Download valence electrons

ATOMS
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All matter is made of atoms, which consist of electrons,
protons and neutrons
An atom is the smallest particle of an element that
retains characteristics of that element
There are 109 elements, each of which has a different
atomic structure
In the classic Bohr model, an atom is visualised as
having a planetary type structure that consists of a
central nucleus, surrounded by orbiting electrons
Nucleus consists of positively charged particles called
protons, and equal number of uncharged particles
called neutrons
Electrons have a negative charge
Each type of atom has a certain number of electrons
and protons that distinguishes it from other atoms of
other elements
The simplest atom is that
of hydrogen, with one proton
and one orbiting electron
Helium has two protons,
two neutrons and two electrons
Electron
6. Introduction to
Semiconductor Devices
Proton
Neutron
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ATOMIC STRUCTURE
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Elements are arranged in the periodic table according
to their atomic number, which is the number of protons
in the nucleus
Hydrogen has an atomic number of 1, and helium 2
In their neutral state, all atoms of an element have an
equal number of electrons and protons
Thus the positive charges cancel out the negative
charges, so an atom is electrically balanced
Electrons orbit the nucleus of an atom at certain
distances from the centre
Electrons near the nucleus have less energy than those
in more distant orbits
Only separate and distinct (discrete) values of electron
energies exist within atomic structures
Thus electrons orbit at discrete distances from nucleus
Each discrete orbit corresponds to an energy level
called a shell
Each shell has a fixed maximum number of electrons at
permissible energy levels (orbits)
Shells are designated 1, 2, 3 and so on, with 1 nearest
the nucleus
Maximum number of electrons permitted in each shell
follows 2N2, where N is number of shell
So first shell has 2 electrons, 2nd has 8, 3rd 18, etc..
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VALENCE ELECTRONS
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Electrons in orbits farthest away from the nucleus have
higher energy, and are less tightly bound to the nucleus
than those closer to it
This is due to force of attraction between positively
charged nucleus and negatively charged electrons,
which decreases with distance
Electrons with the highest energy exist in the outermost
shell of an atom called the valence shell
Electrons in valence shell are called valence electrons
and determine a material’s electrical properties
If an electron absorbs a photon (particle of
electromagnetic radiation) with sufficient energy, it can
escape the atom and becomes a free electron
When an atom is left with a net charge (i.e. when there
are an unequal number of electrons and protons), it is
called an ion
When an electron escapes from a parent atom, the
atom gains a net positive charge as there are now more
protons than electrons than protons – the atoms
becomes a positive ion
When an atom acquires an electron, it becomes a
negative ion – more electrons than protons
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THE COPPER ATOM
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Copper is the most commonly used metal in electrical
applications
It has 29 electrons in orbit around nucleus in 4 shells
In the valence shell, there is only 1 valence electron
When this valence electron gains sufficient thermal
energy, it can break away from the parent atom and
thus becomes a free electron
In a piece of copper at room temperature, several of
these free electrons are present, and are free to move
in the copper material
These free electrons make copper an excellent
conductor and make electrical current possible
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CATEGORIES OF MATERIALS
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Conductors are materials that readily allow current
They have a large number of free electrons, and are
characterised by one to three valence electrons in their
atomic structure
Most metals are good conductors
Silver is the best conductor, followed by copper
Copper is more widely used as it is less expensive than
silver
Semiconductors are classed below conductors in their
ability to carry current as they have fewer free electrons
Semiconductors have four valence electrons, yet
because of their unique characteristics, semiconductor
materials are the basis of electronic devices such as
diodes and transistors
Silicon and germanium are common semiconductors
Insulators are poor conductors of electrical current
They are used to prevent current flow where it is not
wanted
Insulators have very few free electrons and are
characterised by more than four valence electrons
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ELECTRICAL CHARGE
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The charge on an electron and proton are equal in
magnitude
Electrical charge exists because of an excess or
deficiency of electrons (Q)
Static electricity is the presence of a net positive or
negative charge in a material
Materials with charges of opposite polarity are attracted
to each other, those of the same polarity are repelled
A force acts between the charges (attraction or
repulsion), and is called an electric field
Unit of charge is the coulomb, where 1 coulomb is the
total charge possessed by 6.25×10-19 electrons
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POSITIVE AND NEGATIVE CHARGE
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A neutral atom has the same number of electrons and
protons – it has no net charge
When a valence electron gains enough energy to pull
away from the atom, the atom is left with a net positive
charge (more protons than electrons)
It thus becomes a positive ion
If the atom acquires an extra electron, it becomes a
negative ion as there are now more electrons than
protons, and so it has a net negative charge
The amount of energy required to free a valence
electron is related to the number of electrons in the
outer shell
The more complete an outer shell, the more stable the
atom and thus the more energy is required to release
an electron
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Semiconductor Devices
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SILICON AND GERMANIUM ATOMS
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Both silicon and germanium have 4 valence electrons
Silicon has 14 protons, whereas germanium has 32
Valence electrons in germanium are in the 4th shell, and
those of silicon are in the 3rd shell
Germanium valence electrons are at higher energy
levels than those of silicon and thus require a smaller
amount of additional energy to escape from the atom
This makes germanium more unstable than silicon at
high temperatures, and is the main reason silicon is the
most widely used semiconductive material
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ATOMIC BONDING
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When certain atoms combine into molecules to form a
solid material, they arrange themselves into a fixed
pattern called a crystal
Atoms within the crystal structure are held together by
covalent bonds
These are created by the interaction of valence
electrons of each atom
In a silicon crystal, each atom positions itself with four
adjacent atoms
A silicon atom with its four valence electrons shares an
electron with each of its four neighbours
This creates eight valence electrons for each silicon
atom, and hence improves chemical stability
This sharing produces covalent bonds that hold the
atoms together
Each shared electron is attracted equally by two
adjacent atoms
An intrinsic crystal is one without impurities
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CONDUCTION ELECTRONS AND
HOLES
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Here is an energy band diagram for a silicon crystal
with only unexcited atoms (no external energy)
An intrinsic (pure) silicon crystal at room temperature
has enough heat energy for some valence electrons to
jump the gap from the valence band into the conduction
band, becoming free electrons
When an electron jumps to the conduction band, a
vacancy (a hole) is left in the valence band
For every electron raised to the conduction band by
external energy, there is one hole left in the valence
band, creating an electron-hole pair
Recombination occurs when a conduction band
electron loses energy and falls back into a hole in the
valence band
Thus a piece of intrinsic silicon at room temperature
has a number of free, drifting conduction band
electrons, and an equal number of holes in the valence
band
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ELECTRON AND HOLE CURRENT
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When a voltage is applied across a piece of silicon, the
free electrons in the conduction band are attracted to
the positive terminal of the voltage source
The corresponding movement of free electrons is one
type of current in semiconductive material called
electron current
Another type of current occurs at valence level, where
the holes created by the free electrons exist
Electrons that remain in the valence band are still
attached to their parent atoms and are not free to move
randomly in the crystal
Yet a valence electron can move into a nearby hole,
thus leaving another hole where it came from
The hole has effectively (not physically) moved from
one place to another
This is called hole current
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N TYPE AND P TYPE
SEMICONDUCTORS
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Semiconductors do not conduct current well, and are of
little use in their intrinsic state
So intrinsic silicon (or germanium) must be modified by
increasing the free electrons and holes to increase its
conductivity
This is done by adding impurities to form an extrinsic
semiconductive material
There are two types of extrinsic semiconductors
N-type and P-type
Doping is the process where impurities are added to a
semiconductor to increase its conductivity
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N-TYPE SEMICONDUCTOR
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Pentavalent impurity atoms
are added to intrinsic silicon
to increase the number of
conduction band electrons
Such atoms have 5 valence
electrons, and are known as
donor atoms – arsenic (As),
Phosphorus (P), antimony (Sb)
Donor atoms provide an extra electron to the
semiconductor’s crystal structure
Each pentavalent atom forms covalent bonds with 4
adjacent silicon atoms
Four of the pentavalent atom’s valence electrons are
used to form the covalent bonds with silicon atoms,
thus leaving one extra electron
This extra electron becomes a conduction electron as it
is not attached to any atom
In an n-type (n - negative electron charge) semi conductor, most of the current carriers are electrons
Hence in this case the majority carriers are electrons
There are a few holes, but in n-type material they are
minority carriers
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P-TYPE SEMICONDUCTOR
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To increase the number of
holes in intrinsic silicon,
trivalent impurity atoms
are added
Such atoms have three
valence electrons, such
as aluminium (Al), boron
(B), Gallium (Ga), and are
known as acceptor atoms
Acceptor atoms leave a hole in the semiconductor’s
crystal structure
Each trivalent atom forms covalent bonds with four
adjacent silicon atoms
All three of the trivalent valence electrons are used in
the covalent bonds
Since four electrons are required, a hole is thus formed
with each trivalent atom
Here most of the current carriers are holes, which can
be thought of as positive charges
Thus holes are the majority carriers in p-type material,
and electrons are the minority carriers
Silicon doped with trivalent atoms is a p-type
semiconductor
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Semiconductor Devices
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DIODES
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If you take a block of silicon and dope one half of it with
a trivalent impurity and the other half with a pentavalent
impurity, the boundary between the two regions is
formed called the pn junction
A diode consists of an n region and a p region
separated by a pn junction
The n region has many conduction electrons, and the p
region has many holes
There is no movement of electrons (current) through a
diode at equilibrium
A diode has the ability to allow current flow in only one
direction, which is determined by the bias
Bias refers to the use of a DC voltage to establish
certain operating conditions for a device
For a diode, there are two bias conditions: forward and
reverse
These conditions are created by application of a
sufficient external voltage of the proper polarity across
the pn junction
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THE DEPLETION REGION
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With no external voltage, conduction electrons in the n region
randomly drift in all directions
At the instant of junction formation, some of the electrons near
the junction drift into the p region and recombine with holes
close to the junction
For each electron that crosses the junction and recombines
with a hole, a pentavalent atom is left with a net positive
charge in the n region near the junction, making it a positive
ion
When an electron recombines with a hole in the p region, a
trivalent atom acquires a net negative charge, making it a
negative ion
Due to this recombination process a large number of positive
and negative ions build up at the pn junction
Electrons in the n region must overcome attraction of the
positive ions and repulsion of negative ions in order to migrate
into p region
As the ion layers build up, both sides of the junction become
depleted of any conduction electrons or holes, and forms the
depletion region
At equilibrium, the depletion region has widened to a point
where no more electrons can cross the pn junction
The barrier potential is the amount of voltage needed to
move electrons through the depletion region (Silicon = 0.7V,
germanium = 0.3V
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DIODE: FORWARD BIAS
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Forward bias permits diode current flow
Negative terminal of DC bias voltage connected to n
region, positive terminal to p
Negative terminal of bias voltage pushes conduction
band electrons in the n region toward the pn junction
Positive terminal pushes holes in the p region also
toward the pn junction
When the bias voltage is greater than the barrier
potential, there is enough energy for the n region
electrons to penetrate the depletion region, and move
through the junction and recombine with p region holes
As electrons leave the n region, more flow from the
negative terminal of the dc voltage source
Current is thus formed through the n region by the
movement of majority electrons to the pn junction
Once the conduction electrons enter the p region and
combine with holes, they become valence electrons
They move as valence electrons from hole to hole to
the positive terminal of the voltage source
Thus the current in the p region is formed by the
movement of holes toward the pn junction
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DIODE: REVERSE BIAS (1)
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Prevents current flow
Negative terminal of
DC voltage source
connected to p region,
positive to n region
Negative terminal attracts
holes in p region away from pn junction
The positive terminal attracts electrons away from the
pn junction
This causes the depletion region to widen
More positive ions created in the n region, and more
negative ions created in the p region
The depletion region widens until the voltage across it
equals the source bias, and at this point the holes and
electrons stop moving away from the pn junction
When reversed bias, depletion region acts as an
insulator between layers of oppositely charged ions
The depletion region widens with increased reverse
bias voltage
Majority current becomes zero with reverse bias
Small amount of minority current is leaked (nA)
Some electrons manage to diffuse across the pn
junction
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Semiconductor Devices
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DIODE: REVERSE BIAS (2)
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As electrons and holes move away from the pn
junction, the depletion region widens
More positive ions are created in the n region, and
more negative ions in the p region
Initial flow of majority carriers away from the pn junction
is called transient current and lasts only for a very
short time on application if reverse bias
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REVERSE BREAKDOWN
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If the external reverse bias voltage is increased to a
large enough value, reverse breakdown occurs
Most diodes normally are not operated in reverse
breakdown
Diodes can be damaged when reverse breakdown
occurs
Zener diodes are specifically designed for reverse
breakdown operation
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DIODE SYMBOL
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The arrowhead in the symbol points not in the direction
of electron flow, but in the direction of conventional
current flow
Metal contacts are connected to each region, anode to
the p region, cathode to the n region
When the anode is positive with respect to the cathode,
diode is forward biased and current IF is from cathode
to anode
When the diode is forward biased, the barrier potential,
VB, always appears between the anode and cathode
When the anode is negative with respect to the
cathode, diode is reverse biased, and there is no
I
current flow
F
+ VB R
VBIAS
I=0
+ VB R
VBIAS
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IDEAL DIODE MODEL
IF
VR
VF
IR
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PRACTICAL DIODE MODEL (1)
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The ideal model on the previous slide neglected the
effect of the barrier potential, the internal resistances
and other parameters
The practical diode model offers more accuracy
The forward bias diode is represented as a closed
switch in series with a small battery equal to the barrier
potential, VB (0.7V for silicon)
The positive end of the battery is towards the anode
Remember that the barrier potential is not a voltage
source, and can’t be measured with a voltmeter
It only has the effect of a battery when forward bias is
applied because the forward bias voltage must
overcome the barrier potential for the diode to conduct
The reverse biased diode is represented by an open
switch (as ideal case) because the barrier potential
does not affect reverse bias
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PRACTICAL DIODE MODEL (2)
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COMPLETE DIODE MODEL
IF
Slope due to low forward
resistance
VR
VF
0.7V
Small reverse current due
to high reverse resistance
IR
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