1. MATERIALS FOR ELECTRONICS
Electronic Materials are what you find inside the components making up electronic
products. They consist of some stuff that you cannot easily exchange with something else
- not even in principle - without losing the function.
•
What you can change easily for example, is the material for the box, the housing.
Use Al instead of plastic or vice versa for your video recorder - it would still
work, needing at most some minor adjustments.
• You also may change (in principle) the metal for real wires. Using Au, Ag, or Al
instead of - let's say - Cu, makes little difference for the function.
• But exchange any material in a "chip" (i.e. in an integrated circuit) with
something else (even allowing for minor adjustments) - and that definitely will be
the end of your product
Let’s open up a chip. We find
• Packaging material - either some polymer blend or ceramics.
• A "chip" mostly consisting of Si, but interlaced in an intricate pattern with other
materials like P, B, As, SiO2, Si3N4, MoSi2, W, TiN, Al, Cu....
• A lead frame - the little pins sticking out of the package - made of some metal
alloys.
• Tiny wires connecting the leads to the chip or some pretty sophisticated stuff
doing this job.
2. MATTER STRUCTURE AND BONDING
Basic physical concepts
Atoms
All matter is made up of countless tiny particles whizzing around. These particles
are extremely dense; matter is mostly empty space. Matter seems continuous because the
particles are so small, and they move incredibly fast.
Even people of ancient times suspected that matter is made of invisible particles.
They deduced this from observing things like water, rocks, and metals. These substances
are much different from each other. But any given material—copper, for example—is the
same wherever it is found. Even without doing any complicated experiments, early
physicists felt that substances could only have these consistent behaviors if they were
made of unique types, or arrangements, of particles. It took centuries before people knew
just how this complicated business works. And even today, there are certain things that
scientists don’t really know. For example, is there a smallest possible material particle?
The scientists identified different kinds of fundamental substances in nature, and
called them elements. Later, a few more elements were artificially made (now are 118
elements). Each element has its own unique type of particle, known as its atom. Atoms of
different elements are always different.
The slightest change in an atom can make a tremendous difference in its behavior.
You can live by breathing pure oxygen, but you can’t live off of pure nitrogen. Oxygen
will cause metal to corrode, but nitrogen will not. Wood will burn furiously in an
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atmosphere of pure oxygen, but will not even ignite in pure nitrogen. Yet both are gases
at room temperature and pressure; both are colorless, both are odorless, and both are just
about of equal weight. These substances are so different because oxygen has eight
protons, while nitrogen has only seven. There are many other examples in nature where a
tiny change in atomic structure makes a major difference in the way a substance behaves.
Protons, neutrons, and the atomic number
The part of an atom that gives an element its identity is the nucleus. It is made up
of two kinds of particles, the proton and the neutron. These are extremely dense. A
teaspoonful of either of these particles, packed tightly together, would weigh tons.
Protons and neutrons have just about the same mass, but the proton has an electric charge
while the neutron does not.
The simplest element, hydrogen, has a nucleus made up of only one proton; there
are usually no neutrons. This is the most common element in the universe. Sometimes a
nucleus of hydrogen has a neutron or two along with the proton, but this does not occur
very often. These “mutant” forms of hydrogen do, nonetheless, play significant roles in
atomic physics.
The second most abundant element is helium. Usually, this atom has a nucleus
with two protons and two neutrons. Hydrogen is changed into helium inside the sun, and
in the process, energy is given off. This makes the sun shine. The process, called fusion,
is also responsible for the terrific explosive force of a hydrogen bomb.
Every proton in the universe is just like every other. Neutrons are all alike, too.
The number of protons in an element’s nucleus, the atomic number, gives that element its
identity. The element with three protons is lithium, a light metal that reacts easily with
gases such as oxygen or chlorine. The element with four protons is beryllium, also a
metal. In general, as the number of protons in an element’s nucleus increases, the number
of neutrons also increases. Elements with high atomic numbers, like lead, are therefore
much denser than elements with low atomic numbers, like carbon. Perhaps you’ve
compared a lead sinker with a piece of coal of similar size, and noticed this difference.
Isotopes and atomic weights
For a given element, such as oxygen, the number of neutrons can vary. But no
matter what the number of neutrons, the element keeps its identity, based on the atomic
number. Differing numbers of neutrons result in various isotopes for a given element.
Each element has one particular isotope that is most often found in nature. But all
elements have numerous isotopes. Changing the number of neutrons in an element’s
nucleus results in a difference in the weight, and also a difference in the density, of the
element. Thus, hydrogen containing a neutron or two in the nucleus, along with the
proton, is called heavy hydrogen.
The atomic weight of an element is approximately equal to the sum of the number
of protons and the number of neutrons in the nucleus. Common carbon has an atomic
weight of about 12, and is called carbon 12 or C12. But sometimes it has an atomic
weight of about 14, and is known as carbon 14 or C14.
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Electrons
Surrounding the nucleus of an atom are particles having opposite electric charge
from the protons. These are the electrons. Physicists arbitrarily call the electrons’ charge
negative, and the protons’ charge positive. An electron has exactly the same charge
quantity as a proton, but with opposite polarity. The charge on a single electron or proton
is the smallest possible electric charge. All charges, no matter how great, are multiples of
this unit charge.
One of the earliest ideas about the atom pictured the electrons embedded in the
nucleus (like raisins in a cake). Later, the electrons were seen as orbiting the nucleus,
making the atom like a miniature solar system with the electrons as the planets (Fig. 1-1).
Still later, this view was modified further. Today, the electrons are seen as so fastmoving,
with patterns so complex, that it is not even possible to pinpoint them at any given instant
of time. All that can be done is to say that an electron will just as likely be inside a certain
sphere as outside. These spheres are known as electron shells. Their centers correspond to
the position of the atomic nucleus. The farther away from the nucleus the shell, the more
energy the electron has (Fig. 1-2).
Electrons can move rather easily from one atom to another in some materials. In
other substances, it is difficult to get electrons to move. But in any case, it is far easier to
move electrons than it is to move protons. Electricity almost always results, in some way,
from the motion of electrons in a material.
Electrons are much lighter than protons or neutrons. In fact, compared to the
nucleus of an atom, the electrons weigh practically nothing.
Generally, the number of electrons in an atom is the same as the number of
protons. The negative charges therefore exactly cancel out the positive ones, and the atom
is electrically neutral. But under some conditions, there can be an excess or shortage of
electrons. High levels of radiant energy, extreme heat, or the presence of an electric field
can “knock” or “throw” electrons loose from atoms, upsetting the balance.
Ions
If an atom has more or less electrons than protons, that atom acquires an electrical
charge. A shortage of electrons results in positive charge; an excess of electrons gives a
negative charge. The element’s identity remains the same, no matter how great the excess
or shortage of electrons. In the extreme case, all the electrons might be removed from an
atom, leaving only the nucleus. However it would still represent the same element as it
would if it had all its electrons.
A good example of an ionized substance is the atmosphere of the earth at high
altitudes. The ultraviolet radiation from the sun, as well as high-speed subatomic particles
from space, result in the gases’ atoms being stripped of electrons. The ionized gases tend
to be found in layers at certain altitudes. These layers are responsible for long-distance
radio communications at some frequencies.
Ionized materials generally conduct electricity quite well, even if the substance is
normally not a good conductor. Ionized air makes it possible for a lightning stroke to take
place, for example. The ionization, caused by a powerful electric field, occurs along a
jagged, narrow channel, as you have surely seen. After the lightning flash, the nuclei of
the atoms quickly attract stray electrons back, and the air becomes electrically neutral
again.
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An element might be both an ion and an isotope different from the usual isotope.
For example, an atom of carbon might have eight neutrons rather than the usual six, thus
being the isotope C14, and it might have been stripped of an electron, giving it a positive
unit electric charge and making it an ion.
Compounds
Different elements can join together to share electrons. When this happens, the
result is a chemical compound. One of the most common compounds is water, the result
of two hydrogen atoms joining with an atom of oxygen. There are literally thousands of
different chemical compounds that occur in nature.
A compound is different than a simple mixture of elements. If hydrogen and
oxygen are mixed, the result is a colorless, odorless liquid, just like either element is a
gas separately. A spark, however, will cause the molecules to join together; this will
liberate energy in the form of light and heat. Under the right conditions, there will be a
violent explosion, because the two elements join eagerly. Water is chemically illustrated
in next figure.
Compounds often, but not always, appear greatly different from any of the
elements that make them up. At room temperature and pressure, both hydrogen and
oxygen are gases. But water under the same conditions is a liquid. If it gets a few tens of
degrees colder, water turns solid at standard pressure. If it gets hot enough, water
becomes a gas, odorless and colorless, just like hydrogen or oxygen.
Another common example of a compound is rust. This forms when iron joins with
oxygen. While iron is a dull gray solid and oxygen is a gas, rust is a maroon-red or
brownish powder, completely unlike either of the elements from which it is formed.
Molecules
When atoms of elements join together to form a compound, the resulting particles
are molecules. Previous figure is an example of a molecule of water, consisting of three
atoms put together.
The natural form of an element is also known as its molecule. Oxygen tends to
occur in pairs most of the time in the earth’s atmosphere. Thus, an oxygen molecule is
sometimes denoted by the symbol O2. The “O” represents oxygen, and the subscript 2
indicates that there are two atoms per molecule. The water molecule is symbolized H2O,
because there are two atoms of hydrogen and one atom of oxygen in each molecule.
Sometimes oxygen atoms are by themselves; then we denote the molecule simply
as O. Sometimes there are three atoms of oxygen grouped together. This is the gas called
ozone. It is written O3.
3. BONDING
IONIC (ELECTROVALENT) BONDING
The importance of noble gas structures
At a simple level a lot of importance is attached to the electronic structures of
noble gases (like neon or argon) which have eight electrons in their outer energy levels
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(or two in the case of helium). These noble gas structures are thought of as being in some
way a "desirable" thing for an atom to have.
You may well have been left with the strong impression that when other atoms
react, they try to organize things such that their outer levels are either completely full or
completely empty.
Ionic bonding in sodium chloride
Sodium (2,8,1) has 1 electron more than a stable noble gas structure (2,8). If it
gave away that electron it would become more stable.
Chlorine (2,8,7) has 1 electron short of a stable noble gas structure (2,8,8). If it could gain
an electron from somewhere it too would become more stable.
The answer is obvious. If a sodium atom gives an electron to a chlorine atom, both
become more stable.
The sodium has lost an electron, so it no longer has equal numbers of electrons
and protons. Because it has one more proton than electrons, it has a positive charge (of
1+). If electrons are lost from an atom, positive ions are formed.
 Positive ions are sometimes called cations.
The chlorine has gained an electron, so it now has one more electron than protons. It
therefore has a negative charge (of 1-). If electrons are gained by an atom, negative ions
are formed.
 A negative ion is sometimes called an anion.
The nature of the bond
The sodium ions and chloride ions are held together by the strong electrostatic
attractions between the positive and negative charges.
The formula of sodium chloride
You need one sodium atom to provide the extra electron for one chlorine atom, so
they combine together 1:1. The formula is therefore NaCl.
Again, noble gas structures are formed, and the magnesium oxide is held together
by very strong attractions between the ions. The ionic bonding is stronger than in sodium
chloride because this time you have 2+ ions attracting 2- ions. The greater the charge, the
greater the attraction. The formula of magnesium oxide is MgO.
COVALENT BONDING - SINGLE BONDS
A simple view of covalent bonding - The importance of noble gas structures
At a simple level a lot of importance is attached to the electronic structures of
noble gases like neon or argon which have eight electrons in their outer energy levels (or
two in the case of helium). These noble gas structures are thought of as being in some
way a "desirable" thing for an atom to have.
You may well have been left with the strong impression that when other atoms
react, they try to achieve noble gas structures.
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As well as achieving noble gas structures by transferring electrons from one atom
to another as in ionic bonding, it is also possible for atoms to reach these stable structures
by sharing electrons to give covalent bonds.
Some very simple covalent molecules
Chlorine
For example, two chlorine atoms could both achieve stable structures by sharing their
single unpaired electron as in the diagram.
The fact that one chlorine has been drawn with electrons marked as crosses and
the other as dots is simply to show where all the electrons come from. In reality there is
no difference between them.
The two chlorine atoms are said to be joined by a covalent bond. The reason that
the two chlorine atoms stick together is that the shared pair of electrons is attracted to the
nucleus of both chlorine atoms.
Hydrogen
Hydrogen atoms only need two electrons in their outer level to reach the noble gas
structure of helium. Once again, the covalent bond holds the two atoms together because
the pair of electrons is attracted to both nuclei.
Hydrogen chloride
The hydrogen has a helium structure, and the chlorine an argon structure
ELECTRONEGATIVITY
Definition
Electronegativity is a measure of the tendency of an atom to attract a bonding pair
of electrons.
The Pauling scale is the most commonly used. Fluorine (the most electronegative
element) is assigned a value of 4.0, and values range down to cesium and francium which
are the least electronegative at 0.7.
METALLIC BONDING
Metallic bonding in sodium
Metals tend to have high melting points and boiling points suggesting strong
bonds between the atoms. Even a metal like sodium (melting point 97.8°C) melts at a
considerably higher temperature than the element (neon) which precedes it in the Periodic
Table.
Sodium has the electronic structure 1s22s22p63s1. When sodium atoms come
together, the electron in the 3s atomic orbital of one sodium atom shares space with the
corresponding electron on a neighboring atom to form a molecular orbital - in much the
same sort of way that a covalent bond is formed.
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The difference, however, is that each sodium atom is being touched by eight other
sodium atoms - and the sharing occurs between the central atom and the 3s orbitals on all
of the eight other atoms. And each of these eight is in turn being touched by eight sodium
atoms, which in turn are touched by eight atoms - and so on and so on, until you have
taken in all the atoms in that lump of sodium.
Metallic bonding in sodium
All of the 3s orbitals on all of the atoms overlap to give a vast number of
molecular orbitals which extend over the whole piece of metal. There have to be huge
numbers of molecular orbitals, of course, because any orbital can only hold two electrons.
The electrons can move freely within these molecular orbitals, and so each electron
becomes detached from its parent atom. The electrons are said to be delocalised. The
metal is held together by the strong forces of attraction between the positive nuclei and
the delocalised electrons.
Metallic bonding in sodium
This is sometimes described as "an array of positive ions in a sea of electrons".
If you are going to use this view, beware! Is a metal made up of atoms or ions? It is made
of atoms.
Each positive centre in the diagram represents all the rest of the atom apart from
the outer electron, but that electron hasn't been lost - it may no longer have an attachment
to a particular atom, but it's still there in the structure. Sodium metal is therefore written
as Na, not Na+.
VAN DER WAALS FORCES
What are intermolecular attractions?
Intermolecular versus intramolecular bonds. Intermolecular attractions are
attractions between one molecule and a neighbouring molecule. The forces of attraction
which hold an individual molecule together (for example, the covalent bonds) are known
as intramolecular attractions. These two words are so confusingly similar that it is safer
to abandon one of them and never use it.
All molecules experience intermolecular attractions, although in some cases those
attractions are very weak. Even in a gas like hydrogen, H2, if you slow the molecules
down by cooling the gas, the attractions are large enough for the molecules to stick
together eventually to form a liquid and then a solid.
In hydrogen's case the attractions are so weak that the molecules have to be
cooled to 21 K (-252°C) before the attractions are enough to condense the hydrogen as a
liquid. Helium's intermolecular attractions are even weaker - the molecules won't stick
together to form a liquid until the temperature drops to 4 K (-269°C).
Temporary fluctuating dipoles
Attractions are electrical in nature. In a symmetrical molecule like hydrogen,
however, there doesn't seem to be any electrical distortion to produce positive or negative
parts. But that's only true on average.
The lozenge-shaped diagram represents a small symmetrical molecule - H2,
perhaps, or Br2. The even shading shows that on average there is no electrical distortion.
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But the electrons are mobile, and at any one instant they might find themselves
towards one end of the molecule, making that end δ-. The other end will be temporarily
short of electrons and so becomes δ+.
An instant later the electrons may well have moved up to the other end, reversing
the polarity of the molecule.
This constant "sloshing around" of the electrons in the molecule causes rapidly
fluctuating dipoles even in the most symmetrical molecule. It even happens in monatomic
molecules - molecules of noble gases, like helium, which consist of a single atom.
If both the helium electrons happen to be on one side of the atom at the same time, the
nucleus is no longer properly covered by electrons for that instant.
How temporary dipoles give rise to intermolecular attractions
Imagine a molecule which has a temporary polarity being approached by one
which happens to be entirely non-polar just at that moment. (A pretty unlikely event, but
it makes the diagrams much easier to draw! In reality, one of the molecules is likely to
have a greater polarity than the other at that time - and so will be the dominant one.)
As the right hand molecule approaches, its electrons will tend to be attracted by
the slightly positive end of the left hand one.
This sets up an induced dipole in the approaching molecule, which is orientated in
such a way that the δ+ end of one is attracted to the δ- end of the other.
An instant later the electrons in the left hand molecule may well have moved up
the other end. In doing so, they will repel the electrons in the right hand one.
The polarity of both molecules reverses, but you still have + attracting -. As long
as the molecules stay close to each other the polarities will continue to fluctuate in
synchronization so that the attraction is always maintained.
There is no reason why this has to be restricted to two molecules. As long as the
molecules are close together this synchronized movement of the electrons can occur over
huge numbers of molecules.
This diagram shows how a whole lattice of molecules could be held together in a
solid using van der Waals dispersion forces. An instant later, of course, you would have
to draw a quite different arrangement of the distribution of the electrons as they shifted
around - but always in synchronization.
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The strength of dispersion forces
Dispersion forces between molecules are much weaker than the covalent bonds
within molecules. It isn't possible to give any exact value, because the size of the
attraction varies considerably with the size of the molecule and its shape.
HYDROGEN BONDS
A hydrogen bond results from a dipole-dipole force between an electronegative
atom and a hydrogen atom bonded to nitrogen (N), oxygen (O) or fluorine (F) (thus the
name "hydrogen bond", which must not be confused with a covalent bond to hydrogen).
The energy of a hydrogen bond (typically 5 to 30 kJ/mole) is comparable to that of weak
covalent bonds (155 kJ/mol), and a typical covalent bond is only 20 times stronger than
an intermolecular hydrogen bond. These bonds can occur between molecules
(intermolecularly). The hydrogen bond is a very strong fixed dipole-dipole force, but
weaker than covalent, ionic and metallic bonds. The hydrogen bond is somewhere
between a covalent bond and an electrostatic intermolecular attraction. This type of bond
occurs in both inorganic molecules (water) and organic molecules.
The hydrogen bond is often described as an electrostatic dipole-dipole interaction.
However, it also has some features of covalent bonding: it is directional, strong, produces
interatomic distances shorter than sum of van der Waals forces.
The most ubiquitous, and perhaps simplest, example of a hydrogen bond is found
between water molecules. In a discrete water molecule, water has two hydrogen atoms
and one oxygen atom. Two molecules of water can form a hydrogen bond between them;
the simplest case, when only two molecules are present, is often used as a model system.
When more molecules are present, as is the case in liquid water, more bonds are possible
because the oxygen of one water molecule has two lone pairs of electrons, each of which
can form a hydrogen bond with hydrogens on two other water molecules. This can repeat
so that every water molecule is H-bonded with up to four other molecules, as shown in
the figure (two through its two lone pairs, and two through its two hydrogen atoms.)
CRYSTALLINE STRUCTURE OF MATERIALS
Solid materials are classified by the way the atoms are arranged within the solid.
Materials in which atoms are placed randomly are called amorphous. Materials in which
atoms are placed in a high ordered structure are called crystalline. Poly-crystalline
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materials are materials with a high degree of short-range order and no long-range order.
These materials consist of small crystalline regions with random orientation called grains,
separated by grain boundaries.
Of primary interest in this text are crystalline semiconductors in which atoms are
placed in a highly ordered structure. Crystals are categorized by their crystal structure and
the underlying lattice. While some crystals have a single atom placed at each lattice point,
most crystals have a combination of atoms associated with each lattice point. This
combination of atoms is also called the basis.
CRYSTAL LATTICE DEFECTS
The crystals (real ones) at a temperature of T>0K won’t have an ideal structure as
with the growth of the temperature appear the defects of the crystalline network (the
particles are not any more strictly orderly arangement).
Depending on their spatial configuration, the network defects can be clasified in:
• Point defects or zero-dimensional.
• Line defects or one-dimensional.
• Surface defects or two-dimensional.
• Volume defects or three-dimensional.
Point defects
It consist from the absence of a particle from a node (a vacancy), an atom
occupying an interstitial site (occupied interstice) or the existence of an impurity atom
replacing a host atom or an interstitial site.
By the forming energy the punctual faults can be: Schottky defect, or Frenkel
defect.
Schottky defect type fault appears when a particle leaves the node of the
crystalline network and takes place on the surface of the crystal or besides it.
Frenkel defect appear when an atom displaced from a lattice site to a nearby
interstitial site.
4. 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.)
σCond=106-108[S/m]
σSemicond=10-6-105[S/m] σInsulators=10-7-10-18[S/m]
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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.
Schematic of band structures for (a) insulators, (b) semiconductors, and (c)
conductors. (Temperature is 0K.)
Insulator Energy Bands
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).
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.
wI>3eV
wI<3eV
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Semiconductor Energy Bands
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.
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.
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.
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.
5. DIELECTRIC MATERIALS
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
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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
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.
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 polarization. An
electret generates internal and external electric fields, and is the electrostatic equivalent
of a permanent magnet.
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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).
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. The relative permittivity:
C
r 
C0
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.
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]
14
Where χe is the electric susceptibility of the material and:  O 
1
 8,854 10 12 F / m
4  9 109
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.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.
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
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 direct-current
resistance between opposite faces of a one-meter cube of the material (Ohm-m).
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.
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].
15
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.
I
IC
The dielectric loss angle:
δ = 90-φ
δ
φ
IA
U
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.
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
d) The dielectric strength (ESTR) is the minimum electric field that produces
U
breakdown.
ESTR  STR MV / m or kV / cm
d
The dielectric strength is given by the ratio between the breakdown voltage
(USTR) and the thickness (d) of the material between the electrodes.
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.
16
5. 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,
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   r C0
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.
In AC, the relative permittivity of the material isn’t a scalar constant.
D
D and E are the amplitudes of the displacement and
r 
  r'  j r"
0  E
electrical fields in complex representation.


The admittance of capacitor is: Y  j r C0  j  r'   r" C0   r"C0  j r' C0
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.
Cech   r' C0
IC
I
δ
Rech 
φ
IA
U
1
 r"C0
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.
17
Pa U  I a
1
"


 r'
The Dielectric Loss Factor is defined by relation:
Pr U  I C Rech Cech  r
The Q factor or quality factor is the inverse of the dissipation factor (tan δ).
tg h 
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).
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.
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.
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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.
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 electro-optic 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.
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
polarized) 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
19
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.
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.
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.
6. 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:
1
J  E  E

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.
20
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.
σel, σion, σt
σt
The variation of the electric
conductivity of an insulating crystal
with the intensity of electric field.
σion
σel
E[V/m]
5
Ohm’s interval
6
8
10 ÷10
10
Pool’s
Breakdown
interval
interval
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 realized 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.
The electronic conductivity (σel) is the sum of the intrinsic conductivity (σi) and
the extrinsic conductivity given by electrons (σe) and by holes (σh).
 el   i   e   h  Ci  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).
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 covers a distance x, under the effect of the force:
F  q0  E
Wa  q 0  E  x
The electron accumulates an energy:
If we note with Ĩ the medium distance covered by the electron, than the electron
Wa  q 0  E  l
accumulates the energy:
21
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.
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.
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:
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
22
age, the environment where it is used (especially humidity) and the time it experienced
field stress.
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.
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.
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
23
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.
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 high-temperature furnaces
within an oxygen ambient (thermal oxidation).
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 sol-gel 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.
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
24
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
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.
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).
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.
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
25
modified natural materials (e.g., rubber, nitrocellulose, collagen) and finally to
completely synthetic molecules (e.g., bakelite, epoxy, polystyrene, polyvinyl chloride,
polyethylene).
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
favorite 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).
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.
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.
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