electric polarization. The
dielectric loss factor characterizes the total energy loss from the dielectric.
The dielectric loss angle of an insulating material is the difference between ninety
(90°) degrees and the dielectric phase angle.
ELECTRONIC MATERIALS
Lecture 05
I
IC
δ
φ
IA
U
The dielectric loss angle:
δ = 90-φ
represents the complement of the phase angle between the applied voltage U and
the total current I (only between the active current IA and the capacitive one IC) that
flows through the dielectric.
ELECTRONIC MATERIALS
Lecture 05
The dielectric dissipation factor (tan δ) of an insulating material is the tangent of
the loss angle δ. In a perfect dielectric, the voltage wave and the current are
exactly 90° out of phase. As the dielectric becomes less than 100% efficient, the
current wave begins to lag the voltage in direct proportion. The amount the current
wave deviates from being 90° out of phase with the voltage is defined as the
dielectric loss angle. The tangent of this angle is known as the loss tangent or
dissipation factor.
A low dissipation factor is important for plastic insulators in high frequency
applications such as radar equipment and microwave parts; smaller values mean
better dielectric materials. A high dissipation factor is important for welding
capabilities. Both relative permittivity and dissipation factor are measured using the
same test equipment. Test values obtained are highly dependent on temperature,
moisture levels, frequency and voltage.
ELECTRONIC MATERIALS
Lecture 05
d) The dielectric strength (ESTR) is the minimum electric field that produces
breakdown.
The dielectric strength is given by the ratio between the breakdown voltage (USTR)
and the thickness (d) of the material between the electrodes.
E STR 
U STR
d
MV / m
or
kV / cm
Breakdown Voltage (Insulator)
Is a parameter of an insulator that defines the maximum voltage difference that
can be applied across the material before the insulator collapses and conducts.
This may create a weak point in the insulator from a molecular change created by
the current flow.
Two different breakdown voltage measurements of a material are the AC and
impulse breakdown voltages. The AC voltage is the line frequency of the mains
(either 50 or 60 Hz depending on where you live). The impulse breakdown voltage
is simulating lightning strikes, and uses a 1.2 microsecond rise for the wave to
reach 90% amplitude then drops back down to 50% amplitude after 50
microseconds.
ELECTRONIC MATERIALS
Lecture 05
APPLICATIONS OF DIELECTRIC MATERIALS
When an electric field is applied to a dielectric medium, a current flows. The total
current flowing in a real dielectric has two parts: a conduction and a displacement
current. The displacement current can be considered the elastic response of the
dielectric material to the applied electric field. As the magnitude of the electric field
is increased, the additional displacement is stored as potential energy within the
dielectric. When the electric field is decreased, the dielectric releases some of the
stored energy as a displacement current. The electric displacement can be
separated into a vacuum contribution and one arising from the dielectric by
where D and E are the amplitudes of the displacement and electrical fields, P is
the polarization of the medium and χ its electric susceptibility. It follows that the
relative permittivity and susceptibility of a dielectric are related,
ELECTRONIC MATERIALS
Lecture 05
1. Dielectrics in Capacitors
A dielectric material placed between the plates of a parallel plate capacitor causes
an increase in the capacitance in proportion to the relative permittivity of the
material:
C  rC0
This happens because an electric field polarizes the
molecules of the dielectric, producing
concentrations of charge on its surfaces that create
an electric field opposed (antiparallel) to that of the
capacitor. Thus, a given amount of charge produces
a weaker field between the plates than it would
without the dielectric, which reduces the electric
potential. Considered in reverse, this argument
means that, with a dielectric, a given electric
potential causes the capacitor to accumulate a
larger charge polarization.
ELECTRONIC MATERIALS
Lecture 05
In AC, the relative permittivity of the material isn’t a scalar constant.
D
r 
  'r  j "r
0  E
D and E are the amplitudes of the displacement and electrical fields in complex
representation.
The admittance of capacitor is:
Y  j r C 0  j 'r  "r C 0  "r C 0  j 'r C 0
ELECTRONIC MATERIALS
Lecture 05
If the dielectric substance between a capacitor's plates is not a perfect insulator,
there will be a path for direct current (DC) from one plate to the other. This is
typically called leakage resistance, and it is modeled as a shunt resistance to an
ideal capacitance.
IC
I
δ
φ
C ech   'r C 0
R ech
1

 "r C 0
IA
U
ELECTRONIC MATERIALS
Lecture 05
The Dielectric Loss Factor (or dielectric dissipation factor) is related both to the
conduction phenomena and to the electric polarization. The dielectric loss factor
characterize the total energy loss from the dielectric.
The Dielectric Loss Factor is defined by relation:
U  Ia
Pa
 "r
1
tg  h 


 '
Pr
R ech C ech  r
U  IC
The Q factor or quality factor is the inverse of the dissipation factor (tan δ).
1
 'r
Q
 R ech Cech  "
tg h
r
ELECTRONIC MATERIALS
Lecture 05
2. Electrical Insulator
An insulator is a material or object which contains no movable electrical charges.
When a voltage is placed across an insulator, no charges flow, so no electric
current appears.
The term electrical insulator has the same meaning as the term dielectric, but the
two terms are often used in different contexts. Very pure semiconductors are
insulators at low temperatures unless doped with impurity atoms that release extra
charges which can flow in a current. A few materials (such as silicon dioxide) are
almost ideal electrical insulators, a property that is invaluable in flash memory
technology. Teflon is another almost ideal insulator, making it a valuable material
for long term charge storage in electrets. A much larger class of materials, for
example rubber and most plastics are still "good enough" to insulate electrical
wiring and cables even though they may have lower bulk resistivity. These
materials can serve as practical and safe insulators for low to moderate voltages
(hundreds, or even thousands, of volts).
ELECTRONIC MATERIALS
Lecture 05
3. Piezoelectric Transducer
Piezoelectricity is the ability of crystals to generate a voltage in response to
applied mechanical stress. The word is derived from the Greek piezein, which
means to squeeze or press. The piezoelectric effect is reversible in that
piezoelectric crystals, when subjected to an externally applied voltage, can change
shape by a small amount. (For instance, the deformation is about 0.1% of the
original dimension in PZT.) The effect finds useful applications such as the
production and detection of sound, generation of high voltages, electronic
frequency generation, microbalance, and ultra fine focusing of optical assemblies.
ELECTRONIC MATERIALS
Lecture 05
Piezoelectric sensor
The principle of operation of a piezoelectric sensor is that a physical dimension,
transformed into a force, acts on two opposing faces of the sensing element.
Depending on the design of a sensor, different "modes" to load the piezoelectric
element can be used: longitudinal, transversal and shear.
To detect sound, e.g. piezoelectric microphones (sound waves bend the
piezoelectric material, creating a changing voltage) and piezoelectric pickups for
electrically amplified guitars. A piezo sensor attached to the body of an instrument
is known as a contact microphone.
Piezoelectric elements are also used in the generation of sonar waves.
Piezoelectric microbalances are used as very sensitive chemical and biological
sensors.
Piezoelectric transducers are used in electronic drum pads to detect the impact of
the drummer's sticks.
Automotive engine management systems use a piezoelectric transducer to detect
detonation, by sampling the vibrations of the engine block.
ELECTRONIC MATERIALS
Lecture 05
Piezoelectric Actuators
As very high voltages correspond to only tiny changes in the width of the crystal,
making piezo crystals the most important tool for positioning objects with extreme
accuracy — thus their use in actuators.
Loudspeakers: Voltages are converted to mechanical movement of a piezoelectric
polymer film.
Piezoelectric motors: piezoelectric elements apply a directional force to an axle,
causing it to rotate. Due to the extremely small distances involved, the piezo motor
is viewed as a high-precision replacement for the stepper motor.
Inkjet printers: On some high-end inkjet printers, piezoelectric crystals are used to
control the flow of ink from the cartridge to the paper.
Ultrasonic transducers
Piezoelectric materials are used as ultrasonic transducers for imaging applications
(e.g. medical imaging, industrial nondestructive testing, or NDT) and high power
applications (e.g. medical treatment, sonochemistry and industrial processing).
For imaging applications, the transducer can act as both a sensor and an actuator.
Ultrasonic transducers can inject ultrasound waves into the body, receive the
returned wave, and convert it to an electrical signal (a voltage). Most medical
ultrasound transducers are piezoelectric.
ELECTRONIC MATERIALS
Lecture 05
4. Electro-optic Transducer
The electro-optic effect is a change in the optical properties of a material in
response to an electric field that varies slowly compared with the frequency of light.
Electro-optic modulators are usually built with electro-optic crystals exhibiting the
Pockels effect. The transmitted beam is phase modulated with the electric signal
applied to the crystal. Amplitude modulators can be built by putting the electrooptic crystal between two linear polarizers.
Liquid crystals find wide use in liquid crystal displays, which rely on the optical
properties of certain liquid crystalline molecules in the presence or absence of an
electric field. In a typical device, a liquid crystal layer sits between two polarizers
that are crossed (oriented at 90° to one another). The liquid crystal is chosen so
that its relaxed phase is a twisted one. This twisted phase reorients light that has
passed through the first polarizer, allowing it to be transmitted through the second
polarizer and reflected back to the observer.
ELECTRONIC MATERIALS
Lecture 05
ELECTRONIC MATERIALS
Indium tin oxide (ITO, or tindoped indium oxide) is a
mixture of indium(III) oxide
(In2O3) and tin(IV) oxide
(SnO2), typically 90% In2O3,
10% SnO2 by weight. It is
transparent and colorless in
thin layers.
Lecture 05
Reflective twisted nematic liquid crystal display.
1. Vertical filter film to polarize the light as it enters.
2. Glass substrate with ITO electrodes. The shapes
of these electrodes will determine the dark
shapes that will appear when the LCD is turned
on or off. Vertical ridges etched on the surface
are smooth.
3. Twisted nematic liquid crystals.
4. Glass substrate with common electrode film (ITO)
with horizontal ridges to line up with the
horizontal filter.
5. Horizontal filter film to block/allow through light.
6. Reflective surface to send light back to viewer.
ELECTRONIC MATERIALS
Lecture 07
5. Pyroelectric Transducer
Pyroelectricity is the ability of certain materials to generate an electrical potential
when they are heated or cooled. As a result of this change in temperature, positive
and negative charges move to opposite ends through migration (i.e. the material
becomes polarised) and hence, an electrical potential is established.
Although artificial pyroelectric materials have been engineered, the effect was first
discovered in minerals such as quartz and tourmaline and other ionic crystals.
The name is derived from the Greek pyr, fire, and electricity.
Pyroelectric charge in minerals develops on the opposite faces of asymmetric
crystals. The direction in which the propagation of the charge tends toward is
usually constant throughout a pyroelectric material, but in some materials this
direction can be changed by a nearby electric field. These materials are said to
exhibit ferroelectricity. All pyroelectric materials are also piezoelectric, the two
properties being closely related.
Very small changes in temperature can produce an electric potential due to a
materials' pyroelectricity. Motion detection devices are often designed around
pyroelectric materials, as the heat of a human or animal from several feet away is
enough to generate a difference in charge.
ELECTRONIC MATERIALS
Lecture 07
6. Electret
Electret (formed of elektr- from "electricity" and -et from "magnet") is a dielectric
material that has a quasi-permanent electric charge or dipole polarisation. An
electret generates internal and external electric fields, and it is the electrostatic
equivalent of a permanent magnet.
Electrets are only in a metastable state, but may still store excess charge or
polarization for extremely long periods of time.
Manufacture
Bulk electrets can be prepared by cooling a suitable dielectric material within a
strong electric field, after heating it above its melting temperature. The field
repositions the charge carriers or aligns the dipoles within the material. When the
material cools, solidification freezes them in position. Materials used to for electrets
are usually waxes, polymers or resins.
ELECTRONIC MATERIALS
Lecture 07
Electret materials have recently found commercial and technical interest. For
example, they are used in electrostatic microphones and in copy machines. They
are also used in some types of air filters, for electrostatic collection of dust
particles, and in electret ion chambers for measuring ionizing radiation.
An electret microphone is a relatively new type of condenser microphone, which
eliminates the need for a high-voltage bias supply by using a permanently-charged
material.
ELECTRONIC MATERIALS
Lecture 07
ELECTRICAL CONDUCTION
Electrical conduction is the movement of electrically charged particles through a
transmission medium. The movement of charge constitutes an electric current in
response to an electric field. The physical parameters governing this transport
depend upon the material.
Conduction is well-described by Ohm's Law, which states that the current is
proportional to the applied electric field. The ease with which current density
(current per area) j appears in a material is measured by the conductivity σ,
defined as:
J  E 
1
E

ELECTRONIC MATERIALS
Lecture 07
1. Electrical Conduction of Insulating Solids
Materials in which all energy bands are full (i.e. the Fermi energy is between two
bands) are insulators. Bands which are completely full of electrons cannot conduct
electricity, because there is no state of nearby energy to which the electrons can
jump.
If the crystal is with impurities, in the band gap of insulating material the extra
levels have been added by the impurities.
The electrical conduction in insulating solids is given by the intrinsic conduction
and by the extrinsic conduction.
If the insulating material is very pure, the electrical conduction is insignificant.
ELECTRONIC MATERIALS
Lecture 07
σel, σion, σt
σt
σion
σel
E[V/m]
Ohm’s interval
105÷106
108
Pool’s
Breakdown
interval
interval
The variation of the electric conductivity of an insulating crystal with the intensity of
electric field.
ELECTRONIC MATERIALS
Lecture 07
1. In the usual electric fields (E<105-106V/m), the ionic conduction is
predominant, and the total conductivity (σt), practically it does not depend on the
intensity of the electric field. This domain is called Ohm’s interval.
The ionic conduction of the insulating crystals is realized through the convection of
the interstitial own ions (resulting by Frenkel defects), by the convection of the
vacancies (resulting by Frenkel defects or Schottky defects), or through the
convection of the impurities ions (from the nodes or interstices).
The electronic conduction is realised like in the case of extrinsic semiconductor,
through the convection of the electrons that reached the conduction band from the
donor level or from the valence band and through the convection of the holes from
the valence band.
ELECTRONIC MATERIALS
Lecture 07
w
BC
wI
d
wF
a
we
w
whg
BV
x
The band structure for insulators with impurities.
ELECTRONIC MATERIALS
Lecture 07
is the sum of the intrinsic conductivity (σi) and
the extrinsic conductivity given by electrons (σe) and by holes (σh).
The electronic conductivity
(σel)
 el   i   e   h  C i  e

wi
2 kT
 Ce  e

we
2 kT
 Ch  e

wh
2 kT
Where Ci, Ce and Ch are constants and don’t depend by the time and by the
electric field’s intensity. Ce and Ch are dependent by the number of extra levels
(they are proportional with the concentration of the impurities).
ELECTRONIC MATERIALS
Lecture 07
2. In the intense electric fields (105<E<108V/m), the electric conduction is given
by the electrons from the conduction band, that are accelerated by the electric
field. This domain is called Pool’s interval.
The avalanche of electrons theory
If consider an electron from conduction band with the weight m0 and the charge q0,
it cover a distance x, under the effect of the force:
F  q0  E
The electron accumulates an energy:
Wa  q 0  E  x
If we note with Ĩ the medium distance covered by the electron, than the electron
accumulates the energy:
~
Wa  q 0  E  l
ELECTRONIC MATERIALS
Lecture 07
After a collision with a particle, the electron remains with a rest of energy:
~
Wr  q 0  E  l  h  f f
Then, the electron will accumulate a new quantity of energy Wa, and before a new
collision it will have the energy:
~
Wa  Wr  q 0  E  l  Wr
And after a new collision, the electron remains with a 2Wr energy.
By cumulative effect, at a given moment, the energy of the electron will be enough
to ionize the particle.
In the same way, each of the two electrons will ionize a particle. So, there are 4
free electrons.
In this way, will result an avalanche of electrons.
The appearance of the avalanches of electrons is dependent by the electric field,
and explains the variation of the electronic conductivity with the intensity of the
electric field.
The electric conductivity rise with the temperature, with the percent of impurities,
especially with the humidity. So, it is necessary to use pure materials, varnished or
impregnated, in order to protect from the humidity.
ELECTRONIC MATERIALS
Lecture 07
2. Electrical Conduction of Insulating Liquids
Electrical conduction of insulating liquids depends on the type of the molecules
(polar or non-polar) and the purity degree.
In the non-polar liquids, the charge carriers are the ions (positive and negative).
The ions are resulting from the dissociation of the molecules of impurity.
In the polar liquids, the electric conduction is given by the ions of impurity and by
the ions that result from the dissociation of the polar molecules.
In the non-polar liquids, the electric conduction is given by the ions of impurity and
it has a very small value. So, the non-polar liquids have very good insulating
properties.
E.g. The transformer oil (an non-polar liquid)
ρ=2*1010 Ωm
unpurified
ρ=5*1012 Ωm
purified
ρ=1016 Ωm
if it is very pure
ELECTRONIC MATERIALS
Lecture 07
As you know, the first law of Materials science is "Everything can be broken".
Dielectrics are no exception to this rule. If you increase the voltage applied to a
capacitor, eventually you will produce a big bang and a lot of smoke - the dielectric
material inside the capacitor will have experienced "electrical breakdown" or
electrical break-through, an irreversible and practically always destructive sudden
flow of current.
The critical parameter is the field strength E in the dielectric. If it is too large,
breakdown occurs. The (DC) current vs. field strength characteristic of a dielectric
therefore may look this:
E
ESTR
ELECTRONIC MATERIALS
Lecture 07
After reaching ESTR, a sudden flow of current may, within very short times (10–8 s)
completely destroys the dielectric to a smoking hot mass of indefinable structure.
Unfortunately, ESTR is not a well defined material property, it depends on many
parameters, the most notable (besides the basic material itself) being the
production process, the thickness, the temperature, the internal structure (defects
and the like), the age, the environment where it is used (especially humidity) and
the time it experienced field stress.
Material
Critical Field Strength
[kV/cm]
Oil
200
Glass, ceramics
200...400
Mica
200...700
Oiled paper
1800
Polymers
50...900
SiO2 in ICs
> 10 000
ELECTRONIC MATERIALS
Lecture 07
What are the atomic mechanisms by which breakdown occurs or dielectrics fail?
This is a question not easily answered because there is no general mechanism
expressible in formulas. Most prominent are the following disaster scenarios:
Thermal breakdown
A tiny little current that you can't even measure is flowing locally through "weak"
parts of the dielectric. With increasing field strength this current increases,
producing heat locally, which leads to the generation of point defects. Ionic
conductivity sets in, more heat is produced locally, the temperature goes up even
more.... - boooom!
This is probably the most common mechanism in run-of-the-mill materials which
are usually not too perfect.
Thermal breakdown: due to small (field dependent) currents flowing through "weak" parts of
the dielectric.
ELECTRONIC MATERIALS
Lecture 07
Avalanche breakdown
Even the most perfect insulator contains a few free electron. Either because there
is still a non-zero probability for electrons in the conduction band, even for large
band gaps, or because defects generate some carriers, or because irradiation
(natural radioactivity may be enough) produces some.
In large electrical field these carriers are accelerated; if the field strength is above
a certain limit, they may pick up so much energy that they can rip off electrons
from the atoms of the materials. A chain reaction then leads to a swift avalanche
effect; the current rises exponentially ... boom!
Avalanche breakdown due to occasional free electrons being accelerated in the field;
eventually gaining enough energy to ionize atoms, producing more free electrons in a
runaway avalanche.
ELECTRONIC MATERIALS
Lecture 07
Local discharge
In small cavities (always present in sintered ceramic dielectrics) the field strength
is even higher than the average field (E is small)- a microscopic arc discharge may
be initiated. Electrons and ions from the discharge bombard the inner surface and
erode it. The cavity grows, the current in the arc rises, the temperature rises ... boooom!
Local discharge producing micro-plasmas in small cavities, leading to slow erosion of the
material.
Electrolytic breakdown
Not as esoteric as it sounds! Local electrolytical (i.e involving moving ions) current
paths transport some conducting material from the electrodes into the interior of
the dielectric. Humidity (especially if it is acidic) may help. In time a filigree
conducting path reaches into the interior, reducing the local thickness and thus
increasing the field strength. The current goes up....booom!
This is a very irreproducible mechanism because it depends on many details,
especially the local environmental conditions. It may slowly built up over years
before it suddenly runs away and ends in sudden break-through.
Electrolytic breakdown due to some ionic micro conduction leading to structural changes by,
e.g., metal deposition.
ELECTRONIC MATERIALS
Lecture 07
SOME DIELECTRIC MATERIALS
Silicon dioxide (SiO2)
Other names: silica, quartz, sand
The chemical compound silicon dioxide, also known as silica (from the Latin
"silex"), is an oxide of silicon, chemical formula SiO2, and has been known for its
hardness since the 9th century. Silica is most commonly found in nature as sand or
quartz. It is a principal component of most types of glass. Silica is the most
abundant mineral in the earth's crust.
The natural ("native") oxide coating that grows on silicon is hugely beneficial in
microelectronics. It is a superior electric insulator, with high chemical stability. In
electrical applications, it can protect the silicon, store charge, block current, and
even act as a controlled pathway to allow small currents to flow through a device.
At room temperature, however, it grows extremely slowly, and so to manufacture
such oxide layers, the traditional method has been heating of silicon in hightemperature furnaces within an oxygen ambient (thermal oxidation).
ELECTRONIC MATERIALS
Lecture 07
Quartz crystal group
ELECTRONIC MATERIALS
Lecture 08
Silicon nitride (Si3N4)
Silicon nitride is a hard, solid substance. It is the main component in silicon nitride
ceramics, which have relatively good shock resistance and other mechanical and
thermal properties as compared to other ceramics.
Silicon nitride can be obtained by direct reaction between silicon and nitrogen at
high temperatures. Electronic-grade silicon nitride is usually formed using chemical
vapor deposition (CVD), or one of its variants, such as plasma-enhanced chemical
vapor deposition (PECVD). Silicon nitride nanowires can also be produced by solgel method.
Natural existence of silicon nitride is restricted to meteorites, where it very rarely
occurs as mineral nierite.
In electronics, silicon nitride is usually used either as an insulator layer to
electrically isolate different structures or as an etch mask in bulk micromachining.
As a passivation layer for microchips, it is superior to silicon dioxide, as it is a
significantly better diffusion barrier against water molecules and sodium ions, two
major sources of corrosion and instability in microelectronics. It is also used as a
dielectric between polysilicon layers in capacitors in analog chips.
ELECTRONIC MATERIALS
Lecture 08
Metal oxides
Tantalum is a chemical element with the symbol Ta and
atomic number 73. A rare, hard, blue-grey, transition
metal, tantalum is highly corrosion-resistant and occurs
naturally in the mineral tantalite, always together with
the chemically similar niobium.
Tantalum electrolytic capacitors exploit the tendency of
tantalum to form a protective oxide surface layer, using
tantalum powder, pressed into a pellet shape, as one
"plate" of the capacitor, the oxide as the dielectric, and
an electrolytic solution or conductive solid as the other
"plate". Because the dielectric layer can be very thin
(thinner than the similar layer in, for instance, an
aluminium electrolytic capacitor), a high capacitance can
be achieved in a small volume. Because of the size and
weight advantages, tantalum capacitors are attractive
for portable telephones, pagers, personal computers,
and automotive electronics
Tantalite
Tantalum electrolytic
capacitor
ELECTRONIC MATERIALS
Lecture 08
Tantalum pentoxide (Ta2O5) is also known as tantalum (V) oxide.
Ta2O5 is used to make capacitors in cell phones and electronic circuitry; thin-film
components; and high-speed tools. In the 1990s, there was a very strong interest
to do research on tantalum oxide as a high-k dielectric for DRAM capacitor
applications. For example, Elpida Memory (a Japanese company making DRAM)
has put in a strong effort to improve ultra thin tantalum oxide films for DRAM
applications. There is still some interest to use it in capacitors for RF CMOS
integrated circuits.
ELECTRONIC MATERIALS
Lecture 08
Aluminium oxide
Aluminium oxide is an oxide of aluminium with the chemical formula Al2O3. It is
also commonly referred to as alumina or aloxite. It is produced from bauxite. Its
most significant use is in the production of aluminium metal, although it is also
used as an abrasive due to its hardness and as a refractory material due to its high
melting point.
Aluminium oxide is an electrical insulator but has a relatively high thermal
conductivity (40 W/m K).
ELECTRONIC MATERIALS
Lecture 08
Barium titanate
Barium titanate is an oxide of barium and titanium with the chemical formula
BaTiO3. It is a ferroelectric ceramic material, with a photorefractive effect and
piezoelectric properties.
Ferroelectricity is a physical property of a material whereby it exhibits a spontaneous electric polarization
which can be reversed by the application of an external electric field.
The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic
moment.
Barium titanate is used as a dielectric material for ceramic capacitors, and as a
piezoelectric material for microphones and other transducers. As a piezoelectric
material, it was largely replaced by lead zirconate titanate, also known as PZT.
Polycrystalline barium titanate displays positive temperature coefficient, making it a
useful material for thermistors.
ELECTRONIC MATERIALS
Lecture 08
Glass
In the technical sense, glass is an inorganic product of fusion which has been
cooled to a rigid condition without crystallizing. Many glasses contain silica as their
main component and glass former.
Paper
It is produced by pressing together moist fibers, typically cellulose pulp derived
from wood, rags or grasses, and drying them into flexible sheets.
It can be used as capacitor dielectric (permittivity 1.5 - 3).
Plastic
Plastic is the general common term for a wide range of synthetic or semisynthetic
organic solid materials suitable for the manufacture of industrial products. Plastics
are typically polymers of high molecular weight, and may contain other substances
to improve performance and/or reduce costs.
The development of plastics has come from the use of natural plastic materials
(e.g., chewing gum, shellac) to the use of chemically modified natural materials
(e.g., rubber, nitrocellulose, collagen) and finally to completely synthetic molecules
(e.g., bakelite, epoxy, polystyrene, polyvinyl chloride, polyethylene).
ELECTRONIC MATERIALS
Lecture 08
Mica
Micas are a group of aluminium phyllosilicate minerals which are found worldwide.
Mica has a lamellar form with a black luster.
Common micas:
•Phlogopite
•Biotite
•Zinnwaldite
•Lepidolite
•Muscovite
Mica has a high dielectric strength and excellent chemical stability, making it a
favourit material for manufacturing capacitors for radio frequency applications. It
has also been used as an insulator in high voltage electrical equipment.
Mica slices are used in electronics to provide electric insulation between a heatgenerating component and the heat sink used to cool it. The same word is
sometimes used by technicians to designate a synthetised gum (usually blue or
gray) which is used for the same purpose, but which does not actually consist of
silicate mineral (language abuse).
ELECTRONIC MATERIALS
Lecture 08
Mica flakes
Rock with mica
ELECTRONIC MATERIALS
Lecture 08
Mica slices are used in electronics to provide electric insulation between an
electronic component which can generate heat and the heat sink used to cool it.
Mica is used because it can be be split into very thin slices, and this keeps its
thermal resistance low while retaining sufficient dielectric strength to prevent
current from flowing across it at moderate voltages. The insulation is usually
necessary when the heat sink is earthed while the electronic component's metal
surfaces will be connected to a power supply or signal line. If they were in direct
contact this could form a short circuit. Heat sink insulation can also be necessary
to prevent the heat sink from acting like an antenna if the component is
connected to a rapidly varying signal.
ELECTRONIC MATERIALS
Mica insulator slices for TO-3 and TO-264 packages.
Lecture 08
ELECTRONIC MATERIALS
Lecture 08
Epoxy resins
In chemistry, epoxy or polyepoxide is a thermosetting epoxide polymer that cures
(polymerizes and crosslinks) when mixed with a catalyzing agent or hardener.
Most common epoxy resins are produced from a reaction between epichlorohydrin
and bisphenol-A.
Epoxy resin formulations are important in the electronics industry, and are
employed in motors, generators, transformers, switchgear, bushings, and
insulators. Epoxy resins are excellent electrical insulators and protect electrical
components from short circuiting, dust and moisture. In the electronics industry
epoxy resins are the primary resin used in overmolding integrated circuits,
transistors and hybrid circuits, and making printed circuit boards. The largest
volume type of circuit board — an "FR-4 board" — is a sandwich of layers of glass
cloth bonded into a composite by an epoxy resin. Epoxy resins are used to bond
copper foil to circuit board substrates, and are a component of the solder mask on
many circuit boards.
Flexible epoxy resins are used for potting transformers and inductors. By using
vacuum impregnation on uncured epoxy, winding-to-winding, winding-to-core, and
winding-to-insulator air voids are eliminated. The cured epoxy is an electrical
insulator and a much better conductor of heat than air. Transformer and inductor
hot spots are greatly reduced, giving the component a stable and longer life than
unpotted product.
Epoxy resins are applied using the technology of resin casting.
ELECTRONIC MATERIALS
Lecture 08
An epoxy encapsulated hybrid circuit on a printed circuit board.