semicounductors & pn-junction (complete)

7/31/2019 Semicounductors & Pn-junction (Complete)

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Presented By: Mr. MUHAMMAD ABBASE-mail: [email protected]

BASIC

ELECTRONICS

Superior University,


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SEMICONDUCTORS,EXTERINSICSEMICONDUCTORS,

PN JUNCTIONPresented By: Mr. MUHAMMAD ABBAS


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(A)SEMICONDUCTORS

CORE OF AN ATOMCOMPARISON OF A SEMICONDUCTOR AND CONDUCTOR

ATOM

SILICON AND GERMANIUM

COVALENT BONDS

CONDUCTION IN SEMICONDUCTORS

ELECTRON AND HOLE CURRENT

N-TYPE AND P-TYPE SEMICONDUCTORS


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CORE OF AN

ATOMIn order to discuss electrical properties, an atom can be

represented by the valence shell and a CORE thatconsists of all the inner shells and nucleus.

Carbon atom has 4 electrons inthe valence shell and 2electrons in the inner shell.

The core has a net charge of +4 ( +6 for the nucleus and - 2for the two inner-shell electrons).

The nucleus consists of6

protons and 6 neutrons so +6presents the positive chargeof the six protons.

The core iseverythingexceptthe valence electrons.


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COMPARISON OF A

SEMICONDUCTOR

AND CONDUCTOR ATOM

The core of the silicon atom has anet charge of + 4 (14 protons - 10electrons).

One valence electron of Si-atom

feels an attractive force of +4where as one valence electron ofCopper-atom feels an attractiveforce of +1.

The core of the copper atom has anetcharge of + 1 (29 protons - 28electrons).

There is four times more forcetrying to hold a valence electron to

the atom in Silicon than in Copper-atom

Valence electron in Cu has moreenergy than valence electron in Siindicating that it is easier for Cu-valence electron to take part inconduction after obtaining little


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SILICON & GERMANIUM

Silicon is the most widely used material in diodes,

transistors, integrated circuits, and other semiconductordevices.Both silicon and germanium have the characteristic fourvalence electrons.


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The valence electrons in germanium are in the

fourth shell while those in silicon are in the third

shell, closer to the nucleus. This means that the

germanium valence electrons are at higher

energy levels than those in silicon and, therefore.

require a smaller amount of additional energy to

escape from the atom.

This property makes germanium more unstable

at high temperatures, and this is a basic reason

why silicon is the most widely used semi-

conductive material.

Why using Silicon instead ofGermanium?


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COVALENT BONDS

Covalent bonds are formed bysharing of valence electrons of the

atoms.Atoms in the crystal structure are held together by covalentbonds which are created by the interaction of valenceelectrons of atoms.

Each silicon atom positions itself with four adjacent siliconatoms to form a silicon crystal. A silicon (Si) atom with its

four valence electrons shares an electron with each of its fournei hbors.


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An intrinsic crystal is one that has noimpurities.

Covalent bonding in an intrinsic siliconcrystal


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At 0 K there are noelectrons in the

conduction band.Energy band diagram foran unexcited atom in apure (intrinsic) silicon

crystal.

CONDUCTION IN

SEMICONDUCTORS


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An intrinsic (pure) silicon crystal at room temperature hassufficient heat (thermal) energy for some valence electronsto jump the gap from the valence band into the conductionband, becoming free electrons.Free electrons are also called conduction electrons.

When an electron jumps to the conduction band, a vacancy isleft in the valence band within the crystal. This vacancy iscalled a hole.For every electron raised to the conduction band by external

energy, there is one hole left in the valence band, creatingwhat is called an electron-hole pair. Recombination occurs


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A piece of intrinsic silicon at room temperature has, at anyinstant, a number of conduction-band (free) electrons thatare unattached to any atom and are essentially drifting

randomly throughout the material. There is also an equalnumber of holes in the valence band created when theseelectrons jump into the conduction band.

SUMMARY


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Electron and HoleCurrent

When a voltage is applied across a piece of intrinsic silicon,

the thermally generated free electrons in the conduction

band, which are free to move randomly in the crystal

structure, are now attracted towards +ve end.

This movement of free electrons is called as Electron

Current.

A h f i h l b d h


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However, a valence electron can move into a nearby holewith little change in its energy level, thus leaving anotherhole where it came from.

Another type of current occurs in the valence band, wherethe holes created by the free electrons exist.

Electrons remaining in the valence band are still attached totheir atoms and are not free to move randomly in the crystalstructure as are the free electrons.

Effectively the hole has moved from one place to another inthe crystal structure. This is called hole current.


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N-type and P-typeSemiconductorsBasicReason:qSemiconductive materials do not conduct currentwell and are of limited value in their intrinsic state.qThis is because ofthe limited number of freeelectrons in the conduction band and holes in the

valence band.q Intrinsic silicon (or germanium) must be modifiedby increasing the number of free electrons or holesto increase its conductivity and make it useful in

electronic devices.This is done by adding impurities to the intrinsicmaterial.qTwo types ofextrinsic (impure) semiconductivematerials, n-type and p-type, are the key building

blocks for most types of electronic devices.


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SEMICONDUCTORS

IntrinsicSemiconductor

s

Extrinsicsemiconductors

P-typeSemiconducto

rs

N-typeSemiconducto

rs

The conductivity of silicon andgermanium can be significantlyincreased by the controlled

addition of impurities to theintrinsic (pure) semi-conductivematerial.This process, called doping,increases the number ofcurrent carriers (electrons orholes).


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N-typeSemiconductorsTo increase the number of conduction-band electrons in intrinsic silicon,pentavalent

impurity atoms are added. These are atoms with five valence electrons suchas arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb).Each pentavalent atom(antimony, in this case)forms covalent bonds withfour adjacent silicon atoms.

Four of the antimony atom'svalence electrons are usedto form the covalent bondswith silicon atoms, leavingone extra electron.

This extra electron becomesa conduction electronbecauseit is not attached to anyatom.Because the pentavalent

atom gives up an electron, itis often called a donor atom.


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P-typeSemiconductorsTo increase the number of holes in intrinsic silicon, trivalentimpurity atoms are added. These are atoms with three valence

electrons such as aluminum (Al), boron (B), indium (In), and gallium(Ga).

Because the trivalent atom can take an electron, it is often referredto as an acceptor atom. The number of holes can be carefullycontrolled by the number of trivalent impurity atoms added to the

silicon. A hole created by this doping process is not accompanied bya conduction (free) electron.

Each trivalent atom (boron, in

this case) forms covalent bonds

with four adjacent silicon

atoms. All three of the boron

atom's valence electrons are

used in the covalent bonds;

and, since four electrons are

required, a hole results when

each trivalent atom is added.


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The electrons are the majority carriers in n-type material.

Although the majority of current carriers in n-type materialare electrons, there are also a few holes that are createdwhen electron-hole pairs are thermally generated. Theseholes are not produced by the addition of the pentavalentimpurity atoms.

Holes in an n-type material are called minority carriers.

Majority and Minority Carriers inN-Type

Majority and Minority Carriers inP-TypeHoles can be thought of as positive charges because theabsence of an electron leaves a net positive charge on theatom. The holes are the majority carriers in p-type material.Although the majority of current carriers in p-type materialare holes, there are also a few free electrons that are createdwhen electron-hole pairs are thermally generated. These freeelectrons are not produced by the addition of the trivalentimpurity atoms.

Electrons in p-type material are the minority carriers.


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(B) THE PN JUNCTION (DIODE)

FORMATION OF THE DEPLETION REGIONBIASING THE PN JUNCTION

FORWARD BIASING & REVERSE BIASING

CURRENT-VOLTAGE CHARACTERISTIC OF PN-JUNCTIONI-V CHARACTERISTIC FOR FORWARD BIASING

I-V CHARACTERISTIC FOR REVERSE BIASING

TEMPERATURE EFFECTS ON I-V CHARACTERISTIC


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The

DiodeIf a piece of intrinsic silicon is doped so that part is n-typeand the other part is p-type, a PN-junction forms at the

boundary between the two regions and a diode is created. Adiode is a device that conducts current in only one direction.

The n region has many free electrons (majority carriers) fromthe impurity atoms and only a few thermally generated holes(minority carriers).

The p region has manyholes (majority carriers)from the impurity atomsand only a few thermallygenerated free electrons(minority carriers).


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Diffusion

processThe free electrons in the n region are randomly drifting in all

directions.At the instant of the PN-junction formation, free electrons near

the junction in the n region begin to diffuse across the junction

into the p region where they combine with holes near the junction,

as shown in figure.


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These two layers of positive andnegative charges form the

depletion region, as shown infigure.

Formation of Depletion

RegionWhen the pn junction is formed, the n- region loses freeelectrons as they diffuse across the junction. This creates

a layer of positive charges (pentavalent ions) near thejunction.

The term depletion refers to the fact that the region near the pnjunction is depleted of charge carriers (electrons and holes) dueto diffusion across the junction. Keep in mind that the depletion

region is formed very quickly and is very thin compared to the nregion and p region.

As the electrons move acrossthe junction. the p regionloses holes as the electronsand holes combine. This

creates a layer of negativecharges (trivalent ions) nearthe junction.

Barrier


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Barrier

Potential

The barrier potential of a pn-junction depends on several factors:1. Type ofsemi-conductive material.2. the amount ofdoping;3. Temperature: typical barrier potential is approximately 0.7 V for Si &0.3 V for

Ge at 25C.

In the depletion region there are many positive charges and many negativecharges on opposite sides of the pn-junction, The forces between the oppositecharges form a "field of forces" called an electric field. This electric field is a

barrier to the free electrons in the n-region, and energy must be expended tomove an electron through the electric field, i.e., external energy must beapplied to get the electrons to move across the barrier of the electric field inthe depletion region.

The potential difference of the electricfield across the depletion region is the

amount of voltage required to moveelectrons through the electric field. Thispotential difference is called the barrierpotential and is expressed in volts.Stated another way, a certain amount ofvoltage equal to the barrier potential andwith the proper polarity must be appliedacross a pn-junction before electrons willbegin to flow across the junction.


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Energy Diagrams of PN-Junction and Depletion

RegionThe valence and conduction bands in an n-type material areat slightly lower energy levels than the valence and

conduction bands in a p-type material. This is due todifferences in the atomic characteristics of the penta-valentand the trivalent impurity atoms.

After crossing the junction, the electrons quickly lose energyand fall into the holes in the p-region valence band.

Energy Diagrams of PN Junction and Depletion


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Soon, there are no electrons left in the n-region conductionband with enough energy to get across the junction to the p-region conduction band, as shown by the placement of the

top of the n-region conduction band and the bottom of the p-region conduction band.

As the energy level of the n-region conduction band hasshifted downward, the energylevel of the valence band hasalso shifted downward.

At equilibrium; the depletionregion is complete becausediffusion has ceased. There isan energy gradient across thedepletion region which actsas an "energy hill" that ann-region electron must climbto get to the p region.

Energy Diagrams of PN-Junction and Depletion

Region

It still takes the same amount of energy for a valence

electron to become a free electron. So, Eg b/w V.B and C.B.remains the same.

Forward


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Forward

BiasingTo bias a diode, you apply a dc voltage across it.Forward bias is the condition that allows current through the

pn junction.

Negative side of VBlAS is connected to the n region of thediode.Positive side is connected to the p region.

V BlAS must be greater than the barrier potential.

Like charges repel, the negative side of the bias-voltage source


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Like charges repel, the negative side of the bias voltage source"pushes free electrons, (majority carriers in the n region) toward thepn junction. This flow of free electrons is called electron current. Thenegative side of the source also provides a continuous flow ofelectrons through the external connection (conductor) and into the nregion as shown.

The holes in the p region provide the medium or "pathway" for thesevalence electrons to move through the p region. The electrons movefrom one hole to the next toward the left. The holes, which are themajority carriers in the p region, effectively (not actually) move to theright toward the junction. This effective flow of holes is called thehole current. Hole current as being created by the flow of valenceelectrons through the p region, with the holes providing the only

When electrons are inthe valence band in thep region due to loss oftoo much energyovercoming the barrierpotential to remain in

the conduction band.Since unlike chargesattract, the positiveside of the bias-voltagesource attracts thevalence electrons

toward the left end ofthe p region.

Effect of Forward Biasing on Depletion Region and Barrier


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Effect of Forward Biasing on Depletion Region and Barrier

PotentialThe energy that the electrons require in order to passthrough the depletion region is equal to the barrier potentialmeans that electrons give up an amount of energy

equivalent to the barrier potential when they cross thedepletion region. This energy loss results in a voltage dropacross the pn junction equal to the barrier potential (0.7 V).

An additional small voltage

drop occurs across the p and nregions due to the internalresistance of the material.For doped semi-conductivematerial, this resistance, calledthe dynamic resistance, is very

small and can usually beneglected.

Forward bias narrows the depletion region and produces a

voltage


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REVERSE BIAS

Its the condition that

essentially prevents current

through the diode.

Depletion region is much wider than in forward bias or

equilibrium.

Positive side of VBIAS is

connected

to the n region of the diode

and the negative side is

connected to the

p region.

Reverse Current


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Reverse Current

The extremely small current that exists in reverse bias afterthe transition current dies out is caused by the minoritycarriers in the n and p regions that are produced by thermally

generated electron-hole pairs.

However, if theexternal reverse-

bias voltage isincreased to avalue calledbreakdown voltage,the reverse currentwill drasticallyincrease.

Break DownVoltage

Avalanche

Normally, the reverse current is so small that it can beneglected, but if

Avalanche is the rapid multiplication of current carriers inreverse breakdown. It is a very high reverse current that candamage the diode because of excessive heat dissipation.

VI CHARACTERISTICS


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VI-CHARACTERISTICS

With 0 V across the diode, there is no forward current. With the gradualincrease in theforward-bias voltage, the forward current and the voltage across the diode

gradually increase, as shown in Figure (a). A portion of the forward-bias voltageis dropped across the limiting resistor. When the forward-bias voltage isincreased to a value where the voltage across the diode reaches approximately0.7 V (barrier potential), the forward current begins to increase rapidly. asillustrated in Figure (b).

As you continue to increase the forward-bias voltage, the current continues toincreasevery rapidly, but the voltage across the diode increases only gradually above

0.7 V. Thissmall increase in the diode voltage above the barrier potential is due to the

VI-Characteristics for Forward


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VI-Characteristics for Forward

Bias

& Dynamic Resistance

Figure (c) Figure (d)


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Graphing the V-I Curve

If you plot the results of the type of measurements shown in Figure (a) and (b)on a graph, you get the V-I characteristic curve for a forward-biased diode,

as shown in Figure (c). The diode forward voltage (VF ) increases to the rightalong the horizontal axis, and the forward current (IF) increases upward alongthe vertical axis.As you can see in Figure (c), the forward current increases very little until thefor-ward voltage across the p n junction reaches approximately 0.7 V at the kneeof the curve. After this point. the forward voltage remains at approximately0.7 V, but IF increases rapidly. As previously mentioned, there is a slightincrease in VF above 0.7 V as the current increases due mainly to the voltagedrop across the dynamic resistance. Normal operation for a forward-biaseddiode is above the knee of the curve.IF scale is typically in m A.

Three points A, B, and C are shown on the curve in Figure (c). Point Acorrespondsto a zero-bias condition. Point B corresponds to: where the forward voltage isless than the barrier potential of 0.7 V. Point C corresponds to : where theforward voltage approximately equals the barrier potential. As the externalbias voltage and forward current continue to increase above the knee, theforward voltage will increase slightly above 0.7 V. In reality, the forward


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Dynamic ResistanceFigure (d) is an expanded view of the V-I characteristic curve in figure(c) and explains dynamic resistance. Unlike a linear resistance, theresistance of the forward-biased diode is not constant over the entirecurve. Because the resistance changes as you move along the V-Icurve, it is called dynamic or ac resistance. Internal resistances ofelectronic devices are usually designated by lowercase italic l' with aprime, instead ofthe standard R. The dynamic resistance of a diode is designated rd`.

Below the knee of the curve the resistance is greatest because thecurrent increases very little for a given change in voltage

(rd`= VF/ IF ).

The resistance begins to decrease in the region of the knee of thecurve and becomes smallest above the knee where there is a largechange in current for a given change in voltage.

VI-Characteristics for Reverse


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VI Characteristics for Reverse

BiasWhen a reverse-bias voltage is applied across a diode, there is onlyan extremely small reverse current (IR) through the pn junction. With0 V across the diode. there is no reverse current. As you gradually

increase the reverse-bias voltage, there is a very small reversecurrent and the voltage across the diode increases. When the appliedbias voltage is increased to a value where the reverse voltage acrossthe diode (VR ) reaches the breakdown value (VBR ).

The reverse current begins to increase rapidly.As you continue to increase the bias

voltage, the current continues toincrease very rapidly. But the voltageacross the diode increases very littleabove VBR . Breakdown, with exceptions,is not a normal mode of operation formost pn junction diodes.After this point, the reverse voltageremains at approximately VBR , but IRincreases very rapidly, resulting inoverheating and possible damage.

The breakdown voltage for a typical silicon diode can vary, but a

minimum value of 50 V is not unusual.


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The complete VI characteristicCurve


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(C) THE DIODE

DIODE STRUCTURE & SYMBOL

FORWARD BIASING & REVERSE BIASING OF A

DIODE

THE IDEAL DIODE MODEL

THE PRACTICAL DIODE MODEL

THE COMPLEX DIODE MODEL

TESTING A DIODE

Symbol and Biasing of


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Symbol and Biasing of

Diode

Effect of Temperature on VI Characteristics


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Effect of Temperature on VI-Characteristics


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Ideal Diode

Model

VF = 0V

IF = (VBIAS ) /( RLIMIT )

IR = 0AVR =VBIAS


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Practical Diode

Model

VF = 0.7V

IF = (VBIAS - VF) / (RLIMIT )

IR = 0AVR =VBIAS


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Complex Diode

Model

VF = 0.7 V +IF r'd

IF = (VBIAS - 0.7V) /( RLIMIT + r'd)


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Typicaldiodes


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DiodeChecking


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1. Electronic Devices by Floyd.2. Basic Electronics by B.L. Theraja.3. www.google.com.4. Wikipedia.org.

Reference

semicounductors & pn-junction (complete)

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