SEMICONDUCTORS
Overview
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Introduction
What are P-type and N-type semiconductors??
What are Diodes?
Forward Bias & Reverse Bias
Characteristics Of Ideal Diode
Shockley Equation
I – V Characteristics of Diodes
Try this out
6.Cross roads
8.Tri-Cycle
Introduction
Matter Substance
(Resistivity)
Solid
Liquid
Gas
Substance
(Chemical& Physical
Property)
Conductors
Organic
Insulators
Inorganic
Semiconductors
Energy diagrams for the three types of materials.
Insulators, Semiconductors, and Metals
 This
separation of the valence and conduction bands
determines the electrical properties of the material
 Insulators have a large energy gap
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electrons can’t jump from valence to conduction bands
no current flows
 Conductors
(metals) have a very small (or
nonexistent) energy gap
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electrons easily jump to conduction bands due to thermal excitation
current flows easily
 Semiconductors have a moderate energy gap
 only a few electrons can jump to the conduction band
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leaving “holes”
 only a little current can flow
What is a Semiconductor?
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Low resistivity => “conductor”
High resistivity => “insulator”
Intermediate resistivity => “semiconductor”
 Generally, the semiconductor material used in
integrated-circuit devices is crystalline
 In recent years, however, non-crystalline
semiconductors have become commercially very
important
polycrystalline
amorphous
crystalline
Definition
Semiconductors are materials
whose electrical properties lie
between Conductors and
Insulators.
Ex : Silicon and Germanium,
Diagrams of the silicon and germanium atoms.
Semiconductor
Intrinsic
semiconductor
Extrinsic
semiconductor
P-Type
N-Type
Bohr model of an atom
The two simplest atoms, hydrogen and helium.
Energy levels decreases as the distance from the nucleus increases.
Energy band diagram for a pure (intrinsic) silicon crystal with unexcited atoms. There are no electrons in
the conduction band.
Diagrams of the silicon and copper atoms.
The Silicon Atom
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14 electrons occupying the 1st 3 energy levels:
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1s, 2s, 2p orbitals filled by 10 electrons
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3s, 3p orbitals filled by 4 electrons
To minimize the overall energy, the 3s and 3p
orbitals hybridize to form 4 tetrahedral 3sp
orbitals
Each has one electron and
is capable of forming a bond
with a neighboring atom
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2n2
n =1 , K= 2
n = 2, L = 8
n = 3, M= 18
n = 4 , N= 32
n = 5, O = 50
Illustration of covalent bonds in silicon.
Covalent bonds in a three-dimensional silicon crystal.
Creation of electron-hole pairs in a silicon crystal. Electrons in the conduction band are free.
Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine
with holes.
INTRINSIC (PURE) SILICON
At 0 Kelvin Silicon
density is 5*10²³ particles/cm³
Higher temperatures create
free charge carriers.
A “hole” is created in the
absence of an electron.
At 23C there are 10¹º
particles/cm³ of free carriers
Silicon has 4 valence
electrons, it covalently bonds
with four adjacent atoms in
the crystal lattice
Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons.
Hole current in intrinsic silicon.
Improving Conduction by Doping

To make semiconductors better conductors, add
impurities (dopants) to contribute extra electrons
or extra holes
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elements with 5 outer electrons contribute an extra electron to
the lattice (donor dopant)
elements with 3 outer electrons accept an electron from the
silicon (acceptor dopant)
DOPING
There are two types of doping
N-type and P-type.
The N in N-type stands for negative.
A column V ion is inserted.
The extra valence electron is free to
move about the lattice
The P in P-type stands for positive.
A column III ion is inserted.
Electrons from the surrounding
Silicon move to fill the “hole.”
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Energy-band Diagram
A very important concept in the study of semiconductors is the
energy-band diagram
It is used to represent the range of energy a valence electron can
have
For semiconductors the electrons can have any one value of a
continuous range of energy levels while they occupy the valence
shell of the atom
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That band of energy levels is called the valence band
Within the same valence shell, but at a slightly higher energy
level, is yet another band of continuously variable, allowed
energy levels
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This is the conduction band
Band Gap
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Between the valence and the conduction band is
a range of energy levels where there are no
allowed states for an electron
This is the band gap Eg
In silicon at room temperature [in electron
volts]: 1.1eV
Electron volt is an atomic measurement unit, 1
eV energy is necessary to decrease of the
potential of the electron with 1 V.
Impurities
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Silicon crystal in pure form is
good insulator - all electrons are
bonded to silicon atom
Replacement of Si atoms can
alter electrical properties of
semiconductor
Group number - indicates
number of electrons in valence
level (Si - Group IV)
Impurities
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Replace Si atom in crystal with Group V atom
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substitution of 5 electrons for 4 electrons in outer shell
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extra electron not needed for crystal bonding structure
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can move to other areas of semiconductor
current flows more easily - resistivity decreases
many extra electrons --> “donor” or n-type material
Replace Si atom with Group III atom
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substitution of 3 electrons for 4 electrons
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extra electron now needed for crystal bonding structure
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“hole” created (missing electron)
hole can move to other areas of semiconductor if electrons
continually fill holes
again, current flows more easily - resistivity decreases
electrons needed --> “acceptor” or p-type material
Doping
By substituting a Si atom with a special impurity atom (Column V
or Column III element), a conduction electron or hole is created.
Donors: P, As, Sb
Acceptors: B, Al, Ga, In
Dopant concentrations typically range from 1014 cm-3 to 1020 cm-3
What are P-type and N-type ?
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Semiconductors are classified in to P-type and
N-type semiconductor
P-type: A P-type material is one in which
holes are majority carriers i.e. they are
positively charged materials (++++)
N-type: A N-type material is one in which
electrons are majority charge carriers i.e. they
are negatively charged materials (-----)
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P-type semiconductor
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Ntype semiconductor
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SiB,SiAl,SiGa,SiIn
GeB,GeAl,GeGa,GeIn
SiN,SiP,SiAs,SiSb
GeN,GeP,GeAs,GeSb
Compound
Semiconductor
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GaP,GaAs,
P-Type
Trivalent impurity atom in a silicon crystal structure. A boron (B) impurity atom is shown in the center.
N-Type
Pentavalent impurity atom in a silicon crystal structure. An antimony (Sb) impurity atom is shown in the center. The
extra electron from the Sb atom becomes a free electron.
Extrinsic semiconductors II
Doping
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N-Type
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P-Type
THE P-N JUNCTION
THE P-N JUNCTION
Diode structure and schematic symbol.
p-n junction formation
p-type material
n-type material
Contains
NEGATIVELY
charged acceptors
(immovable) and
POSITIVELY charged
holes (free).
Contains POSITIVELY
charged donors
(immovable) and
NEGATIVELY
charged free electrons.
Total charge = 0
Total charge = 0
p-n junction formation
What happens if n- and p-type materials are in close contact?
p-type material
n-type material
Contains
NEGATIVELY
charged acceptors
(immovable) and
POSITIVELY charged
holes (free).
Contains POSITIVELY
charged donors
(immovable) and
NEGATIVELY
charged free electrons.
Total charge = 0
Total charge = 0
The basic diode structure at the instant of junction formation
showing only the majority and minority carriers.
Formation of the depletion region. The width of the depletion region is
exaggerated for illustration purposes.
Energy diagrams illustrating the formation of the pn junction and
depletion region.
Diodes
Electronic devices created by bringing
together a p-type and n-type region within the
same semiconductor lattice. Used for
rectifiers, LED etc
Diodes
It is represented by the following symbol,
where the arrow indicates the direction of
positive current flow.
Forward Bias and Reverse Bias
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Forward Bias : Connect positive of the Diode
to positive of supply…negative of Diode to
negative of supply
Reverse Bias: Connect positive of the Diode
to negative of supply…negative of diode to
positive of supply.
Forward-bias and reverse-bias connections showing the diode symbol.
Biasing the P-N Diode
THINK OF THE
DIODE AS A
SWITCH
Forward Bias
Reverse Bias
Applies - voltage
to the n region
and + voltage to
the p region
Applies + voltage to
n region and –
voltage to p region
CURRENT!
NO CURRENT
A diode connected for forward bias.
A forward-biased diode showing the flow of majority carriers and the voltage
due to the barrier potential across the depletion region.
The depletion region narrows and a voltage drop is produced across the pn junction
when the diode is forward-biased.
A diode connected for reverse bias.
The diode during the short transition time immediately after reverse-bias
voltage is applied.
The extremely small reverse current in a reverse-biased diode is due to the
minority carriers from thermally generated electron-hole pairs.
Forward-bias measurements show general changes in VF and IF as VBIAS is increased.
P-N Junction - V-I characteristics
Voltage-Current relationship for a p-n junction (diode)
V-I characteristic curve for forward bias. Part (b) illustrates how the dynamic resistance r’d
decreases as you move up
V-I characteristic curve for reverse-biased diode.
The complete V-I characteristic curve for a diode.
I-V characteristics of Ideal diode
Current-Voltage Characteristics
THE IDEAL DIODE
Positive voltage yields
finite current
Negative voltage yields
zero current
REAL DIODE
Characteristics of Diode
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Diode always conducts in one direction.
Diodes always conduct current when
“Forward Biased” ( Zero resistance)
Diodes do not conduct when Reverse Biased
(Infinite resistance)
I-V Characteristics of Practical Diode
Temperature effect on the diode V-I characteristic. The 1 mA and 1µA marks on the vertical
axis are given as a basis for a relative comparison of the current scales.
Junction Breakdown
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Zener break down-Heavily doped
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Narrow Barrier
10 power 8 V/m electric field
Low resistance in breakdown region
Avalanche break down-Lightly doped
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Wide Barrier- minority charge carriers collide
Very low resistance in breakdown region
Diode Ratings
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Peak Reverse Voltage
( VR)
Average Forward Current
(I FM, IFMax,IF av)
Forward surge current
(IFS)
Maximum Forward
Voltage (VFM)
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Forward Voltage (VF)
Reverse Current
(IR , I RM)
Reverse Recovery
time(trr)
Power Dissipation (P)
Diode Ratings
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Peak Inverse Voltage
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Max reverse voltage
can be applied
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Peak reverse voltage
(PRV)
Reverse breakdown
voltage (VR)
Maximum reverse
voltage (VRM)
Diode Ratings
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Average Forward
Current
@ 250C
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Also called as
maximum steady state
forward current (I FM)
Or Repetitive forward
current (Irep)
Current should be
reduced for operation
at high temperature
Diode Ratings
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Forward Surge current
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Large current which a
diode can take for a
very short time period
IFS
Diode Ratings
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Maximum Forward
Voltage
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Maximum forward
voltage- without
burnout
VFM
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Forward Voltage
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Voltage at a given temp
and for a specific value
of forward current
VF
Diode Ratings
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Reverse Recovery
Time
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Maximum time taken to
switch from ON to OFF
trr
Nanoseconds
Diode Ratings
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Reverse Current
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Maximum saturation
current at maximum
reverse voltage
IR or IRM
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Power dissipation
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Maximum power that
the diode can dissipate
on a continuous basis in
free air at 250C
Rectification
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Converting ac to dc is accomplished by the
process of rectification.
Two processes are used:
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Half-wave rectification;
Full-wave rectification.
Half-wave Rectification
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Simplest process used
to convert ac to dc.
A diode is used to clip
the input signal
excursions of one
polarity to zero.
Shockley Equation
  vD
iD  I s exp 
  nVT
 
  1
 
kT
VT 
q
VT  26 mV
Is is the saturation current ~10 -14
Vd is the diode voltage
n – emission coefficient (varies from 1 - 2 )
k = 1.38 × 10–23 J/K is Boltzmann’s constant
q = 1.60 × 10–19 C is the electrical charge of an
electron.
At a temperature of 300 K, we have
Types of Diodes and Their Uses
PN Junction
Diodes:
Are used to allow current to flow in one direction while blocking
current flow in the opposite direction. The pn junction diode is the
typical diode that has been used in the previous circuits.
A
K
P
Schematic Symbol for a PN
Junction Diode
Zener Diodes:
n
Representative Structure for a PN
Junction Diode
Are specifically designed to operate under reverse breakdown
conditions. These diodes have a very accurate and specific reverse
breakdown voltage.
A
Schematic Symbol for a Zener
Diode
K
Kristin Ackerson, Virginia Tech EE
Spring 2002
Types of Diodes and Their Uses
These diodes are designed to have a very fast switching time which
makes them a great diode for digital circuit applications. They are
very common in computers because of their ability to be switched
on and off so quickly.
Schottky Diodes:
A
K
Schematic Symbol for a Schottky
Diode
Shockley Diodes:
A
The Shockley diode is a four-layer diode while other diodes are
normally made with only two layers. These types of diodes are
generally used to control the average power delivered to a load.
K
Schematic Symbol for a fourlayer Shockley Diode
Kristin Ackerson, Virginia Tech EE
Spring 2002
Types of Diodes and Their Uses
Light-Emitting
Diodes:
Light-emitting diodes are designed with a very large bandgap so
movement of carriers across their depletion region emits photons of
light energy. Lower bandgap LEDs (Light-Emitting Diodes) emit
infrared radiation, while LEDs with higher bandgap energy emit
visible light. Many stop lights are now starting to use LEDs because
they are extremely bright and last longer than regular bulbs for a
relatively low cost.
A
K
The arrows in the LED
representation indicate
emitted light.
Schematic Symbol for a LightEmitting Diode
Kristin Ackerson, Virginia Tech EE
Spring 2002
LED - Light Emitting Diodes
When a light-emitting diode is
forward biased, electrons are
able to recombine with holes
within the device, releasing
energy in the form of photons.
This effect is called
electroluminescence and the
color of the light (corresponding
to the energy of the photon) is
determined by the energy gap of
the semiconductor.
Source http://en.wikipedia.org/wiki/Light-emitting_diode
LED - Light Emitting Diodes
UV – AlGaN
Blue – GaN, InGaN
Red, green – GaP
Red, yellow – GaAsP
IR- GaAs
LED - Colors & voltage drop
Color
Wavelength
(nm)
Voltage (V)
Semiconductor Material
Infrared
λ > 760
ΔV < 1.9
Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs)
Red
610 < λ < 760
1.63 < ΔV < 2.03
Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide
(GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III)
phosphide (GaP)
Orange
590 < λ < 610
2.03 < ΔV < 2.10
Gallium arsenide phosphide (GaAsP) Aluminium gallium indium
phosphide (AlGaInP)Gallium(III) phosphide (GaP)
Yellow
570 < λ < 590
2.10 < ΔV < 2.18
Gallium arsenide phosphide (GaAsP) Aluminium gallium indium
phosphide (AlGaInP) Gallium(III) phosphide (GaP)
Green
500 < λ < 570
1.9 < ΔV < 4.0
Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN) Gallium(III)
phosphide (GaP)Aluminium gallium indium phosphide (AlGaInP)
Aluminium gallium phosphide (AlGaP)
Blue
450 < λ < 500
2.48 < ΔV < 3.7
Zinc selenide (ZnSe), Indium gallium nitride (InGaN), Silicon carbide
(SiC) as substrate, Silicon (Si)
Violet
400 < λ < 450
2.76 < ΔV < 4.0
Indium gallium nitride (InGaN)
Purple
multiple types
2.48 < ΔV < 3.7
Dual blue/red LEDs,blue with red phosphor,or white with purple plastic
Ultraviolet
λ < 400
3.1 < ΔV < 4.4
diamond (235 nm), Boron nitride (215 nm) , Aluminium nitride (AlN)
(210 nm) Aluminium gallium nitride (AlGaN) (AlGaInN) — (to 210 nm)
White
Broad
spectrum
ΔV = 3.5
Blue/UV diode with yellow phosphor
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pn-junction laser
Light
Amplification by
Stimulated
Emission of
Radiation
Diode Lasers are Small!
http://faculty.uml.edu/carmiento/Special%20Lectures/Intro%20to%20EE%20Lecture.pdf