3 - semiconductors

7/24/2019 3 - Semiconductors

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SemiconductorsSemiconductors

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Three Types of MaterialsThree Types of Materials

CONDUCTOR

is any material that supports a generous flow of charge when a

voltage source of limited magnitude is applied across its terminal

less than 4 valence electrons in the outermost shell

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NSU!"TOR#D$!$CTRC

is any material that offers a very low level of conductivity under

pressure from an applied voltage source

material that resists flow of charge

more than 4 valence electrons in the outermost shell

resistivity %&' is up to ()(*+,cm

e-amples are paper. dry wood. plastic. ru//er. mica. etc0

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S$MCONDUCTOR

is a material that has aconductivity level somewhere

/etween the e-tremes of an

insulator and a conductor0

it acts as an insulator at

a/solute ) %) 1' and conducts atroom temperature

has 4 valence electrons in the

outermost shell

shows a reduction in

resistance with increase intemperature and is said to have

a negative temperature

coefficient


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Semiconductor MaterialsSemiconductor Materials

Common materials used in the development of semiconductor devices2

Silicon %Si'Silicon %Si' & 3 ) - ()& 3 ) - ()55+,cm+,cm is widely used in diodes. transistors and integrated circuitsis widely used in diodes. transistors and integrated circuits 6est 7uality silicon is o/tained /y growing crystals in a la/oratory6est 7uality silicon is o/tained /y growing crystals in a la/oratory

which is then fa/ricated intowhich is then fa/ricated into waferswafersoror chipschips

8ermanium %8e'8ermanium %8e'

& 3 ) +,cm& 3 ) +,cm 9ure germanium is a poor electrical conductor and /ecomes a9ure germanium is a poor electrical conductor and /ecomes a

semiconductor when impurities are addedsemiconductor when impurities are added :as used e-tensively in the early years /ut has /een shifted towards:as used e-tensively in the early years /ut has /een shifted towards

SiliconSilicon ;as a low voltage drop of )05


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Selenium %Se'Selenium %Se' has a resistance that varies depending on the intensity of light that fallshas a resistance that varies depending on the intensity of light that falls

on it. a property =nown ason it. a property =nown asphotoconductivityphotoconductivity is used in photocellsis used in photocells the main advantage of selenium over silicon is that it can withstandthe main advantage of selenium over silicon is that it can withstand

/rief/rief transientstransientsoror surgessurgesof a/normally high voltageof a/normally high voltage

8allium "rsenide %8a"s'8allium "rsenide %8a"s' in comparison with Silicon. it needs less voltage and functions at higherin comparison with Silicon. it needs less voltage and functions at higher

fre7uencies /ecause the charge carriers move fasterfre7uencies /ecause the charge carriers move faster are relatively immune to the effects of ioni>ing radiation such as ?raysare relatively immune to the effects of ioni>ing radiation such as ?rays

and gamma raysand gamma rays is used in light@emitting diodes %!$D'. infrared@emitting diodes. laseris used in light@emitting diodes %!$D'. infrared@emitting diodes. laser

diodes. visi/le light and infrared detectors. ultra@high@fre7uencydiodes. visi/le light and infrared detectors. ultra@high@fre7uency

amplifying devices and a variety of integrated circuits %C'amplifying devices and a variety of integrated circuits %C' is more e-pensive to produce than siliconis more e-pensive to produce than silicon

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Metal O-ideMetal O-ide Needs almost no power to function /ecause it draws so littleNeeds almost no power to function /ecause it draws so little

current that a /attery in a MOS %Metal O-ide Semiconductor'current that a /attery in a MOS %Metal O-ide Semiconductor'

device lasts longdevice lasts long

;as high speed which can /e used in operation at high;as high speed which can /e used in operation at highfre7uencies and ma=es it possi/le to perform calculations perfre7uencies and ma=es it possi/le to perform calculations per

secondsecond "re easily damaged /y static electricity"re easily damaged /y static electricity

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$nergy !evels$nergy !evels

The more distant the electrons from the nucleus. the higher the energy state and

any electron that has left its parent atom has a higher energy state than any

electron in the atomic structure0

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oni>ation is the mechanism where/y an electron can a/sor/ sufficient energy

to /rea= away from the atomic structure and enter the conduction /and

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$nergy 8ap %Ug or $g' or Aor/idden 6and is the energy difference /etween theconduction /and and valence /and0 t is the energy re7uired to move or transfer a

valence electron at the valence /and to the conduction /and


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(0 ;ow much energy in Boules is re7uired to move a charge of C through adifference in potential of *


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Types of "tomic 6ondingTypes of "tomic 6onding

onic or $lectrovalent or $lectrostatic 6ondingonic or $lectrovalent or $lectrostatic 6onding Refers to the /onding resulting from the attractive forces of oppositelyRefers to the /onding resulting from the attractive forces of oppositely

charged ions %positive or negative'charged ions %positive or negative'

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Ionic compounds resu! "#en me!$s re$c! "i!# nonme!$s

%e!$s loseeec!rons !o m$!c# !#e number of valence electrons

o& !#eir ne$res! no'e ($s

Positive ions &ormwhen !#e num'er o& eec!rons $re ess !#$n

!#e num'er o& pro!ons

)roup 1 me!$s ion 1*

)roup 2 me!$s ion 2*

In ionic compounds+ nonme!$s in 15+ 16+ $nd 17 ($in eec!rons

&rom me!$s

,onme!$ $dd eec!rons !o $c#iee !#e oc!e! $rr$n(emen!


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Ionic Bond

Between atoms of metals and nonmetals

with very different electronegativity

Bond formed by transfer of electrons

Produce charged ions all states.

Conductors and have high melting point.

Examples !aCl" CaCl#" $#%


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Ionic onds/ ne i( )reed #ie& o(


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1). Ionic bond& electron from !a is transferred to Cl" this

causes a charge imbalance in each atom. 'he !a becomes

(!a)*and the Cl becomes(Cl+*" charged particles or ions.


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Metallic 6ondingMetallic 6onding

Refers to the type of /onding that is a product of theRefers to the type of /onding that is a product of theattractive forces of group of positive ions andattractive forces of group of positive ions and

electrons that are generally free to move a/outelectrons that are generally free to move a/out

among its ionsamong its ions

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,ormed between atoms of metallic elements

Electron cloud around atoms

-ood conductors at all states" lustrous" very high

melting points

Examples !a" ,e" l" u" Co


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%e!$ic onds/ %eo" do(s "i!#

pen! o& 'ones !o (o $round.


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%e!$ic ond+ e$ o& ec!rons


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/etals ,orm lloys

/etals do not combine with metals. 'hey form

lloys which is a solution of a metal in a metal.

Examples are steel" brass" bron0e and pewter.


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Covalent 6ondingCovalent 6onding

s when atoms of materials share electrons with another atoms0s when atoms of materials share electrons with another atoms0The shared electrons are attracted simultaneously to the atomsThe shared electrons are attracted simultaneously to the atoms

resulting to a force that /inds them togetherresulting to a force that /inds them together

2020

Between nonmetallic elements of similar

electronegativity.

,ormed by sharing electron pairs

1table non+ioni0ing particles" they are not

conductors at any state

Examples %#

" C%#

" C#

23

" 2#

%" 1iC


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"#en eec!rons$re s#$red equally

,,:

;V, ,


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2. Covalent bonds- 'wo atoms share one or more pairs of outer+shell

electrons.

Oxygen AtomOxygen Atom Oxygen AtomOxygen Atom

Oxygen Molecule (OOxygen Molecule (O22))

;
http://www.usfca.edu/fac-staff/Courses/BIOL104_USF/104_Fall03_ppt/Text%20Chapter%2002/OxgnMol.swfhttp://www.usfca.edu/fac-staff/Courses/BIOL104_USF/104_Fall03_ppt/Text%20Chapter%2002/OxgnMol.swfhttp://www.usfca.edu/fac-staff/Courses/BIOL104_USF/104_Fall03_ppt/Text%20Chapter%2002/OxgnMol.swfhttp://www.usfca.edu/fac-staff/Courses/BIOL104_USF/104_Fall03_ppt/Text%20Chapter%2002/OxgnMol.swfhttp://www.usfca.edu/fac-staff/Courses/BIOL104_USF/104_Fall03_ppt/Text%20Chapter%2002/OxgnMol.swfhttp://www.usfca.edu/fac-staff/Courses/BIOL104_USF/104_Fall03_ppt/Text%20Chapter%2002/OxgnMol.swfhttp://www.usfca.edu/fac-staff/Courses/BIOL104_USF/104_Fall03_ppt/Text%20Chapter%2002/OxgnMol.swfhttp://www.usfca.edu/fac-staff/Courses/BIOL104_USF/104_Fall03_ppt/Text%20Chapter%2002/OxgnMol.swfhttp://www.usfca.edu/fac-staff/Courses/BIOL104_USF/104_Fall03_ppt/Text%20Chapter%2002/OxgnMol.swf


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"#en eec!rons$re s#$red 'u!

s#$red unequally

: ;V,

,


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o$r ;o$en! onds/ neen

m$!c#ed+ 'u! "iin( !o s#$re.


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+ water is apolarmoleculebecause oxygen is more electronegative than

hydrogen" and therefore electrons are pulled closer to oxygen.


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Covalent 6onding of SemiconductorsCovalent 6onding of Semiconductors

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Covalent Bonding of 1ilicon Crystal Covalent Bonding of -as Crystal


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Type of Semiconductor MaterialsType of Semiconductor Materials

NTRNSC M"T$R"!

" semiconductor that has /een carefully refined to reduce the impurities toa very low level H essentially as pure as can /e made availa/le through

modern technology

Relative Mo/ility is the a/ility of the free carriers to move throughout the

material

Note2 an increase in temperature of a semiconductor can result in a

su/stantially increase in the num/er of free electrons in the material

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emiconduc!or In!rinsic ;$rriers>per cu'ic cen!ime!er? :e$!ie %o'ii! @$c!or >An?>cm2BVCs?

-as 4.5 x 463 4766

1i 4.7 x 4646 8966

-e #.7 x 4648 766

$?TRNSC M"T$R"!" semiconductor material that has /een su/Iected to the doping process


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Boylestad and !ashels;y

Electronic Devices and Circuit TheoryCopyright


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n-typen-typeMaterialMaterial

is created /y introducing impurity elements that have five valence electrons

%pentavalent' such as antimony. arsenic. phosphorus and /ismuth

3030n!imon impuri! in n-!pe m$!eri$

,o!e/'he remaining

unassociated electron"

loosely bound to its

parent atom" isrelatively free to move

within the newly

formed n+type

material.

onor $!oms

diffused impurities

with five valence

electrons.


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p-typep-typeMaterialMaterial

is created /y introducing impurity elements that have three valence

electrons %trivalent' such as /oron. gallium. indium and aluminum

3131

,o!e/'he resultingvacancy will readily

accept a Afree

electron.

ccep!or $!oms

diffused impurities

with three valence

electrons.

oron impuri! in p-!pe m$!eri$


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$lectron vs ;ole Alow$lectron vs ;ole Alow

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Two sets of currents flow in a diode2

Majority and Minority CarriersMajority and Minority Carriers

MaIority CarriersMaIority Carriers

The maIority carriers in n@type materials are electrons0 The maIority carriers inp@type materials are holes0

Minority CarriersMinority Carriers

The minority carriers in n@type materials are holes0 The minority carriers inp@type materials are

electrons0

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n-type and p-typen-type and p-typeMaterialMaterial

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