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7/31/2019 Semicounductors & Pn-junction (Complete)
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7/12/12
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