EE3950 Class Notes Chapter 10 Hambley 1-21 (1)

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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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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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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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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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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Typical electronicsystemrequirements:

Digital electronics:

5 volts dc;

Analog electronicsrequires twosupplies:

+15 volts dc;

-15 volts dc.


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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.

EE3950 Class Notes Chapter 10 Hambley 1-21 (1)

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