Electronics Lect2

7/31/2019 Electronics Lect2

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Conductors

Conductors have loosely bound electrons in their outer shellThese electrons require a small amount of energy to free themfor conduction

Lets apply a potential difference across the conductor above

The force on each electron is enough to free it from its orbitand it can jump from atom to atom the conductor conducts

Conductors are said to have a low resistivity / resistance


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Conductors

Good conductors have low resistance soelectrons flow through them with ease.

Best element conductors include: Copper, silver, gold, aluminum, & nickel

Alloys are also good conductors: Brass & steel

Good conductors can also be liquid: Salt water


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Conductor Atomic Structure The atomic structure of

good conductors usuallyincludes only one electronin their outer shell.

It is called a valence electron. It is easily striped from the

atom, producing currentflow.

CopperAtom


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The Silicon, Si, AtomSilicon has a valency of 4 i.e. 4electrons in its outer shell

Each silicon atom shares its 4outer electrons with 4neighbouring atoms

These shared electrons bonds are shown ashorizontal and vertical linesbetween the atoms

This picture shows theshared electrons


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Silicon the crystal latticeIf we extend thisarrangement throughout apiece of silicon

We have the crystal lattice of silicon

This is how silicon lookswhen it is cold

It has no free electrons it cannot conduct electricity therefore it behaves like aninsulator


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Electron Movement in SiliconHowever, if we apply a littleheat to the silicon.

An electron may gainenough energy to breakfree of its bond

It is then available forconduction and is free to

travel throughout thematerial


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Hole Movement in SiliconLets take a closer look atwhat the electron has leftbehind

There is a gap in the bond what we call a hole


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Hole Movement in SiliconThis hole can also move

An electron in a nearbybond may jump into thishole

Effectively causing the holeto move

Like this


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Semiconductor Valence Orbit

The main characteristicof a semiconductorelement is that it hasfour electrons in itsouter or valence orbit.


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Crystal Lattice Structure The unique capability of

semiconductor atoms istheir ability to linktogether to form aphysical structure called

a crystal lattice. The atoms link together

with one another sharingtheir outer electrons.

These links are calledcovalent bonds. 2D Crystal Lattice

Structure


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Semiconductors

An intrinsic semiconductor, also called anundoped semiconductor or i-typesemiconductor, is a pure semiconductorwithout any significant dopant species present.

An extrinsic semiconductor is asemiconductor that has been doped .


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Doping Relying on heat or light for conduction does not

make for reliable electronics To make the semiconductor conduct electricity,

other atoms called impurities must be added. Impurities are different elements. This process is called doping .


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Semiconductors can be Conductors

An impurity, or element

like arsenic, has 5 valenceelectrons. Adding arsenic (doping)

will allow four of thearsenic valence electronsto bond with theneighboring silicon atoms.

The one electron left overfor each arsenic atombecomes available toconduct current flow.


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The Phosphorus AtomPhosphorus is number 15 in the periodic table

It has 15 protons and 15electrons 5 of theseelectrons are in its outershell


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Doping Making n-type Silicon

Suppose we remove asilicon atom from thecrystal lattice

and replace it with a

phosphorus atom

We now have an electron that is not bonded it is thus free for conduction


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Doping Making n-type SiliconLets remove another silicon atom

and replace it with aphosphorus atom

As more electrons areavailable for conduction wehave increased theconductivity of the material

If we now apply a potential difference across thesilicon

Phosphorus is called thedopant


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Extrinsic Conduction n-type Silicon

A current willflow

Note:The negativeelectrons move

towards thepositiveterminal


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From now onn-type will beshown likethis.

N-type Silicon

This type of silicon is called n-type

This is because the majority charge carriers arenegative electronsA small number of minority charge carriers holes will exist due to electrons-hole pairs being created inthe silicon atoms due to heat The silicon is still electrically neutral as the number of protons is equal to the number of electrons


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The Boron AtomBoron is number 5 in theperiodic table

It has 5 protons and 5electrons 3 of theseelectrons are in its outershell


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Doping Making p-type SiliconAs before, we remove asilicon atom from thecrystal lattice

This time we replace it witha boron atom

Notice we have a hole in a bond this hole is thus free for conduction


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P-type Silicon

This type of silicon is called p-type

This is because the majority charge carriers are positive holesA small number of minority charge carriers electrons will exist due to electrons-hole pairs being created in thesilicon atoms due to heat The silicon is still electrically neutral as the number of protons is equal to the number of electrons

From now onp-type will beshown likethis.


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The p-n Junction

Suppose we join a piece of p-type silicon to a pieceof n-type silicon

We get what is called a p-n junction

Remember both pieces are electrically neutral


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The p-n JunctionWhen initially joined electronsfrom the n-type migrate intothe p-type less electrondensity there

When an electron fills ahole both the electronand hole disappear as thegap in the bond is filled

This leaves a region with no free charge carriers the depletion layer this layer acts asan insulator


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The p-n JunctionAs the p-type has gained electrons it is left with anoverall negative charge

As the n-type has lost electrons it is left with anoverall positive charge

Therefore there is a voltage across the junction the junction voltage for silicon this isapproximately 0.6 V

0.6 V


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The Reverse Biased P-N JunctionTake a p-n junction

Apply a voltage acrossit with the

p-type negative

n-type positive

Close the switch

The voltage sets up anelectric field throughout the

junction The junction is said to be reverse biased


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The Reverse Biased P-N JunctionNegative electrons in then-type feel an attractiveforce which pulls themaway from the depletionlayer

Positive holes in the p-type also experience anattractive force which

pulls them away from thedepletion layer

Thus, the depletion layer ( INSULATOR) is widened and nocurrent flows through thep-n junction


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The Forward Biased P-N JunctionTake a p-n junction

Apply a voltage acrossit with the

p-type postitive

n-type negative

Close the switch

The voltage sets up anelectric field throughout the

junction The junction is said to be forward

biased


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The Forward Biased P-N JunctionNegative electrons in then-type feel a repulsiveforce which pushes theminto the depletion layer

Positive holes in the p-type also experience arepulsive force whichpushes them into thedepletion layer

Therefore, the depletion layer is eliminated and a currentflows through the p-n junction


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The Forward Biased P-N JunctionAt the junction electronsfill holes

They are replenished bythe external cell and

current flows

Both disappear as theyare no longer free for

conduction

This continues as long as the external voltage is greater than the junction voltage i.e. 0.6 V


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The Forward Biased P-N JunctionIf we apply a higher voltage

The electrons feel agreater force and move

faster

The current will begreater and will looklike

The p-n junction is called a DIODEand isrepresented by the symbol

The arrow shows thedirection in which itconducts current

this.

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Electronics Lect2