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Resistance
Charges passing through any conducting medium collide with
the material at an extremely high rate and, thus, experience
friction.
The rate at which energy is lost depends on the wire thickness
(area), length and physical parameters like density and
temperature as reflected through the resistivity
R
l
A
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Resistance
Charges passing through any conducting medium collide with
the material at an extremely high rate and, thus, experience
friction.
The rate at which energy is lost depends on the wire thickness
(area), length and physical parameters like density and
temperature as reflected through the resistivity
R
l
A
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Drift الجرف
Applying an electric field across a
semiconductor material, results in both
types of carrier moving in opposite
directions thus creating current flow.
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Semiconductors
Extrinsic semiconductors can be doped with
both types of impurities, and their respective
concentrations determine the type material
they will become:
N-type when ND > NA
Majority carriers are free electrons and minority
carriers are holes.
P-type when ND < NA
Majority carriers are holes and minority carriers
are free electrons.
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Drift الجرف
The magnitude of the electric field in volts/cm is
given by:
And the effective velocity of the carrier moving by the
drift action of an applied electric filed is given by:
Where n = 1350 cm2/V-s and p = 480 cm2/V-s are
the electron and hole mobility constants respectively.
L
V
E
Ennv Eppv
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P-N Junction
Created by bringing together a p-type
and n-type region within the same
semiconductor lattice.
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The Diode
n
p
p
n
B A
SiO2
Al
A
B
Al
A
B
Cross-section of pn-junction in an IC process
One-dimensional
representation diode symbol
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At the junction, free
electrons from the N-type
material fill holes from the
P-type material. This
creates an insulating layer
in the middle of the diode
called the depletion zone.
الوصلة نظريةP-N
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Conductivity أنصاف في الناقلية
النواقل
Property of a material.
It is a measure of the material’s ability
to to carry electric current.
It is given in Semiconductor by:
Measured in S-cm.
pn pnq
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Potential Barrier الكموني الحاجز
The charge barrier creates a state of balance
with the diffusion process, and this barrier
can be represented as a voltage or potential
barrier.
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Resistivity المقاومية
Measured in -cm it is the reciprocal of conductivity:
The resistance of a material with constant cross section
can be calculated by:
1
A
L
R
ne المعادن في الناقلية
االلكترونات حركيةn الحرة االلكترونات تركيز e االلكترون شحنة
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التيار كثافةCurrent
Density
Current per unit cross-sectional area.
Measured in A/cm2.
Given by:
n الحرة اإللكترونات تركيز , p الثقوب تركيز
الثقوب تركيز
* The direction of current flow vector is the
same direction as the electric field vector.
qEp p )(nEJ n
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Diffusion اإلنتشار
Diffusion current occurs because of the
physical principle that over time particles
undergoing random motion will show a
movement from a region of high
concentration to a region of lower
concentration.
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Diffusion
Current density is directly proportional to the
gradient of carrier concentration.
Dn and Dp are the diffusion constants for
electrons and holes respectively.
dx
dn
qDJ nn
dx
dp
qDJ pp
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Diodes الناقل نصف الوصلة ثنائي
A diode can be considered to be an
electrical one-way valve.
They are made from a large variety of
materials including silicon, germanium,
gallium arsenide, silicon carbide …
ANODE
D1
DIODE
CATHODE
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Diode V-I Characteristic
For ideal diode, current flows only one way
Real diode is close to ideal
Ideal Diode
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Piecewise Linear Model
The real diode can
be approximated by
a model which uses
two connected line
segments.
Note that the turn
on voltage, VF ,
marks the point
where the two line
segments meet.
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The Diode Equation
The diode equation can be derived based on the
assumption that carriers move by diffusion.
ID – Current through diode.
IO – Reverse saturation current.
VD – Voltage across the diode.
k – Boltzmann’s Constant.
n – Ideality factor (n = 1 for silicon).
T – Temperature in degrees Kelvin.
1nkT
qV
OD
D
eII 39
kT
q
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الناقل نصف الثنائي مقاومة
1nkT
qV
OD
D
eII
T
o
T
v
v
o
V
II
V
eI
dv
dI
g
T
الديناميكية الناقلية
الديناميك المقاومة تكون وبالتالي جدا صغيرة الناقلية تكون العكسي اإلنحياز حالة فيجدا كبيرة ية
يكون األمامي اإلنحياز حالة فيIo<<Iيكون وبالتالي:
I
V
g
r T
1
I
mv
I
V
r T 26
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الحمل خط مفهومLoad Line
Vo
Io
Vs
r
R
Io
Vs Vo
Io
Vo=R . Io
Vo=Vs - IoRs
o
Q point
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الديناميكي الحمل خطAC- Load Line
Vs+vs
R1 R2
C
iD
vD
-1/Rdc
-1/Rac
Q
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Graphical Solution
Simplify the circuit
connected to the diode
to a Thevenin’s
equivalent circuit.
Analyze two cases:
iD = 0;
vD = 0.
This two points
identifies the
Thevenin’s circuit load
line, and this lines
intersects the diode plot
at the operating point.
البياني الحل
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Full-wave Rectification
The output of a full-
wave rectifier is
driven by both the
positive and
negative cycles of
the sinusoidal input,
unlike the half-wave
rectifier which uses
only one cycle.
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التعرج حساباتRipple
Full-wave ripple frequency is twice AC frequency
Ripple Factor=Vr(rms)/Vdc
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Filtering
The reduction in
voltage between
charging cycles is
dependent on the
time constant stated
below:
t
m
L
eVtv
CR
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Ripple Factor
Ripple is the small voltage variation
from the filter’s output.
Good power supplies produce as little
ripple as possible.
Ripple is usually specified as Ripple
Factor, RF :
valuedc
rippleofvaluerms
RF
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Voltage Doubler
Voltage doublers allow you to develop higher
voltages without a transformer.
Stages can be cascaded to produce triplers,
quadruplers, etc.
Voltage multipliers usually supply low currents to a
high-resistance load.
Output voltage usually drops quickly as load current
increases.
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Diodes - Simple Applications
Why would you want the equivalent of an electronic check-
valve?
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Diode Current-Voltage relationship
The characteristics of all silicon based
diodes are effectively the same,
showing a sharp exponential increase
in current at forward bias and only a
few As at reverse bias.
Note that this “knee” is temperature-
dependent as observed in lab 1.
The non-ideal behavior shows a slower
than exponential increase due to a
series resistance, like the inductor.
At large reverse-bias (not shown)
current starts to flow. We will discuss
this region later.
1N4148 characteristics
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Diode models
A diode is often approximated as a
2-terminal device with ~0.7 v
drop as long as 1-100 mA are
flowing in the forward direction.
The curves for a typical switching
signal diode show why this is OK
(note the log scale for current).
When it is either reverse or
weakly forward biased it is
modeled as an open circuit.
Real diodes and the symbol
are shown. The arrow points
in the direction of forward
current flow.
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More Diode Physics
Pure semiconducting materials like Silicon have very high resistivity at room
temperature. However it can be made conductive by adding extra electrons. In
n-type material this is done via atoms with one more electron than silicon
(phosphorus). It can also be more conductive if one electron is removed,
allowing the resulting “holes” to move about (they are really absences of
valence or bonding electrons that hop from one site to the next). This is done in
p-type material by adding elements with one less electron (aluminum). This is
known as doping.
An interesting thing happens if p- and n-type material are in contact. A region
that has no holes on the p-side and no electrons on the n-side forms, called the
depletion region. This area is like the original pure semiconductor and does not
conduct, making the junction a very poor conductor again. However if it is
biased (meaning a voltage is applied) with the p-type positive relative to the n-
type, the depletion region shrinks.
This is the origin of the one-way flow. The next slides describe why this is,
using images from the website http://jas.eng.buffalo.edu/
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When material with lots of electrons (n-type) is brought into
contact with material with lots of holes (p-type) the electrons and
holes diffuse, resulting in a large current. However the loss of
these carriers leaves a depleted region that is charged (because of
the P and Al ions left behind) and reduces the current since the
electric field repels the carriers.
Diode Current-Voltage relationship (1)
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Diode Current-Voltage relationship (2)
At zero bias electrons
(holes) flow left (right)
because of diffusion and
an equal amount flow right
(left) due to the electric
field, resulting in no
current. As the depletion
region increases with
reverse bias (not shown),
the diffusion current to the
left decreases leaving only
a small reverse saturation
current due to the field.
Notice that for these doping levels the
depletion region is 0.15 microns thick.
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Diode Current-Voltage relationship (3)
At positive bias the depletion
region is reduced and many
electrons (holes) diffuse left
(right) while only a small amount
flow right (left) due to the
electric field, resulting in a large
current (sum of holes and
electrons) flowing right.
When an electron gets to the p-
type material it recombines with
a hole. If it does this in the
depleted region the ideality
factor N is 2, while it is 1 if
recombination occurs outside.
The depletion region is now only 0.05
microns thick.
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Types of diodes
Power diodes (or rectifiers) - designed to handle large currents,
typically used on power supplies or for switches. Typical values
(for 1N4002)
Max Average current: 1 A Peak current: 30 A
Reverse voltage: 100 v
Signal diodes - low powered, often faster switching (smaller
depletion region). Typical values (for 1N4148)
Max Average current: 100 mA Peak current: 450 mA
Reverse voltage: 75 v Recovery time: 4 ns
Zener diodes - voltage regulation, similar to power diodes but
has any important specification: the zener voltage, Vz (more later).
Designed to breakdown at precise voltage (range 1-400 v).
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Clamp (or clipper)
Vin
R
Vout
What does this circuit do? As
mentioned in the text it
“clamps” or “clips” the output
voltage. If the magnitude of
the input exceeds Vbatt+0.7 v,
either negative or positive, the
diode starts conducting. This
limits the maximum voltage
(this is similar to surge
protectors).
Vbat
One limitation of this is that by design it will distort a time-
varying voltage input, cutting it off at a max and a min
value.
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Stiff Clamp
Vin
C
Vout
This circuit on the other hand works
very well at higher frequencies in
limiting the minimum voltage on
the output. Treating the diode like a
simple one-way switch we see that
the output can never fall below -0.7
v, because then the diode begins
conducting.
How does the circuit do this? If the input voltage is less than this
minimum, current flows counterclockwise, charging the left
capacitor plate negative and the right plate positive. This capacitor
voltage adds to the input, raising it to a minimum of -0.7 v, when
current stops flowing. Note that it is the opposite if the diode is
reversed because the currents flow clockwise.
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Zener diode - IV characteristics
All diodes show a breakdown under
reverse bias when the internal electric
field gets very high, ripping electrons
from atoms. This avalanche breakdown
occurs typically at -50 to -1000 v.
If the depleted region that acts like a
barrier to current is thin there is another
possibility. The electrons can quantum
mechanically tunnel from the n- to the p-
side, resulting in a reverse current. This
can be engineered to occur at a precise
voltage called the Zener voltage, VZ.
V
I
VZ
IIIIII
The graph shows three distinct regions, as labeled. Region III is
where zener diodes normally operate.
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Zener diode - voltage regulator (1)
How can a zener regulator act to control the
output voltage? In the circuit shown (note
that the zener is connected so that it is
reverse biased) current flows through the
zener and it is in region III, so the voltage
drop across it is VZ.
A zener only works as a regulator because
it ensures that enough current is drawn
through the series resistor so that the
voltage drop results in Vout=VZ.
If no current flows through the zener
(actually less than a few mAs) it no longer
regulates because it is in region II.
Vout
UnregulatedVin
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Zener diode - voltage regulator (2)
Rload
UnregulatedVin
Rload
UnregulatedVin
The voltage across the load is
VZ since there is current
through the zener (Region III)
RR
The voltage on the load is
Rload/(R+Rload)Vin because
there is no current through the
zener.