ch6 semiconductors

7/21/2019 Ch6 Semiconductors

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BITSPilaniDubai Campus

Ch 6:

SEMICONDUCTORS


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Dr. G. AMARANATH, BITS Pilani, Dubai Campus

1. INTRODUCTION

2. N- TYPE AND P-TYPE

3. PN- JUNCTION

4. DIODE AND ITS CHARACTERISTICS

5. DIODE CIRCUITS

6. ENER DIODES

CONTENTS:


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D!"!#$%&!'( $) E#!*(r$'+*

1stGeneration: 2ndGeneration:

Vacuum diodes Semiconductor diodes & single transistor


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3rdGeneration: 4thGeneration:

Small and edium Scale !ntegrated Circuits "SS!# or chip $S!: 1 % 1' %

"less than 1'' , 1''1''' (ransistors per !ntegrated Circuit # V$S!: 1' % 1


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)thGeneration: *thGeneration:

+1 billion (ransistors

Can u name it

-ltra $arge Scale !ntegration: + 1


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. (he /lo0 o/ charge "current# results /rom the moement o/

electrons

. !n case o/ a metal the atom consists o/ outer most alence

electrons 0hich are /ree to moe and positiel charged

ions 0hich consist o/ nucleus and tightl bound electrons. ecause o/ thermal energ, the /ree electrons continuall

moe and collide 0ith the stationar positie ion and as a

result o/ this collision the electron changes its direction o/

motion. (he aerage distance bet0een collisions is called the ean

/ree path

CONDUCTORS


7/57Dr. G. AMARANATH, BITS Pilani, Dubai Campus

.When a voltage V is appliedto the metal, E results. E =V/d.The electrons then go from a

lower potential to higherpotential at a speed u calledthe drift speed, given byu = E,

where is the mobility ofelectronsonsider a metallic conductor with cross section area ! and " be the lehe conductor.# is the number of free electrons



. Suppose these electrons moe to the le/t 0ith a dri/t

speed u

. (he motion o/ 5 electrons to the le/t results in a current i

to the right

. $et ( be the time ta%en b an electron to trael adistance o/ $ m (hen u 6 $7(

( 6 $7u

. 8e de/ine current, i 6 597( 6 597"$7u# 6 59u7$

and current densit 6 i7; 6 59u7$;



. (he olume o/ the conductor is $;

. (hen the densit o/ /ree electrons n is

n 6 57$;

(hus 6 n9u

0here n9 is the charge densit in C7 m3

8 and 6 n9u

(hus 6 n9=> 6 ? > 0here

? 6 n9u is called conductiit o/ the metal

0ith units "@m#1

Aence ? is proportional to n 0hich is the densit o/ /ree

electrons



. (he reciprocal o/ conductiit is called resistiit B

/ 10

5o0 + / JA +'*! J / +0A

+ / EA / EA0 / A"0 "since E / " 0here

is the oltage across the conductor#

. (hus + / A"0 or b hms la0 + /"0R "0here

resistance o/ the conductor is R / 0 A / 0;

. n alue /or: conductor 6 1'2Eto 1'2F

insulator 6 1'

semiconductor 6 1' to 1'2F at 3''lectron densit 6 hole densit 6 1) H1'1)cm3

"e# in metals the conduction is unipolar

0here as in semiconductos, it is bipolar "due to electron

and hole currents#



8hen a oltage is applied across

a piece o/ Si, electron current

Aole current


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;t 3'' < , the intrinsic concentration o/ silicon is 1)H 1'1*

m3 (he /ree electron mobilit is '13 m27 Vs and the

hole mobilit is '')m27 Vs 8hat is the conductiit o/

Si

E7&%#! 6.2


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. (he conductiit o/ sc increases 0ith increase in tempsince more electron hole pairs are produced

. (o /urther increase the conductiit , a small amount o/

impurit is added to the sc (hese are %no0n as doped

sc. -suall a pentaalent " arsenic or phosphorus# or

trialent impurit " oron or Gallium# is added to the sc

such as silicon and germanium

. (here are t0o tpes o/ doped sc: 5tpe and P tpe

DOPED SEMICONDUCTORS


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N (9%! !&+*$'8*($r.

.Each impurity atom has +valence electrons, only fourof which are used forcovalent bonding.

.There is one ecess freeelectron for each impurityatom..?ince such an impurity

atom donates a freeelectron, it is called donorimpurity and is responsiblefor conductivity.

.The majority carriers


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P T9%! !&+*$'8*($r:

.Each impurity atom 6 here@oron3 has * valence electrons,hence each impurity atomproduces an ecess hole.

.This hole will accept a freeelectron and is hence called!cceptor impurity.

.0n A type sc, the majoritycharge carriers are holesand minority charge carriersare electrons.

.Both P and N typesemicondutors are


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-nder thermal e9uilibrium o/ a semiconductor,

8here n is the /ree electron concentration, p is the hole

concentration and niis the intrinsic concentration o/ the

semiconductor

$et 5D6 Donor atom concentration and 5;6 ;cceptor atom

concentration

;s the atom is electricall neutral, 5D I p 6 5; I n

Jor n tpe semiconductor 5;6 ' "since n tpe is donor #(hus 5D I p 6 n or 5D6 np

5o0 /or n tpe n ++ p hence 5D6n

or p 6 ni27n 6 ni27 5D

MASS ACTION A;:

2

innp =


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0n summary, for an n B type semiconductor

n C#8and

?imilarly for p B type semiconductor

p C#!and

! good approimation for conductivity of an ntype sc is 6nDDp3 ie = nn9and for p type sc

= pp9

D

i

N

np

2

=

A

i

N

nn

2

=


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; pn Kunction is a Kunction /ormed

b Koining ptpe and n tpe semi

conductor together in er close

contact

J'*(+$' the region 0here the p and

n tpe semiconductors meet ie,

the boundar bet0een the t0o sides

(he % 8$%!8 7'8 (h! ' 8$%!8 * 7r!

r!#7(+"!#9 *$'8*(+'< and the='*(+$' +

'$'-*$'8*(+'


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. (he nonconducting laer is called the depletionLone Aere the charge carriers in the doped n

tpe and p tpe attract each other in a process

called recombination

. manipulating the nonconducting laer , pn

Kunctions are commonl used as diodes

. Diodes allo0 /lo0 o/ electricit in $'! 8+r!*(+$'

onl

Th! J'*(+$' D+$8!:


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A %-' ='*(+$' 7'8 7 ='*(+$'

8+$8!: !r$ >+7

.A) type is to the left and contains ecess holes and n)typeis to the right containing ecess electrons as maoritycharge carriers..?ince there is a concentration gradient across theunction diFusion resultsB electrons diFuse to the left andholes diFuse to the right .Goles leave behind Bve ions andelectrons leave behind Hve ions

.The net eFect is a diFusion current to the right since

.


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. ;h!' (h! h$#! 7'8 !#!*(r$' &!!( +' (h! "+*+'+(9 $) (h! $)(h! ='*(+$' (h!9 *$&>+'! 7'8 7r! '!(r7#+?!8.

. S+'*! (h! r!+( + *7##!8 (h! 8!%#!(+$' r!


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. ccurs 0hen p side "anode# is connected the positie terminalo/ the batter and ntpe "cathode# is connected to the negatie

terminal

. 8ith a batter connected li%e this, the holes in the p side and

electrons in the n side are pushed to0ards the Kunction

. Aence the 0idth o/ the depletion region decreases

. (he depletion region is narro0 enough that electrons can cross

the Kunction and inKect into the p side"le/t# 0here the

recombine 0ith holes and similarl holes cross oer the Kunction

and inKect into the n side "right#

. 5et result is hole current and electron current are both to the

right

@$r7r8 B+7:


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R!"!r! B+7:

.The positive terminal of the battery is connected to thecathode 6n side3 and the negative terminal is connected

to the anode 6p side3.Gere the holes in the p type material drift to the left andare pulled away from the unction..The electrons in the n) type material drift to the right andare also pulled away from the unction..-esult is a widened depletion region.


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. ; potential o/ VI V is deeloped across the Kunction0here V is the batter potential and V is the barrier

potential

. !n n tpe, holes are minorit charge carriers and the dri/t

into the depletion region 0here the are pushed across b Eto the le/t

. !n p tpe , electrons are minorit charge carriers and their

dri/t is to the right

. N!( !))!*( + 7 &7## *rr!'( IS *7##!8 R!"!r! S7(r7(+$'*rr!'( $r 7(r7(+$' *rr!'( (hr$


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Th + h + + )


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Th! +-" *h7r7*(!r+(+* *r"! $)

7 8+$8!:.V is the voltage across thediode and i is the currentthrough it.

The relation between v and i is %

Iorwardbias

-everse bias

Where 0?is the reverse saturationcurrentVT= T/&&,+17 is the volt

e9uivalent of temp.

J = emission constant = & for $eK for ?i..0n the graph in I@ as V DD , i DD..0n -@ there is a small saturationcurrent due to minority charge

carriers and as V = )VLthere is a

E9n &( )1/ = TVvS eIi


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. !/ the diode is in J and V + '2 V, then at room temp to a goodapproHimation,

.0f the diode is -@and )VL OV O (. V, then to a good

appro i = )0?

.0f V = )VLthen diode breaMs down in -@ and a very largenegative current Nows.

.!hen i " #$% then &oltage across the diode is

constant then '= '(

Irom e9n &,

?o E9n valid fori D )0?

TVv

SeIi

/

D+ 8 B h +


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Jor a case 0hen diode is J "i +',+'# in Ge and Si

D+$8! B!h7"+$r:

I- *h7r7*(!r+(+*

.Ior a given voltage $e diode has a much larger currentthan ?i..Ior $e the current at O(. volts is very small. Pnly whenvoltage eceeds (. volts the current DDD. Ior ?i it is


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. (his oltage at 0hich the current starts increasing iscalled *(-+' "$#(7

. Jor Ge, V6 '2 and /or Si, V6')

. (he saturation current is depends on temperature

. !Sis a /unction o/ ( in M

. Melation bet0een !S and ( is :

0here !S "(a# is the saturation current at temperature (a

and !S "(b# is the saturation current at temperature (b

T ( 8 8 )


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. Jor a gien oltage , diodecurrent ++ as ( ++

. Solid cure is i characteristic o/

a diode at temp (a

and dashed cure is at (b 0here(b + (a

. !/ 62/or (a then i6i1and i 6i2 /or

(b + (a

. (o maintain current ! 6 i1 0hentemp ++ /rom (a to (bthen diode

oltage should be lo0ered /rom 2

to 1

T!&%!r7(r! 8!%!'87'*! $)

8+$8! I- *h7r7*(!r+(+*:


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!n mathematical terms:


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(he relation bet0een i and in a diode is :


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$raphical analysis of a diode circuit

.The load line and the i)vcharacteristic curve of thediode intersect at a pointQ.

.Q is called the 9uiescentoperating point or )#point.

.0Qand VQ are the current

and voltage

corresponding to the Q)


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; Si diode has a reerse saturation current o/ 1n; at 3''%Jind i0hen is ' olts

Soln:

DRI E: 6.5(($r+7#

mAi

x

ee

eIia

xx

Vv

ST

742.0

000742.0104486.741921

)1(10)1(10

)1()

9

52.1393002/586,117.09

/

=

=

=

=

Vvx

xxv

I

iVv

nAiwhenvFindkat

nAofcrrentsatrationreversehasdiodeSiAb

S

T

035.01101

105.0ln

586,11

3002

)1ln(

.5.0.300

1)

9

9

=

+=

+=

=


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Gien that the current through a !541)3 silicon diode is 1'm; at3''


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. !deal diode is a t0o terminal

deice 0hose i characteristics

are sho0n belo0:

THE IDEA DIODE:

6sc3 6oc3


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@$r 7' +8!7# 8+$8!:. 8hen current is in )$r7r8 8+r!*(+$' )$r7r8 >+7 the oltage

across the diode is Lero, 0hich means 8+$8! >!h7"! 7 7 h$r(

*+r*+(

. 8hen oltage across an ideal diode is '$'-%$+(+"! )$r r!"!r!

>+7, the current through the diode is Lero 0hich means 8+$8!

>!h7"! 7 7' $%!' *+r*+(

S(!% ($ >! )$##$!8 )$r $#"+'< %r$>#!&:

1 8hen diode is J 0e sa its ON.

2 8hen diode is M 0e sa its O@@3 ;/ter assuming diodes are ON$r O@@0e replace them b h$r(

*+r*+( $r $%!' *+r+( respectiel

4 8e then appl %no0n analsis techni9ues to the resulting linear

circuits


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RUES:1 !/ current through an 5 "J# diode is calculated to be

nonpositie, that means 0e assumed 0rong it should

hae been JJ

2 (hen change assumptions and repeat analsis3 Similarl i/ oltage across an JJ "M# diode is

calculated to be positie, then our assumption 0as

incorrect !t should hae been assumed 5 "J#

4 ;gain redo analsis) !/ all diodes assumed to be 5 are /ound to hae

positie currents and i/ all diodes assumed to be JJ

are /ound to hae nonpositie oltages then our

assumptions 0ere right


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D+$8! 7 7 h7#) 7"! r!*(+)+!r:

.*onsider the case o+ a &e input &oltage ie &%-

&.

.%ince current goes through a resistor +rom higherto lo/er potential let us assume that diode 0 is1N..Replace 0 2y a short circuit

.By 1hm3s la/ i= &% 4R


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.5o0 consider Vs Q '. ;ssume D is JJ

. Meplace D b open ciruit

. Since ! 6 ' , b


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D+$8! 7 7 *#+%%!r:


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D+$8! 7 7 *#+%%!r:

C#+%%!r *+r*+(

.There are two voltage sources.

.0f v?D ( then v?will forward bias the diode.

.The * volts source tends to reverse bias the diode 86since cathode connected to Hve of *volts source..We Rnd which voltage source dominates.

.!ssume 8 is P# 6ie 8 is sc3. Thecircuit is


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(hus b hms la0:

;nd b


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. (he output oltage 'is:

ppose if the input voltage was a sinusoid wave with an amplitude of 7volts theoutput voltage v(when plotted will be clipped as shown%

ch6 semiconductors

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