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7/29/2019 EE3950 Class Notes Chapter 10 Hambley 1-21 (1)
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Objectives
Understand diode operation.
Select diodes for various applications.
Use the graphical load-line technique toanalyze nonlinear circuits.
Analyze and design simple voltage
regulator circuits.
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Objectives
Use the ideal diode and piecewise-linearmodels to analyze circuits.
Understand rectifier and wave shapingcircuits.
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Diodes
A two terminal nonlinear electronicdevice.
It conducts current in one direction,but not in the opposite direction.
Arrow indicates the direction of flow ofpositive carriers
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Diodes
A diode is said to beforward biasedwhen it isconducting current,and it is said to bereversed biased
when it is notconductingsignificant current.
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Semiconductors
Crystalline solidmaterials whoseresistivity's are valuedbetween those ofconductors andinsulators.
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Semiconductors
Silicon atoms join together to form a regularthree dimensional structure called a crystallattice.
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Intrinsic Semiconductor
Intrinsic semiconductorsare puresemiconductor materials.
They are neutral in totalcharge, and are poorconductors of electricity.
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Extrinsic Semiconductors
The process that adds small amounts ofimpurities, dopants, to a semiconductor iscalled doping.
Impurities are classified as either: Donor.
Acceptor.
After the doping process the materialsgenerated are termed extrinsicsemiconductors.
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N-Type Semiconductor
Donor impurities:
Antimony, Arsenic and Phosphorus.
They have 5 electrons in their outer electronshell. Four of them will be used in thecovalent bonds to the neighboring siliconatoms, but the fifth can be easily freed from
their original atoms by thermal energy evenat room temperatures.
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N-Type Semiconductor
Semiconductor materials doped to containexcess free electrons are considered n-typesemiconductors.
Even though the added impurity createdexcess free electrons, the material is stillneutral in charge.
The free electron concentration isapproximately equal to the donor atom dopingdensity:
nND
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N-Type Semiconductor
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P-Type Semiconductor
Acceptor impurities:
Boron, Gallium and Indium.
They have 3 electrons in their outerelectron shell, and they are not enoughto fill all the orbitals around it. This
leaves a bond site empty, and thisempty place is called a hole.
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P-Type Semiconductor
Semiconductor materials doped to containexcess holes are considered p-typesemiconductors.
Even though the added impurity createdexcess holes, the material is still neutral incharge.
The hole concentration is approximately equal
to the donor atom doping density: pNA
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P-Type Semiconductor
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Semiconductors
Extrinsic semiconductors can be doped withboth types of impurities, and their respectiveconcentrations determine the type material
they will become: N-type when ND> NA
Majority carriers are free electrons and minoritycarriers are holes.
P-type when ND< NAMajority carriers are holes and minority carriers
are free electrons.
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Semiconductors
In pure, intrinsic semiconductors, freeelectrons and holes are created in pairs.
The intrinsic carrier concentrationisdefined as:
ni= n = p
For silicon at 300K,ni 1.6 X 10
10 electrons/cm3
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Current Conduction inSemiconductors
At temperatures above absolute zero the freecarriers are in constant random motion due totheir thermal energy, however their netmotion in any particular direction is zero,
therefore there is no net current flow.
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Current Conduction inSemiconductors
There are two mechanisms by whichcharge move in a particular direction,
thus creating an electric current: Drift.
Diffusion.
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Drift
Applying an electric field across asemiconductor material, results in both
types of carrier moving in oppositedirections thus creating current flow.
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Drift
The magnitude of the electric field in volts/cm isgiven by:
And the effective velocity of the carrier moving bythe 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
VE
Ennv
Eppv
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Conductivity
Property of a material.
It is a measure of the materials ability
to allow electric current to flow.It is given by:
Measured in S/m.
pn
pnq
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Resistivity
Property of a material.
Measured in -m it is the reciprocal of
conductivity:
1
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Resistance
Resistance measured in it is the reciprocalof conductance measured in S.
The resistance of a material with constantcross section can be calculated by:
GA
LR
1
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Current Density
Current per unit cross-sectional area,measured in A/cm2.
Given by:
The direction of current flow vector is thesame direction as the electric field vector.
EJ
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Diffusion
Diffusion current occurs because of thephysical principle that over time particlesundergoing random motion will show a
movement from a region of highconcentration to a region of lowerconcentration.
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Diffusion
Current density is directly proportional to thegradient of carrier concentration.
Dnand Dpare the diffusion constants forelectrons and holes respectively.
dx
dnqDJ nn
dx
dpqDJ pp
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P-N Junction Diode
Created by bringing together a p-typeand n-type region within the same
semiconductor lattice.
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P-N Junction
At the instant this junction is created freeelectrons and holes start diffusing from theirregions of high concentration to regions of
low concentration.
This diffusion process is stopped very quicklydue to the fact that the movement of the free
electrons and holes leave behind uncoverednegative and positive charges bound in thelattice (dopant atoms).
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Depletion Region
The diffusion processbuilds up chargelayers in a region,
called depletionregion, which isdepleted of carriers.
The charge layerprevents furtherdiffusion.
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Potential Barrier
The charge barrier creates a state of balance withthe diffusion process, and this barrier can berepresented as a voltage or potential barrier.
The height of the potential barrier across the p-njunction can be modified by applying an externalvoltage across the junction.
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Potential Barrier
The diffusion of carriers across thejunction is exponentially related to thebarrier height: Change in voltage incurs an exponential
change in current due to carrier diffusion.
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Forward Bias
If the p-region is made more positive thanthe n-region then the height of barrier isreduced and more carriers can diffuse
through the junction. The junction is said tobe forward biased.
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Reverse Bias
If the p-region is made more negative thanthe n-region then the height of barrier isincreased and very few carriers can diffuse
through junction. This is called reverse bias.
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VI Characteristic Curve
Turn on voltage:
0.6 to 0.7 volts for Si.
0.3 volts for Germanium.
Breakdown voltage:
Varies depending on thetype of diode.
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Turn-on Voltage
Arbitrarily defined by manufacturers tobe the externally applied voltage
(forward bias) required to obtain 1 Aof current flow.
It is designated by VF
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Breakdown Voltage
The minimum reversevoltage to makethe diode conduct in reverse.
It is designated by VR
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The Shockley Equation
1T
D
nV
V
SD eII
IS Saturation current
n Emission coefficient
VT Thermal voltage
q
kTVT
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Zener Diode
Diode designed to operate in the breakdown region.
The breakdown voltage, is also known as theavalancheor zener breakdown voltage.
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Graphical Solution
Simplify the circuit connected to the diode to aThevenins equivalent circuit.Analyze two cases: i
D= 0 diode is behaving as an open circuit;
vD= 0 diode is behaving as a short circuit.
This two points identifies the Thevenins circuit loadline, and this lines intersects the diode plot at theoperating point.
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Graphical Solution
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Diode Circuit Models
Diode models that predict the relationbetween the dc voltage across the diode, VD,and the current through the diode, ID, areused to analyze circuits containing this non-linear device. Three models will be discussedhere:
The ideal diode model; The diode equation model;
The piecewise linear diode model.
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Diode Circuit Models
Which model should you use?
Ask yourself:
What do I know about the problem? Which is the simplest model that will give
me results with accuracy I desire?
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Ideal Diode
Idealized two terminaldevice which passescurrent in one direction(zero resistance) and
passes no current in theopposite direction(infinite resistance).Its v-i plot, which showsthe relationship of the
voltage across the diodeand the current flowingthrough it, contains adiscontinuity.
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Ideal Diode
If the diode is forward biased then the ideal diodeconducts current as a closed switch.
If the diode is reverse biased then the ideal diode will
not conduct current, and it will appear as an openswitch.
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Ideal Diode
When analyzing circuits using thismodel, replace the diode with a verysmall test resistance, R, and solve forthe voltage across the test resistance. Ifthe polarity of the voltage across thetest resistance would forward bias the
diode replace it with a closed switchotherwise replace it with an openswitch.
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The Diode Equation and Model
The diode equation can be derived based on theassumption that carriers move by diffusion.
ID Current through diode.
IO Reverse saturation current.
VD Voltage across the diode. kBoltzmanns Constant.
n Ideality factor (n= 1 for silicon).
T Temperature in degrees Kelvin.
1nkT
qV
OD
D
eII
39kT
q
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The Diode Equation and Model
1nkT
qV
OD
D
eII
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Piecewise Linear Model
The real diode canbe approximated bya model which uses
two connected linesegments.
Note that the turnon voltage, VF ,
marks the pointwhere the two linesegments meet.
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Power Supply Circuits
Power supply circuits are used toconvert ac to dc for the purpose of
operating electronic circuits.Typical residential ac powerdistribution:
110-120 volts; 220-240 volts.
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Power Supply Circuits
Typical electronicsystemrequirements:
Digital electronics:
5 volts dc;
Analog electronicsrequires twosupplies:
+15 volts dc;
-15 volts dc.
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Power Supply Circuits
To achieve its purpose a power supplymust:
Step down the voltage supplied; Convert ac to dc by rectifying the ac.
A transformer is used to step down the
magnitude of the voltages from the wallreceptacle.
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Transformer
A transformer consists of two coils of wire ona common iron core. The voltages on thesetwo coils are related by the turns ratio, whichis the ratio of the number of turns of wire inthe secondary coil to that in the primary coil.
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RMS Values
Note that the 110-120 volts and 220-240 volts are RMS values.
The actual amplitude of that sinusoidalsignal is a factor of 2 larger.
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Rectification
Converting ac to dc is accomplished bythe process of rectification.
Two processes are used: Half-wave rectification;
Full-wave rectification.
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Half-wave Rectification
Simplest processused to convert acto dc.
A diode is used toclip the input signalexcursions of one
polarity to zero.
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Full-wave Rectification
The output of a full-wave rectifier isdriven by both the
positive andnegative cycles ofthe sinusoidal input,
unlike the half-waverectifier which usesonly one cycle.
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Filtering
Process used tosmooth out theoutput of the
rectifier circuit.
One of the mostcommon filter is the
RC network.
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Filtering
The reduction involtage betweencharging cycles is
dependent on thetime constant statedbelow:
t
m
L
eVtv
CR
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Ripple Factor
Ripple is the small voltage variationfrom the filters output.
Good power supplies produce as littleripple as possible.
Ripple is usually specified as Ripple
Factor, RF:
valuedc
rippleofvaluermsRF
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Clipper Circuits
Used to limit thevoltage excursionsof a signal at some
particular positivevalue, negativevalue or both.
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Clamper Circuits
Used to generate anoutput waveformwhich appears like
the input one exceptthat the DC levelhas either shiftedpositively or
negatively withrespect to the inputwaveform.
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Voltage Multiplier Circuits
A voltage multiplier is an electrical circuitthat converts AC electrical power from alower voltage to a higher DC voltage by
means of capacitors and diodes combinedinto a network.
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Zener Diode
Analyzing a diode operating in the reverse bias regionwill show that the current through it remainsessentially constant until the breakdown voltage, also
called the avalancheor zener breakdown voltage, isreached. At this point the current will increase veryrapidly for a small voltage change.
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Voltage Regulation
This characteristic of the zener diode isvery useful for voltage regulation
circuits. The zener diode provides aneffective way to clamp or limit thevoltage at a relatively constant valuethus creating a voltage regulationcapability.
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Voltage Regulation
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Photo Diodes and LEDs
Photodiodesconvert incident radiation toelectric current.
The suns radiation creates electron-hole pairs
in the depletion region of a large p-ndiode,and the electric field in this region sweeps thecarriers to the terminals thus generatingcurrent.
The magnitude of the current approximatelyproportional to the light incidence on thediode.
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Photo Diodes and LEDs
Light Emitting DiodesLEDsare pnjunctions fabricated from special
semiconductors materials, like galliumarsenide. They are useful because theyallow direct recombination of electronsand holes, thus releasing energy in theform of light.