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Introduction To Semiconductors
Chapter 1
Atomic Structure
Semiconductor,Insulators and Conductors
Covalent Bonds
Conduction in Semiconductors
N-Type and P-Type Semiconductors
The PN-Junction
Biasing the PN-Junction
The Diode
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Atomic Structure
• All matter is made of atoms; and all atoms are made of
electrons, Protons, and Electrons.
• Structure of the atom
• Electron Orbit Shells
• Valence electrons, Ions
• Two Semiconductive materials (Silicon and Germanium)
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Conductors
• Conductor is a material that easily conducts electrical current.
• The best conductors are single-element materials, such as
copper, silver, gold, and aluminum, which are characterized by
atoms with only one valence electron very loosely bound to the
atom.
• These loosely bound valence electrons can easily break away
from their atoms and become free electrons.
• Therefore, a conductive material has many free electrons that,
when moving in a net direction, make up the current.
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Insulators
• An insulator is a material that does not conduct electrical
current under normal conditions.
• Most good insulators are compounds rather that singleelement materials.
• Valence electrons are tightly bound to the atoms; therefore,
there are very few free electrons in an insulator.
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Semiconuctors
• A semiconductor is a material that is between conductors and
insulators in its ability to conduct electrical current.
• A semiconductor in its pure (intrinsic) state is neither a good
conductor nor a good insulator.
• The most common single-element semiconductors are silicon,
germanium, and carbon.
• Compound semiconductors such as gallium arsenide are also
commonly used.
• The single-element semiconductors are characterized atoms
with four valence electrons.
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Energy diagrams for the three types of
materials
Energy
Energy
Energy
Conduction band
Energy gap
Conduction band
Energy gap
Valence band
Insulator
Valence band
Semiconductor
Conduction band
Valence band
Conductor
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Covalent Bonds
• When certain atoms combine to form a solid material, they
arrange themselves in a fixed pattern called a crystal.
• The atom within the crystal structure are are held together
by covalent bonds, which are created by the interaction of
the valence electrons of the atoms. Silicon is a crystalline
material.
Si
Si
Si
Si
Si
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Electron and Hole Current
• When a voltage is applied across a piece of intrinsic silicon,
the thermally generated free electrons in the conduction band,
which are free to move randomly in the crystal structure, are
now easily attracted toward the positive end.
• This movement of free electrons is one type of current in a
semiconductor material and is called electron current.
• A valence electron can move into a nearby hole, with little
change in its energy level, thus leaving another hole where it
came from.
• Effectively the hole has moved from one place to another in
the crystal structure. This is called hold current.
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N-TYPE AND P-TYPE SEMICONDUCTORS
• Semiconductor materials do not conduct current well and are of
little value in their intrinsic state.
• This is because of the limited number of free electrons in the
conduction band and holes in the valence band.
• Intrinsic silicon (or germanium) must be modified by increasing
the free electrons and holes to increase its conductivity and make
it useful in electronic devices. This is done by adding impurities
to the intrinsic material.
•Two types of extrinsic (impure) semiconductor materials N-type
and P-type, are the key building block for all types of electronic
devices.
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N-TYPE AND P-TYPE SEMICONDUCTORS
Continue
Doping: The conductivity of silicon and germanium can be
drastically increased by the controlled addition of impurities
to the intrinsic (pure) semiconductor material. This process,
called doping, increases the number of current carriers
(electrons or holes), thus increasing the conductivity and
decreasing the resistively. The two categories of impurities
are n-type and p-type.
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N-TYPE SEMICONDUCTORS
• To increase the number of conduction-band electrons in intrinsic
silicon, pentavalent impurity atoms are added.
• These are atoms with five valence electrons such as arsenic
(As), phosphorus (P), bismuth (Bi), and antimony (Sb).
• Each pentavalent atom (antimony, in this case) forms covalent
bonds with four adjacent silicon atoms. Four of the antimony
atom’s valence electrons are used to form the covalent bonds with
silicon atoms, leaving one extra electron. This extra electron
becomes a conduction electron because it is not attached to any
atom. Because the pentavalent atom gives up an electron, it is
often called a donor atom.
• The number of conduction electrons can be carefully controlled
by the number of impurity atoms added to the silicon.
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N-TYPE SEMICONDUCTORS Continue
• Majority and Minority Carriers: Since most of the current
carriers are electrons, silicon (or germanium) doped with
pentavalent atoms is an n-type semiconductor material (the n
stands for the negative charge on an electron). The electrons are
called the majority carriers in n-type material.
• There are also a few holes that are created when electron-hole
pairs are thermally generated. These holes are not produced by
the addition of the pentavalent impurity atoms. Holes in an ntype material are called minority carriers.
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P-type Semiconductor
• To increase the number of holes in intrinsic silicon, trivalent
impurity atoms are added. These are atoms with three valence
electrons such as aluminum (AI), boron (B), indium (In) and
gallium (Ga).
• Because the trivalent atom can take an electron, it is often
referred to as an acceptor atom.
• The number of holes can be carefully controlled by the
number of trivalent impurity atoms added to the silicon.
• A hole created by this doping process is not accompanied by
a conduction (free) electron.
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P-type Semiconductor Continue
• Majority and Minority Carriers. Since most of the current
carriers are holes, silicon (or germanium) doped with trivalent
atoms is called a p-type semiconductor material.
• Holes can be thought of as positive charges because the
absence of an electron leaves a net positive charge on the
atom. The holes are the majority carriers in p-type material.
• There are also a few free electrons that are created when
electron-hole pairs are thermally generated. These free
electrons are not produced by the addition of the trivalent
impurity atoms.
• Electrons in p-type material are the minority carriers.
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THE PN JUNCTION
• If you take a block of silicon and dope half of it with a
trivalent impurity and the other half with a pentavalent
impurity, a boundary called the pn junction is formed between
the resulting p-type and n-type portions .
• The pn junction is the feature that allows diodes, transistors,
and other devices to work.
• If a piece of intrinsic silicon is doped so that half is n-type and
the other half is p-type, a pn junction forms between the two
regions as indicated.
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Formation of the Depletion Region
• When the pn junction is formed, the n region loses free
electrons as they diffuse across the junction. This creates a
layer of positive charges (pentavalent jons) near the junction.
As the electrons move across the junction, the p region loses
holes as the electrons and holes combine. This creates a layer
of negative charges (trivalent jons) near the junction. These
two layers of positive and negative charges form the depletion
region.
• After the initial surge of free electrons across the pn junction,
the depletion region has expanded to a point where equilibrium
is established and there is no further diffusion of electrons
across the junction.
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Barrier Potential
• The forces between the opposite charges form a “field of
forces” called an electric field.
• This electric field is a barrier to the free electrons in the n
region, and energy most be expended to move an electron
through the electric field. That is, external energy must be
applied to get the electrons to move across the barrier of the
electric field in the depletion region.
• The potential difference of the electric field across the
depletion region is the amount of energy required to move
electrons through the electric field. This potential difference is
called the barrier potential and is expressed in volts.
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Barrier Potential
• The barrier potential of a pn junction depends on several
factors, including the type of semiconductor material, the
amount of doping, and the temperature.
• The typical barrier potential is approximately 0.7 V for
silicon and 0.3 V for germanium at 250C.
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Biasing The PN Junction
• In electronics, the term bias refers to the use of a dc voltage
to establish certain operating conditions for an electronic
device.
• In relation to a pn junction, there are two bias conditions:
forward and reverse..
• Either of these bias conditions is established by connecting
a sufficient dc voltage of the proper polarity across the pn
junction.
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Forward Bias
• Forward Bias is the condition that allows current through
a pn junction a dc voltage source connected by conductive
material (contacts and wire) across a pn junction in the
direction to produce forward bias. This external bias voltage
is designated as VBIAS.
• The negative side of VBIAS is connected to the n region of
the pn junction and the positive side is connected to the p
region. A second requirement is that the bias voltage, VBIAS,
must be greater than the barrier potential.
P region
N region
++++
+ Vbias
__ _ _
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Forward Bias
• Because like charges repet, the negative side of the biasvoltage source “pushes” the free electrons, which are the
majority carriers in the n region, toward the pn junction.
• This flow of free electrons is called electron current.
• The negative side of the source also provides a continuous
flow of electrons through the external connection (conductor)
and into the n region as shown.
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Reverse Bias
• Reverse bias is the condition that prevents current through
the pn junction a dc voltage source connected across a pn
junction in the direction to produce reverse bias.
• This external bias voltage is designated as VBIAS just as was
for forward bias. The positive side of VBIAS is connected to the
n region of the pn junction and the negative side is connected
to the p region.
Depletion Region
P
--
++
N
- Vbias +
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Reverse Bias
• What happens when a pn junction is reverse-biased. Because
unlike charges attract, the positive side of the bias-voltage
source “pulls” the free electrons, which are the majority
carriers in the n region, away from the pn junction.
• Reverse Current The small number of free minority
electrons in the p region are “pushed” toward the pn junction
by the negative bias voltage.
• When these electrons reach the wide depletion region, they
“fall down the energy hill” and combine with the minority
holes in the n region as valence electrons and flow toward the
positive bias voltage, creating a small hole current.
• Therefore, the minority electrons easily pass through the
depletion region because they require no additional energy.
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The Diode
• The diode is a PN junction devices and learn its electrical
symbol.
• One half of the diode is an n-type semiconductor and the
other half is a P-type semiconductor.
VBIAS
+
-
VF
+
-
I= 0A
R
R
VBIAS
Forward Bias
VBIAS
Reverse Bias
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The Ideal Diode Model
• The ideal model of diode is a simple switch.
• When the diode is forward-biased, it acts like a closed (on)
switch. VF = 0, IF = VBIAS/ RLIMIT
• When the diode is reverse-biased, it acts like an open (off)
switch. IR = 0 A, VR = VBIAS
Ideal Diode
A
K
Ideal Diode
A
R
K
R
VBIAS
F.B
VBIAS
R.B
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The Practical Diode Model
• The practical model, the one we will use most often, adds
the barrier potential to the ideal switch model.
• When the diode is forward-biased, it acts as a closed
switch in series with a small voltage (0.7 or 0.3) equal to
the barrier potential with the positive side toward the
anode.
• IF = VBIAS - VF
RLIMIT
• The Reverse current is neglected , the diode is assumed to
have zero reverse current. IR = 0A, VR = VBIAS
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The Practical Diode Model
VF
-
+
R
VBIAS
F.B
VBIAS
R.B
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