Ch6 Semiconductors
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Transcript of 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
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.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
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. 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$;
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. (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
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. (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#
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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
@$r7r8 B+7:
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R!"!r! B+7:
.The positive terminal of the battery is connected to thecathode 6n side3 and the negative terminal is connected
to the anode 6p side3.Gere the holes in the p type material drift to the left andare pulled away from the unction..The electrons in the n) type material drift to the right andare also pulled away from the unction..-esult is a widened depletion region.
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Dr. G. AMARANATH, BITS Pilani, Dubai Campus
. ; potential o/ VI V is deeloped across the Kunction0here V is the batter potential and V is the barrier
potential
. !n n tpe, holes are minorit charge carriers and the dri/t
into the depletion region 0here the are pushed across b Eto the le/t
. !n p tpe , electrons are minorit charge carriers and their
dri/t is to the right
. N!( !))!*( + 7 &7## *rr!'( IS *7##!8 R!"!r! S7(r7(+$'*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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Dr. G. AMARANATH, BITS Pilani, Dubai Campus
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 $e the current at O(. volts is very small. Pnly whenvoltage eceeds (. volts the current DDD. Ior ?i it is
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Dr. G. AMARANATH, BITS Pilani, Dubai Campus
. (his oltage at 0hich the current starts increasing iscalled *(-+' "$#(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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Dr. G. AMARANATH, BITS Pilani, Dubai Campus
. 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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Dr. G. AMARANATH, BITS Pilani, Dubai Campus
; Si diode has a reerse saturation current o/ 1n; at 3''%Jind i0hen is ' olts
Soln:
DRI E: 6.5(($r+7#
mAi
x
ee
eIia
xx
Vv
ST
742.0
000742.0104486.741921
)1(10)1(10
)1()
9
52.1393002/586,117.09
/
=
=
=
=
Vvx
xxv
I
iVv
nAiwhenvFindkat
nAofcrrentsatrationreversehasdiodeSiAb
S
T
035.01101
105.0ln
586,11
3002
)1ln(
.5.0.300
1)
9
9
=
+=
+=
=
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Dr. G. AMARANATH, BITS Pilani, Dubai Campus
Gien that the current through a !541)3 silicon diode is 1'm; at3''
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Dr. G. AMARANATH, BITS Pilani, Dubai Campus
. !deal diode is a t0o terminal
deice 0hose i characteristics
are sho0n belo0:
THE IDEA DIODE:
6sc3 6oc3
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Dr. G. AMARANATH, BITS Pilani, Dubai Campus
@$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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Dr. G. AMARANATH, BITS Pilani, Dubai Campus
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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Dr. G. AMARANATH, BITS Pilani, Dubai Campus
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%