Transcript Electron

Atomic Structure
Major parts of an atom.
Proton: Protons are positively charged particles
found in the atomic nucleus. Protons were
discovered by Ernest Rutherford.
Neutron: Neutrons are uncharged particles
found in the atomic nucleus. Neutrons were
discovered by James Chadwick in 1932.
Electron: Electrons are negatively charged
particles that surround the atom's nucleus.
Electrons were discovered by J. J. Thomson in 1897.
http://www.bmb.psu.edu/courses/bisci004a/chem/basechem.htm
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The nucleus was discovered by Ernest Rutherford in 1911 and is the
central part of an atom. It is composed of protons and neutrons and
contains most of an atom's mass.
Electrons circle nucleus in defined
Electrons are arranged in Energy Levels
shells
K
2 electrons
or Shells around the nucleus of an atom.
L
8 electrons
M
18 electrons
• first shell
a maximum of
N
32 electrons
2 electrons
Within each shell, electrons are
further grouped into subshells
• second shell
a maximum of
s
2 electrons
8 electrons
p
6 electrons
• third shell
a maximum of
d
10 electrons
8 electrons
f
14 electrons
electrons are assigned to shells and
subshells from inside out
Si has 14 electrons: 2 K, 8 L, 4 M
2
Electronic Configuration
Dot & Cross Diagrams
With electronic configuration elements
With Dot & Cross diagrams elements
are represented numerically by the
and compounds are represented by
number of electrons in their shells and
Dots or Crosses to show electrons, and
number of shells. For example;
circles to show the shells. For example;
Ca
20
40
2,8,8,2
B
X
5
11
2,3
8
O
X
X
X
O
X
X
X
X
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The goal of electronic materials is to generate and control the flow of an
electrical current.
Electronic materials include:
1-
Conductors: have low resistance which allows electrical current flow
.Good conductors have low resistance so electrons flow through them
with ease (Copper, silver, gold, aluminum, nickel, steel).
2-
Insulators: have high resistance which suppresses electrical current
flow. Insulators have a high resistance so current does not flow in them
(Glass, ceramic, plastics, wood).
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3-
Semiconductors: can allow or suppress electrical current flow.
Semiconductors are materials that essentially can be conditioned to act as
good conductors, or good insulators, or any thing in between (carbon,
silicon, and germanium). The atoms in a semiconductor are materials from
either group IV of the periodic table, or from a combination of group III and
group V (called III-V semiconductors), or of combinations from group II
and group VI (called II-VI semiconductors). Silicon is the best and most
widely used semiconductor. as it forms the basis for integrated circuit (IC)
chips and is the most mature technology and most solar cells are also
silicon based
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Grou
p
Perio
d
1
2
3
4
5
6
7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1
2
H
He
3
4
5
Li
Be
B
11
12
13
Na
Mg
Al
Si
19
20
21
22
23
24
25
26
27
28
29
30
31
32
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
37
38
39
40
41
42
43
44
45
46
47
48
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Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
55
56
57-71
72
73
74
75
76
77
78
79
80
Cs
Ba
Hf
Ta
W
Re
Os
Ir
Pt
Au
87
88
104
105
106
107
108
109
110
Fr
Ra
Rf
Db
Sg
Bh
Hs
Mt
57
58
59
60
61
62
La
Ce
Pr
Nd
89
90
91
92
93
94
Ac
Th
Pa
U
Np
Pu
89-103
Pm Sm
9
10
O
F
Ne
16
17
18
P
S
Cl
Ar
33
34
35
36
As
Se
Br
Kr
50
51
52
53
54
In
Sn
Sb
Te
I
Xe
81
82
83
84
85
86
Hg
Tl
Pb
Bi
Po
At
Rn
111
112
113
114
115
116
117
118
Ds
Rg
Cn
Uut
Fl
63
64
65
66
67
68
69
70
71
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
95
96
97
98
99
100
101
102
103
Bk
Cf
Es
Fm
Md
No
Lr
Am Cm
6
7
8
C
N
14
15
Uup Lv Uus Uuo
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All the elements used to make semiconductors appear in Column IV of
the Periodic Table or are a combination of elements in columns at equal
distance of Column IV on each side .
http://enpub.fulton.asu.edu/widebandgap/NewPages/SCbasics.html
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Semiconductors are materials whose electrical properties lie between
Conductors and Insulators. Ex : Silicon and Germanium
The name “ Semiconductor ” implies that it conducts somewhere
between the two cases (conductors or insulators)
Conductivity :
σ conductors ~1010 /Ω-cm
The conductivity (σ) of a semiconductor (S/C) lies
between these two extreme cases.
σinsulators ~ 10
-22
/Ω-cm
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http://www.electronics-tutorials.ws/diode/diode_1.html
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Drift and Diffusion
• Current Flow:
•Drift: charged particle motion in response to an electric field.
•Diffusion: Particles tend to spread out or redistribute from areas
of high concentration to areas of lower concentration
•Recombination: Local annihilation of electron-hole pairs
•Generation: Local creation of electron-hole pairs
Drift
Direction of motion:
•Holes move in the direction of the electric field (from + to -)
•Electrons move in the opposite direction of the electric field (from - to +)
•Motion is highly non-directional on a local scale, but has a net direction on a
macroscopic scale
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Drift and Diffusion
It is well known that current is the rate of flow of charge, thus if the number
density of charge carriers (electrons and holes) present in a semiconductor
material are known currents flowing in such devices can be calculated using two
current mechanisms: Drift and Diffusion
Electron and holes will move under the influence of an applied electric field
since the field exert a force on charge carriers (electrons and holes).
F  qE
These movements result a current of Id
Id :
n:
q:
I d  nqVd A
drift current
number of charge carriers per unit volume
charge of the electron
Vd : Drift velocity of charge carrier
A:
area of the semiconductor
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Carrier Mobility
Vd   E

:
E:
:
mobility of charge carrier
applied field
Vd 
 
E
2


cm
   

V  Sec 
is a proportionality factor
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Drift and Diffusion
•Average net motion is described by the drift velocity, vd with
units cm/second
•Net motion of charged particles gives rise to a current
http://users.ece.gatech.edu/~alan/ECE3080/Lectures/ECE3080-L-7-Drift%20-%20Diffusion%20Chap%203%20Pierret.pdf
The other difference between drift current and diffusion current, is that
the direction of the diffusion current depends on the change in the carrier
concentrations, not the concentrations themselves
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Generation:
Is the
movement of an electron from
the valence band to the
conduction band. This will lead
to the creation of an electronhole pair.
Recombination: Is the
movement of an electron from
the conduction band to the
valence band. This will lead to
the destruction of and electronhole pair.
The recombination processes can be reversed
resulting in generation processes.
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SEMICONDUCTORS
INTRINSIC
123-
EXTRINSIC
Chemically very pure
Possesses poor conductivity
Has equal numbers of
negative carriers (electrons)
and positive carriers (holes)
1Improved intrinsic
semiconductor with a small amount of
impurities added by a process, known
as doping
2Alters the electrical
properties of the semiconductor and
3Improves its conductivity
DOPING
the negative charge
conductor (n-type )
the positive charge
conductor (p-type )
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DOPING
Doping is the process by which small amounts of
selected additives, called impurities, are added to
semiconductors to increase their current flow.
Semiconductors that undergo this treatment are
referred to as Extrinsic Semiconductors.
The negative charge
conductor (n-type )
This type of semiconductor
has a surplus of electrons,
the electrons are
considered the majority
current carriers, while the
holes are the minority
current carriers.
The positive charge
conductor (p-type )
This type semiconductor
holes are present in the
greatest quantity they
are majority current
carriers while electrons
are the minority current
carriers.
Available as either elements( Si and Ge) or compounds
( GaAs, SiC, GaN, GaP )
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In the atoms, the larger the radius, the higher the electron potential
energy. Hence , electron position can be described either by radius or by
its potential energy.
In the semiconductor crystal: the atom orbits overlap; radius-based
description becomes impractical. Energy-based description works well:
The highest orbit filled with electrons becomes the valence band,
The higher orbit (nearly empty ) becomes the conduction band.
( Serway book) www.physics.qc.edu/.../10%20Semiconductors..ppt
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http://hyperphysics.phy-astr.gsu.edu/hbase/solids/band.html#c3
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The term junction (formed by joining p-type anf n-type semiconductors together in
a very close contact) means the boundary interface where the two regions of the
semiconductor meet.
Filling a hole makes a negative ion
and leaves behind a positive ion on
the N side. The positive and negative
charges form the depletion region.
The electric field formed in the
depletion region acts as barrier and
an external energy must be applied to
get the electrons to move across the
barrier of the electric field, such
potential difference needed to move
the electrons through the electric field
is called barrier potential and
depends on the type of
semiconductor, the amount of doping
and temperature.
The value is around 0.7v for silicon
and 0.3
http://www.tpub.com/neets/book7/24h.htm
v for germanium
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Work function is the energy required to extract an electron from a solid in other words the
work function is the energy required to remove an electron from the highest filled level in the
Fermi distribution of a solid so that it is stationary at a point in a field-free zone just outside the
solid, at absolute zero. An estimate of the work function can be obtained thermionically from
Richardson's equation
VBM: valence bands maximum
CBM: conduction bands maximum
Eg :band gap
EF: Fermi energy
http://rsl.eng.usf.edu/Documents/Tutorials/TutorialsWorkFunction.pdf
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http://www.tpub.com/neets/book7/24h.htm
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http://www.tpub.com/neets/book7/24h.htm
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http://www.tpub.com/neets/book7/24h.htm
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zener breakdown occurs due to strong electric field across the
diode. When a lower voltage is given ,higher electric field is generated
.due to strong electric field ,the covalent bonds breakdown and free
electrons are generated when they are sufficient there is sharp rise in
current and breakdown occurs.
avalanche breakdown occurs due to higher velocity of
minority carriers. due to less doping the width of depletion region is
more. So when a high voltage is given, velocity of minority carriers
increases towards opposite type semiconductor. When they collide
with the walls of semiconductor, free electrons are generated. When
free electrons are sufficient ,then there is sharp rise in currents and
breakdown occurs.
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http://www.electronics-tutorials.ws/diode/diode_3.html
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by
Prof. Dr. Ali S. Hennache
P-N JUNCTION
PN Junction ( 04 H ) :
_
Depletion region – Junction capacitance – Diode equation 1H
_
Effect of temperature on reverse saturation current – construction, working 1H,
_
V-I characteristics and simple applications of :Junction diode, Zener diode 1H,
_
Tunnel diode and Varactor diode. Filter considerations 1H.
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Formation of a P-N junction
P-N junctions are formed by joining n-type and p-type semiconductor materials, as shown
below. Since the n-type region has a high electron concentration and the p-type a high
hole concentration, electrons diffuse from the n-type side to the p-type side. Similarly,
holes flow by diffusion from the p-type side to the n-type side.
When the electrons and holes move
to the other side of the junction, they
leave behind exposed charges on
dopant atom sites, which are fixed in
the crystal lattice and are unable to
move. On the N-type side, positive
ion cores are exposed. On the P-type
side, negative ion cores are exposed.
An electric field Ê forms between the
positive ion cores in the N-type
material and negative ion cores in the
P-type material. This region is called
the depletion region.
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html
Depleted region is thus, the part of a PN junction in which there are no electrons or holes
and thus, this latter prevents current from flowing.
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Capacitor = Two conductors with a dielectric in between
Diode in reverse bias mode = Two semiconductors with a depletion region in between
PN junction: putting a P-type material next to N-type material to form the PN junction
P-type is where we have more "holes"; N-type is where we have more electrons in
the material. Initially, when we put them together to form a junction, holes near the
junction tends to "move" across to the N-region, while the electrons in the N-region
drift across to the p-region to "fill" some holes. This current will quickly stop as the
potential barrier is built up by the migrated charges. So in steady state no current
flows.
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When we put a potential different across the terminals we have two cases:
1. Positive end to P-type, Negative end to N-type: The electric field from the external
potential different can easily overcome the small internal field (in the so-called depletion
region, created by the initial drifting of charges): usually anything bigger than 0.6V would
be enough. The external field then attracts more e- to flow from n-region to p-region and
more holes from p-region to n-region and we have a forward biased situation. the diode
is ON.
2. Positive end to N-type, Negative end to P-type: in this case the external field pushes eback to the n-region while more holes into the p-region, as a result we get no current
flow. Only the small number of thermally released minority carriers (holes in the n-type
region and e- in the p-type region) will be able to cross the junction and form a very
small current, but for all practical purposes, this can be ignored (the diode is
somehow off)
If the reverse biased potential is large enough we get avalanche break down and current
flow in the opposite direction. In many cases, except for Zener diodes, we most likely will
destroy the diode.
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Diode Junction Capacitance
The concept of junction capacitance is a diode acting as a capacitor.
Forward bias: Putting the power source such that charge is able to flow through the diode
Reverse bias: Putting the power source such that charge is not able to flow through the
diode
As it is well known that capacitance is two conductors separated by an insulator/dielectric.
In this part of the lecture we will be focusing on reverse biasing when talking about junction
capacitance.
a depletion region
a dielectric
http://tymkrs.tumblr.com/post/6976329612/diode-junction-capacitance
Capacitor = Two conductors with a dielectric in between
Diode in reverse bias mode = Two semiconductors with a depletion region in between
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Ideal Diode
The diode equation gives an expression for the current through a diode as a function of
voltage. The Ideal Diode Law, expressed as:
where:
I = the net current flowing through the diode;
I0 or (Is)= "dark saturation current", the diode leakage current density in the absence of light;
V = applied voltage across the terminals of the diode;
q = absolute value of; electron charge
k = Boltzmann's constant; and
T = absolute temperature (K).
The "dark saturation current" (I0) is an extremely important parameter which differentiates
one diode from another. I0 is a measure of the recombination in a device. A diode with a
larger recombination will have a larger I0.
Non-Ideal Diode
For actual diodes, the expression becomes:
where: n = ideality factor, a number between 1 and 2 which typically increases as the current
decreases.
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Temperature effects on p-n diode characteristics.
Temperature can have a marked effect on the characteristics of a silicon semiconductor
diode as shown in Figure below . It has been found experimentally that the reverse
saturation current Io (Is) will just about double in magnitude for every 10°C increase in
temperature.
http://www.griet.in/ece/qna/EDCQNAUNITI.pdf
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Looking at the above equation (given in the previous lecture) it would appear that the
current should decrease as the temperature increases. The exact opposite is what really
occurs. The reverse saturation current, IS, is a strong positive function of temperature as
discussed below. The increase in IS with temperature more than offsets the effect of T in
the exponential above.
Forward Bias: These curves show the characteristics of diode for different temperatures
in the forward bias. As it can be seen from the figure given above, that curve moves
towards left as we increase the temperature. We know with increase in temperature,
conductivity of semiconductors increase. The intrinsic concentration (ni) of the
semiconductors is dependent on temperature as given by:
When temperature is high, the electrons of the outermost shell take the thermal energy
and become free. So conductivity increases with temperature. Hence with increase in
temperature, the forward curve would shift towards left i.e. curve would rise sharply and
the breakdown voltage would also decrease with increase in temperature.
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Reverse Bias: This curve shows the characteristics of diode in the reverse biased
region till the breakdown voltage for different temperatures. We know ni concentration
would increase with increase in temperature and hence minority charges would
increase with increase in temperature. The minority charge carriers are also known as
thermally generated carriers and the reverse current depends on minority carriers only.
Hence as the number of minority charge carriers increase, the reverse current would
also increase with temperature as shown in the figure given on the previous page.
The reverse saturation current gets double with every 10° C increase in temperature.
In a PN junction diode, the reverse current is due to the diffusive flow of minority electrons
from the p-side to the n-side and the minority holes from the n-side to the p-side. Hence IS,
reverse saturation current depends on the diffusion coefficient of electrons and holes. The
minority carriers are thermally generated so the reverse saturation current is almost
unaffected by the reverse bias but is highly sensitive to temperature changes.
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THE DIODE DC OR STATIC RESISTANCE
IV characteristics for forward bias
Point A corresponds to zero-bias
condition.
Point B corresponds to where the
forward voltage is less than the
barrier potential of 0.7 V (For Silicon).
Point C corresponds to where the
forward voltage approximately equals
the barrier potential and the external
bias voltage and forward current
have continued to increase.
VD
RD 
ID
The diode DC or static resistance
VF
RF 
IF
VR
RR 
IR
forward biased
reverse biased
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The dynamic. resistance of a diode is designated rd
VF
rd 
I F
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Example
Determine the dc resistance for a diode with the following operating point:
A) ID =2 mA and VD = 0.5 V
B) ID =20 mA and VD = 0.8 V
C) ID =-1 μA and VD = -10 V
VF
0.5
RF 

 250
3
I F 2 *10
VF
0.8
RF 

 40
3
I F 20 *10
VR
 10
RR 

 10 M
6
I R  1*10
37
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The p-n junction plays an important role as the basic device structure for
fabricating a wide variety of electronic and photonic devices and is specially
used in more advanced electronic technologies such as in Information and
Communication Technology (ICT).
For example, p-n junction structures have been used in fabricating the
switching diodes, diode rectifiers, solar cells, light emitting diodes (LEDs),
laser diodes (LDs), photo detectors, bipolar junction transistors (BJTs),
heterojunction bipolar transistors (HBTs), and junction field-effect transistors
(JFETs), metal–semiconductor field-effect transistors (MESFETs), highelectron mobility transistors (HEMTs), and tunnel diodes.
The p-n heterojunctions can be formed from a wide variety of elemental and
compound semiconductors such as n-Si/p-SiGe, n-ZnSe/p-GaAs, p-AlGaAs/nGaAs, p-Ge/n-GaAs, n-InGaAs/n-InP, p-InAlAs/n-InGaAs, p-GaN/n-InGaN,
and p-AlGaN/n-InGaN semiconductor electronic and photonic devices.
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Zener diodes
are semiconductor diodes which have been manufactured to have their reverse
breakdown occur at a specific, well-defined voltage (its “Zener voltage”), and that are
designed such that they can be operated continuously in that breakdown mode.
Commonly available Zener diodes are available with breakdown voltages (“Zener voltages”)
anywhere from 1.8 to 200 V.
Zener Diode Allows current flow in one direction, but also can flow in the reverse direction
when above breakdown voltage
http://www.evilmadscientist.com/2012/basics-introduction-to-zener-diodes/
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.
Uses of Zener Diodes
Since the voltage dropped across a Zener Diode is a known and fixed value,
Zener diodes are typically used to regulate the voltage in electric circuits.
Using a resistor to ensure that the current passing through the Zener diode is
at least 5mA (0.005 Amps), the circuit designer knows that the voltage drop
across the diode is exactly equal to the Zener voltage of the diode.
41
Breakdown voltage is a term used to describe the level of AC or DC voltage that results
in the failure of a semiconductor device
Avalanche Breakdown is A process that occurs in a diode when high voltage causes
free electrons to travel at high speeds, colliding with other electrons and knocking them
out of their orbits. The result is a rapidly increasing amount of free electrons.
Zener Breakdown
1.This occurs at junctions which being
heavily doped have narrow depletion
layers
2. This breakdown voltage sets a
very strong electric field across
this narrow layer.
3. Here electric field is very strong
to rupture the covalent bonds thereby
generating electron hole pairs. So even
a small increase in reverse voltage is
capable of producing large number of
current carriers. Ie why the junction
has a very low resistance. This leads to
Zener Breakdown.
Avalanche breakdown
1. This occurs at junctions which
being lightly doped have wide
depletion layers.
2. Here electric field is not strong
enough to produce Zener breakdown.
3. Her minority carriers collide
with semi conductor atoms in the
depletion region, which breaks the
covalent bonds and electron-hole pairs
are generated. Newly generated
charge carriers are accelerated
by the electric field which results in
more collision and generates avalanche
of charge carriers. This results in
avalanche breakdown.
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Circuit symbol of tunnel diode is :
It was introduced by Leo Esaki in 1958
Tunnel diode structure
The tunnel diode is similar to a standard p-n junction in many respects except
that the doping levels are very high. Impurity concentration is 1 part in 10³ as
compared to 1 part in 10⁸ in p-n junction diode.
Also the depletion region, the area between the p-type and n-type areas,
where there are no carriers is very narrow . Typically it is in the region of
between five to ten nano-metres - only a few atom widths. The tunnel diode is
generally made up of Ge and GaAs.
43
Under Forward Bias
Step 1: At zero bias
there is no current flow
Step 2: A small forward bias is
applied. Potential barrier is still
very high – no noticeable
injection and forward current
through the junction.
Step 4: As the forward bias
continues to increase, the
number of electrons in the n
side that are directly
opposite to the empty states
in the valence band (in terms
of their energy) decrease.
Therefore decrease in the
tunneling current will start.
Step 3: With a larger voltage the
energy of the majority of electrons
in the n-region is equal to that of the
empty states (holes) in the valence
band of p-region; this will produce
maximum tunneling current
Step 5: As more forward
voltage is applied, the
tunneling current drops
to zero. But the regular
diode forward current
due to electron – hole
injection increases due
to lower potential
barrier.
44
- Ve Resistance
Region
I
v
Vp
Reverse
voltage
VP
Vv
Forward
Voltage
Vf
Reverse
Current
Ip:- Peak Current, Iv :- Valley Current
Ip
Forward
Current
Because of heavy doping depletion layer
width is reduced, reverse breakdown
voltage is also reduced to a very small value
resulting in appearance of the diode to be
broken for any reverse voltage and a
negative resistance section is produced in
V-I characteristic of diode.
Reduced depletion region can result in
carrier ‘punching through’ the junction with
the velocity of light even when they do not
possess enough energy to overcome the
potential barrier. The result is that large
current (forward) is produced relatively low
forward voltage (< 100 mV). Such a
mechanism of conduction in which charge
carriers punch through a barrier directly
instead of climbing over it is called
tunneling. That is why these diodes are
tunnel diodes. Because of heavy doping, it
can conduct in both forward as well as
reverse direction.
Vp:- Peak Voltage, Vv:- Valley Voltage
,Vf:- Peak Forward Voltage
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Varicap Diode (Variable Capacitor Diode)
VARACTOR DIODE(VARIABLE REACTANCE DIODE)
Varicap is a p-n junction with a special impurity profile, and its capacitance variation is very
sensitive to reverse-biased voltage. Thus its reactance can be varied in a controlled manner
with a bias voltage.
Varactor diode symbol
Varactor diodes are always operated under reverse bias conditions, and in this way there
is no conduction. They are effectively voltage controlled capacitors.
46
The transmission capacitance (Ct) established by the isolated uncovered
charges is determined by
CT= 
C(pF)
A
______
Wd
Where  is the permitivity of the
semiconductor materials, A the p-n junction
area and Wd the depletion width.
VR(V)
(VR = applied reverse bias)
47
commons.bcit.ca/cbennie/files/1207chtpp.03
They are widely used in parametric amplification, harmonic generation, mixing, detection,
and in electronic tuning, voltage controlled oscillators in radio ,TV, cellular and wireless
receivers .
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Rectifiers ( 04 H ):
Half wave rectifier 1H , full wave and bridge rectifiers 1H
Power, efficiency and ripple factor for half wave and full wave rectifiers 1H
Regulation , Harmonic components in rectified output 1H
49
A rectifier is an electrical device that converts alternating current (AC) to direct
current (DC), a process known as rectification. Rectifiers are semiconductor
diodes that conduct in only one direction. Today, most rectifier diodes are made
of silicon.
Rectifiers have many uses including as components of power supplies and as
detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum
tube diodes, mercury arc valves, and other components.
https://ssel.montana.edu/downloads/general/GCtpp.4.6
50
Rectifier circuit are divided into three types
1- Uncontrolled
2- The half
3- The fully controlled
rectifiers
controlled rectifier
rectifiers
make use of diodes. The
output D.C. voltage is fixed
and is decided by the
amplitude of the A.C. input
voltage. The direction of the
power flow is only from the
source to the load.
circuits comprise of
diodes and SiliconControlled Rectifiers
(SCRs). The D.C. load
voltage can be
controlled by changing
the firing angle of the
SCR. The control is
limited in comparison
with the fully controlled
rectifier circuits.
make use of SCRs only as
the rectifying elements. The
control of output voltage is
obtained by changing the firing
angle. as in the half controlled
rectifiers. One important
difference between This type
of rectifier and half controlled
type rectifier is that in fully
controlled rectifier circuits
direction of flow of power can
be reversed i.e. it can be
made to flow from D.C. to A.C.
side.
Types 2 and 3 are beyond the scope of this course
51
Center-Tapped
Full-Wave Rectifier
Half-Wave Rectifier (HWR)
Full-Wave Rectifier (FWR)
Center-tapped
Full-Wave Rectifier
Full-Wave
Bridge Rectifier
52
Half-wave rectification A type of current conversion that uses only one half
of an AC waveform to convert into intermittent DC. This can be the positive half
or negative half of an AC wave, depending on how the diode is connected to
the circuit.
Full-wave rectification A type of current conversion that uses both parts of
the AC sine wave, both positive and negative, to produce a DC output with a
single polarity.
53
HALF WAVE RECTIFIER
https://www.classle.net/book/half-wave-rectifier
The primary of the transformer is connected to ac supply. This induces an ac
voltage across the secondary of the transformer.
During the positive half cycle of the input voltage the polarity of the voltage
across the secondary forward biases the diode. As a result a current IL flows
through the load resistor, R. The forward biased diode offers a very low
resistance and hence the voltage drop across it is very small. Thus the
voltage appearing across the load is practically the same as the input voltage
at every instant.
During the negative half cycle of the input voltage the polarity of the
secondary voltage gets reversed. As a result, the diode is reverse biased.
Practically no current flows through the circuit and almost no voltage is
developed across the resistor. All input voltage appears across the diode itself.
54
Hence we conclude that when the input voltage is going through its positive half
cycle, output voltage is almost the same as the input voltage and during the
negative half cycle no voltage is available across the load. This explains the
unidirectional pulsating dc waveform obtained as output. The process of
removing one half the input signal to establish a dc level is called half wave
rectification.
A half-wave rectifier will only give one peak per cycle and for this and other
reasons is only used in very small power supplies.
A full wave rectifier achieves two peaks per cycle and this is the best that can be
done with single-phase input.
In half wave rectification, either the positive or negative half of the AC wave is
passed, while the other half is blocked. Because only one half of the input
waveform reaches the output
55
In order to produce steady DC from a rectified AC supply, a smoothing circuit,
sometimes called a filter, is required. In its simplest form this can be what is
known as a reservoir capacitor, filter capacitor or smoothing capacitor, placed
at the DC output of the rectifier. There will still remain an amount of AC ripple
voltage where the voltage is not completely smoothed.
Peak Inverse Voltage
When the input voltage reaches its maximum value Vm during the negative half
cycle the voltage across the diode is also maximum. This maximum voltage is
known as the peak inverse voltage. Thus for a half wave rectifier
http://www.visionics.ee/curriculum/Experiments/HW%20Rectifier/Half%20Wave%20Rectifier1.html
56
PIV is the maximum (peak) voltage that appears across the diode when
reverse biased. Here, PIV = Vm.
-
-
PIV +
+
www.faculty.umassd.edu/xtras/catls/.../ tpp.1851
57
A Full Wave Rectifier is a circuit,
which converts an ac voltage into a
pulsating dc voltage using both half
cycles of the applied ac voltage.
It uses two diodes of which one
conducts during one half cycle while
the other conducts during the other
half cycle of the applied ac voltage.
During the positive half cycle of the
input voltage, diode D1 becomes
forward biased and D2 becomes
reverse biased. Hence D1 conducts
and D2 remains OFF. The load current
flows through D1 and the voltage drop
across RL will be equal to the input
voltage.
58
An ideal half-wave rectifier
only "uses" half of the AC
waveform (hence the name
half-wave).
An ideal full-wave bridge
rectifier will use the entire
AC waveform.
An ideal full-wave rectifier
(with a center-tapped
transformer) will also use
the entire AC waveform.
59
A full-wave rectifier uses a diode bridge, made of four diodes
60
Half Wave
A half wave rectifier removes one of the positive or the negative half cycle of the wave
and only either half of the cycle appears in the output
Where
Vmax is the maximum or peak voltage value of the AC sinusoidal supply, and
VS is the RMS (Root Mean Squared) value of the supply.
The outcome of the above equation is given in more details in appendix 1
61
Full Wave
In the full wave rectifier both the cycles appear in the positive or negative cycle of the
output.
The efficiency of a full wave rectifier (81.2%) is double of a half wave rectifier(40.6%)
because the r.m.s. value in case of a full wave rectifier is Maximum current divided by
1.41 (under root of 2) whereas in case of a half wave rectifier the r.m.s current is half of
maximum current during the wave cycle.
The outcome of the above equation is given in more details in appendix 2
62
1 T
i dt

0
T
1 2
I d .c 
i d
2 0
i  I m sin 
i0
Vm
Im 
Rf  R
I av 
but
I d .c
2
1 

[ I m sin  d   0 d ]

2 0
1
I d .c 
[ I m ( cos  )0
2
I
I d .c  m
0i 
  i  2
If diode is ideal and/or R>>Rf then

Vd .c  I d .c * R
I R
Vd .c  m

Vd .c 
Vm

 0.318Vm
63
1 T
i dt
T 0
1 2
I d .c 
i d
2 0
i  I m sin 
i  I m sin 
2Vm
Im 
Rf  R
I av 
but
2
1 
I d .c 
[  I m sin  d   I m sin  d ]

2 0
1
1
I d .c 
[ I m ( cos  )0 
[ I m ( cos  )2
2
2
2I
I d .c  m

Vd .c  I d .c * R
2I R
Vd .c  m

0i 
  i  2
If diode is ideal and/or R>>Rf then
Vd .c 
2Vm

 0.637Vm
64