Download - Chapter 1 - Additional Notes Lecture
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8/13/2019 Chapter 1 - Additional Notes Lecture
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EMT111CHAPTER 1
Introduction to Semiconductor
By
Pn. Aini Syuhada Md Zain
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Introduction to Semiconductor -
Chapter Outline :
1.8 Voltage Current Characteristic of a Diode
1.9 Diode Models
1.10 Testing a Diode
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-When a forward bias voltage isappliedcurrent called forwardcurrent,
-In this case with the voltageapplied is less than the barrier
potential so the diode for allpractical purposes is still in anon-conducting state. Current isvery small.
-Increase forward bias voltage
current also increase
FI
FIGURE 1-26 Forward-bias measurements
show general changes in VFand IFas VBIAS isincreased.
1.8 Voltage-Current Characteristic of a Diode
( V-I Characteristic for forward bias)
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-With the applied voltageexceeding the barrierpotential (0.7V), forward
current begins increasingrapidly.
-But the voltage across thediode increase only above0.7 V.
FIGURE 1-26 Forward-bias measurements
show general changes in VFand IFas VBIAS is
increased.
1.8 Voltage-Current Characteristic of a Diode
( V-I Characteristic for forward bias)
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-Plot the result ofmeasurement in Figure 1-26, you get the V-Icharacteristic curve for aforward bias diode
- Increase to the right
- increase upward
FFd IVr /
'
dynamic resistance rddecreases as you move up thecurve
FV
FI
VVF
7.0
zero
biasVV
F 7.0
1.8 Voltage-Current Characteristic of a Diode
( V-I Characteristic for forward bias)
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Reverse
Current
Breakdown
voltage
-not a normaloperation of pn
junction devices
-the value can be
vary for typical Si
1.8 Voltage-Current Characteristic of a Diode
( V-I Characteristic for Reverse bias)
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Combine-Forward bias
& Reverse bias CompleteV-Icharacteristic curve
1.8 Voltage-Current Characteristic of a Diode
( Complete V-I Characteristic curve)
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Forward biased
dioed :for a given value
of
For a given
Barrier potential
decrease as T
increase
Reverse currentbreakdown
small & can be
neglected
FIT ,
FV
FF
VI ,
1.8 Voltage-Current Characteristic of a Diode
( Temperature effect on the diode V-I Characteristic)
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Directional of current
cathodeanod
1.9 Diode Models
( Diode structure and symbol)
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DIODE
MODEL
The Ideal
Diode Model
The Complete
Diode Model
The Practical
Diode Model
1.9 Diode Models
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Assume
Forward
current, by
Ohms law
Ideal model of diode-simple switch:
Closed (on) switch -> FB
Open (off) switch -> RBVV
F 0
LIMIT
BIAS
FR
VI
BIASR
R
VV
AI
0
(1-2)
1.9 Diode Models
( The ideal Diode model)
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Adds the barrier potentialto the ideal switch model
is neglected
From figure (c):
The forward current [byapplying Kirchhoffs voltagelow to figure (a)]
Ohms Law
dr'
Equivalent to close
switch in series with a
small equivalent voltage
source equal to the barrier
potential 0.7V
Represent by
produced across the pnjunction
FV
Same as ideal diode
model
)(3.0
)(7.0
GeVV
SiVV
F
F
0LIMIT
RFBIAS VVV
LIMITFR RIV
LIMIT
LIMIT
FBIAS
F
R
VVI
BIASR
R
VV
AI
0
(1-3)
1.9 Diode Models ( The Practical Diode model)
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Complete model of diodeconsists:
Barrier potential
Dynamic resistance,
Internal reverse resistance,The forward voltage:
The forward current:
dr'
Rr'
acts as closed switch inseries with barrier
potential and small dr'
Rr'
acts as openswitch in parallel
with the large
'7.0dFF
rIVV
'
7.0
dLIMIT
BIAS
F
rR
VVI
(1-4)
(1-5)
1.9 Diode Models ( The Complete Diode model)
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10V
1.0k
5V
1.0k
(1) Determine the forward voltage and forward current
[forward b ias] for each of the diode model also find thevoltage across the limiting resistor in each cases.
Assumed rd = 10 at the determined value of forwardcurrent.
1.9 Diode Models ( Example)
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a) Ideal Model:
b) Practical Model:
(c) Complete model:
VARIV
mAV
R
VI
V
LIMITFR
BIAS
F
F
LIMIT
10)101)(1010(
10100010
0
33
VARIV
mAVV
R
VVI
VV
LIMITFR
LIMIT
FBIAS
F
F
LIMIT
3.9)101)(103.9(
3.91000
7.010)(7.0
33
VkmARIV
mVmAVrIVV
mAk
VV
rR
VVI
LIMITFR
dFF
dLIMIT
BIAS
F
LIMIT
21.9)1)(21.9(
792)10)(21.9(7.07.0
21.9101
7.0107.0
'
'
1.9 Diode Models ( Example)
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Diodes come in a variety of sizes and shapes. The design and structure is
determined by what type of circuit they will be used in.
1.9 Diode Models ( Typical Diodes)
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Testing a diode is quite simple, particularly if the multimeter
used has a diode check function. With the diode check functiona specific known voltage is applied from the meter across thediode.
K A A K
With the diode check
function a good diode willshow approximately .7 V or.3 V when forward biased.
When checking in reversebias the full applied testingvoltage will be seen on thedisplay.
1.10 Testing A Diodes ( By Digital multimeter)
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Defective Diode
1.10 Testing A Diodes ( By Digital multimeter)
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Select OHMs range
Good diode:
Forward-bias:
get low resistance reading (10 to 100ohm)
Reverse-bias:
get high reading (0 or infinity)
1.10 Testing A Diodes ( By Analog multimeterohmfunction )
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P-materials are doped with trivalent impurities
N-materials are doped with pentavalent impurities
Pand Ntype materials are joined together to form aPNjunction.
A diode is nothing more than a PNjunction.
At the junction a depletion region is formed. Thiscreates barrier which requires approximately .3 V for a
Germanium and .7 V for Silicon for conduction to takeplace.
Diodes, transistors, and integrated circuits areall made of semiconductor material.
Summary
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When reversed biased a diode can only withstandso much applied voltage. The voltage at which
avalanche current occurs is called reverse breakdownvoltage.
There are three ways of analyzing a diode. Theseare ideal, practical, and complex. Typically we use a
practical diode model.
A diode conducts when forward biased and does notconduct when reverse biased
Summary