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BIPOLARTRANSISTOR
1
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Bibliography:
2
• H. Mathieu, « Physique des semi-conducteurs et des composantsélectroniques », 4° édition, Masson 1998.
• D.A. Neamen, « semiconductor physics and devices », McGraw-Hill, Inc 2003.• P. Leturcq et G.Rey, « Physique des composants actifs à semi-conducteurs »,
Dunod Université, 1985.• J. Singh, « semiconductors devices :an introduction », McGraw-Hill, Inc 1994.• Y. Taur et T.H. Ning, « Fundamentals of Modern VLSI devices », Cambridge
University Press, 1998.• K.K. Ng, « complete guide to semiconductor devices », McGraw-Hill, Inc 1995.• D.J. Roulston, « Bipolar semiconductor devices », McGraw-Hill, Inc 1990.
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Plan• Geometry• Principle of operating• Static characteristics• Ebers-Moll Equations• Static parameters ( gain…)• Second order effects• Switching performances• Transistor in HF domain• Hetero-junction Bipolar Transistor (HBT or TBH)
3
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Geometry
• Geometry:• Lateral• Vertical
• For digital circuits, vertical design
4
lateral
vertical
npn
BC
E
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Geometry for IC
5
Muller et Kamins, « device electronics for IC »,2nd Ed., Wiley, 1986
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Geometry with insulating oxide
6
Muller et Kamins, « device electronics for IC »,2nd Ed., Wiley, 1986
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BJT always present: why?
• speed• Low noise• High gain• Low output resistance
• Still present in mobile phone ( analog part)• Low density, mainly in power stage• BiCMOS
7
Analogueamplifier
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Working principle
• 2 PN Junctions with one common region (base)• The first Junction (EB) will
inject carriers• The second one (CB) will
collect carriers• The base must be thin (lower than diffusion )length
8
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• Reverse biased Junction:• Low current due to empty
« tank »• En modulant le remplissage
du réservoir, modulation du courant inverse collecté (collecteur)
• On remplit le réservoir (la base) en polarisant en direct la jonction EB
9
Working principle
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• The reverve biasing CB junctioncreates a favorable electric fieldfor the collect
• Conditions: • Thin base:
• Avoid recombination effects• Ligthly doped base compared to
emitter• favors one type of injected
carriers (better injection efficiency)
10
Working principle
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Statics characteristics
11
NPN Transistor NPN Transistor
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12
Minority carrier distribution in a forward biased npntransistor
ideal
With recombination
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13
PNP Transistor
No recombination in the Base ! ( )
1D Approximation
Homogeneous doping in the Base
Low Injection
Statics characteristics + simplifying hypotheses
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Current components in the NPN Transistor
• In the neutral Base region:• Continuity equation
• and ,
• Integrating from E-B to C-B:
• finally:
• In forward active regime, Jn is negative ( e- vers x<0)
14
dxnpd
eDJ
neDJ
pp
p
n
n ).(
dxnpd
eDJ
neDJ
pp
p
n
n ).(
pn JJ pn
dxnpd
eDJ
pn
n ).(
dxnpd
eDJ
pn
n ).(
'
'
)(
)exp()exp(2
C
E
BCBE
iBnbn
dxxp
kTeV
kTeV
neDJ
'
'
)(
)exp()exp(2
C
E
BCBE
iBnbn
dxxp
kTeV
kTeV
neDJ
'
'
)(
)1()1(2
C
E
kTeV
kTeV
iBnbn
dxxp
eeneDJ
BCBE
'
'
)(
)1()1(2
C
E
kTeV
kTeV
iBnbn
dxxp
eeneDJ
BCBE
![Page 15: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/15.jpg)
15
)1(
)()1(
)()(
)1()1(
'
'
'
'
'
'
222 kT
eV
C
E
iBnbkTeV
C
E
iBnbC
E
kTeV
kTeV
iBnbn
BCBE
BCBE
edxxp
neDedxxp
neD
dxxp
eeneDJ
)1(
)()1(
)()(
)1()1(
'
'
'
'
'
'
222 kT
eV
C
E
iBnbkTeV
C
E
iBnbC
E
kTeV
kTeV
iBnbn
BCBE
BCBE
edxxp
neDedxxp
neD
dxxp
eeneDJ
and:BeffA
C
E
WNdxxpB
'
'
)( BeffA
C
E
WNdxxpB
'
'
)(
So:
)1()1()1()1(
22kT
eVkT
eV
SnkT
eV
BeffA
nbiBkTeV
BeffA
nbiBEn
BCBEBC
B
BE
B
eeIeWNDene
WNDenAI
)1()1()1()1(
22kT
eVkT
eV
SnkT
eV
BeffA
nbiBkTeV
BeffA
nbiBEn
BCBEBC
B
BE
B
eeIeWNDene
WNDenAI
with :B
iE
BeffA
nbiBESn G
enAWNDenAI
B
22
B
iE
BeffA
nbiBESn G
enAWNDenAI
B
22
Saturation current of electrons in the PN narrowjunction ( or without recombination in the Base).
BeffnbiB
iBeff
nb
A
iB
iB W
Dp
nnW
DN
nnG B
2
2
2
2
BeffnbiB
iBeff
nb
A
iB
iB W
Dp
nnW
DN
nnG B
2
2
2
2
Gummel number in the Base (s/cm4)
Current components in the NPN Transistor
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• Emitter current • Collector current
16
1)exp(
kTeVJJ BE
spEpE
1)exp(
kTeVJJ BC
spCpC
finally
pCpEECB
npCC
npEE
IIIII
III
III
E B C
JpE Jn JpCIE
IB
IC
NPN
Current components in the NPN Transistor
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• And (!) :
17
)1(exp)(
)1](exp)(
[ '
'
'
'
22
kT
eV
dxxp
DAenkT
eVIdxxp
DAenIII BCC
E
nbiBEspEC
E
nbipEnE
)1](exp)(
[)1(exp)(
'
'
'
'
22
kT
eVIdxxp
DAenkT
eV
dxxp
DAenIII BCspCC
E
nbiBEC
E
nbipCnC
1)exp(1)exp(
kTeVI
kTeVIIIIII BC
spCBE
spEpCpECEB
Isn
Current components in the NPN Transistor
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• Forward active region:• E‐B (forward) and C‐B (reverse)
18
kTeVII
kTeVI
QQDnAeI BE
SpESnBE
SpESB
nBiE exp)(exp)(
22
kTeVI
kTeV
QQDnAeI BE
SnBE
SB
nBiC expexp)(
22
kTeVIIII BE
SpECEB exp*
Statics characteristics
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• Emitter injection efficiency:
• DC common base current gain:
• DC common emitter current gain :
19
Ep
nE I
I
SpSn
Sn
nBi
SBSpE
C
III
DneQQJI
I
22
).(1
1
1B
C
II NB:if we neglect recombination
process and are equivalent E
Statics characteristics
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• Base transport factor:• Take into account recombination
in neutral base region
20
n
ppB
n
srB
nnAeXQI eff
2/))0((
t
sC
QI
12
2
2
effB
n
t
n
rB
C
XL
II
Statics characteristics
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• Recombination factor:• Take into account recombination in
depleted base region
21
kTeVW
AenI BE
Ti
rD 2exp
2
withWT, width of E‐B space charges.
When we add up all the contribution to the base currentwe get:
rDrBBB IIII *
Statics characteristics
![Page 22: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/22.jpg)
• The common emitter gain can be expressed as:
• intriseque base current (no recombination)• recombination current in the neutral base region• recombination current in the depletion zone of EB junction
22
C
rD
EC
rDrBB
C
B
II
IIII
II
111 *
*BI
rBIrDI
Statics characteristics
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Other modes of operation
• Saturation mode (regime):• The two junctions are forward biased.
23
kTeV
BS
nBikTeV
spEBS
nBiE
BCBE
eQQDnAe
eIQQDnAe
I
2222
kTeV
spCBS
nBikTeV
BS
nBiC
BCBE
eIQQDnAee
QQDnAeI
2222
n(x)
)0(exn
)( Bex Wn
QS1
QS2
Base
0 WB
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• Low injection : (QS<<QB):• Current is due to saturation charge injected into the base, ie
QST = QS1 +QS2
• If we deal with « narrow » junction, this charge is simplygiven by the surface of ½ trapeze (linear decay)
24
kTeV
sntkT
eV
A
iBBBS
kTeV
sntkT
eV
A
iBBS
BCBC
BEBE
eJeNnWWxenWQ
eJeNnWxenWQ
2
2
2
1
21)(
21
21)0(
21
Saturation mode
![Page 25: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/25.jpg)
Saturation mode
• Low injection: (QS<<QB):• An other « view » of saturation charge (from Ablard):
• We consider transistor in active mode with a charge QSN and a charge QSAT (we have to determine) which supply the samesaturation current Icsat.
25
0 WB
n(x)
)0(exn
)( Bex Wn
Base
)()0( Bexex Wnn
QSN
QSAT
Impossible d’afficher l’image.Impossible d’afficher l’image.
We get :Impossible d’afficher l’image.
Responsible for the degradation of dynamic performance
QST = QSN+QSAT
![Page 26: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/26.jpg)
• High injection level
• In this case, injected electron density reaches holes density into the base ( )
• Equivalent study leads to the following result :
• .
26
BkT
eVkT
eV
iS WeeenQBCBE
)(21 22 B
kTeV
kTeV
iS WeeenQBCBE
)(21 22
kTeV
iB
nBkTeV
iB
nBn
BCBE
enWeDen
WeDJ 22 22
kTeV
iB
nBkTeV
iB
nBn
BCBE
enWeDen
WeDJ 22 22
pn
Saturation mode
![Page 27: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/27.jpg)
Nonideal Effects
• « Gummel plot »:• Graph of IC and IB versusVBE
27
1
2
3
![Page 28: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/28.jpg)
• Early effect / collector punchthrough
• Base – collector junction breakdown
• Emitter and Base serie resistance
• Ic « collapse » fort high current
• « crowding effect »
• Early effect / collector punchthrough
• Base – collector junction breakdown
• Emitter and Base serie resistance
• Ic « collapse » fort high current
• « crowding effect »
28
Nonideal Effects
![Page 29: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/29.jpg)
• At "first glance" Ic is independent of VCB
• In fact we have the modulation of the width of the neutral region in the base, so in the same wayQB+QS , and so Ic !
29
kTeV
QQDnAeI BE
SB
nBiC exp)(
22
if VBC WB QB+QS
Ic
ZCE B-C
Early effect / collector punchthrough
![Page 30: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/30.jpg)
• At the limit:• The space charge BC
« depletes » totaly the neutral base
• collector injects directlycurrent into the emitter.
• Current only limited by serieresistance Rserie form E and C
30
BCZCE
SC
BB
dBC
pBA
W
WeNCQ
V
C
CBB
DSC
DAABpt N
NNNeWV
2)(2
Early effect / collector punchthrough
![Page 31: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/31.jpg)
Base - Collector junction breakdown
• Avalanche of BC:• Often occurs before punchthrough
• How to prevent it?• Lowering the electric field:
• Reduce the doping gradient in the collector
• Lightly doped layer between base and collector
31
Ionisation par impacts
BrBfB III BrBfB III
![Page 32: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/32.jpg)
Emitter and Base resistance (effet 3)
• At low current negligeableeffects
• In high speed circuits, B-C allways reverse biased (rc2 and rc3 as low as possible)• Resistances rc have no
effects on current flow• only re and rb play a major role .
• IR drop Voltage
32
)exp(
)(
'
'
kTVeII
VVV
rrIIrrIIrV
BEBB
BEBEBE
beBCebBEeBE
0 BEC III
Measured current:IB’
![Page 33: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/33.jpg)
Collector « collapse » for high injection level (effect 1)
• Numbers of physicalmechanisms can cause thisfalloff of IC0:• Increase of the charge (holes)
into the base (maintainneutrality)
• Increase the width of the quasineutral base layer (push of the space charge into the collector): Kirk effect
33
kTeV
dxxpneDAI
kTeVJAI
BEC
E p
ieBnBEC
BECEC
exp)(
exp
'
'
2
0
Nc
Nb
x)
Wb0
E’ C’
Nc-n
Nb+n
x)
Wb0
E’ C’
![Page 34: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/34.jpg)
34
And the effect #2 ????????????????
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« crowding effect »
• The view of a 1D devices is an approximation
• Drop voltage IR in the base. • The edge of the emitter contact is
more biased than the core• Favour a high density of current
essentially along the edges• Not a good thing for high power
devices
• Solutions: interdigitatedapproach
35
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Bipolar Transistor: a switch ?
• ON state: the switch isclosed (saturation mode)
• OFF state: the switch is open (cutoff mode)
• ON state: the switch isclosed (saturation mode)
• OFF state: the switch is open (cutoff mode)
36
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• Input signal as low as possible
• input power as low as possible
common emitter
• Input signal as low as possible
• input power as low as possible
common emitter
37
Bipolar Transistor: a switch ?
![Page 38: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/38.jpg)
• Switching velocity?• Limiting factors?
38
Turn on transient: Continuity equation fir
the charge:
IdQdt
Qn
B B
n
Base charge can bewritten:
Collector current is givenby:
transit time
in the base (narrow)
)]exp(1[)(n
nBBtItQ
t
BC
tQtI
)(
)(
)]exp(1[)(
nt
nBc
t
B tIItQ
Bipolar Transistor: a switch ?
![Page 39: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/39.jpg)
• Turn on:• IC increases until the saturation
regime is reached:(we neglect VCEsat)
• The limit charge QB(ton) to saturate the transistor is givenby :
• The on state time is given by:
39
C
DDC R
VIsat
C
DDC R
VIsat
nB
pBCS D
dIQ sat
2
2
nB
pBCS D
dIQ sat
2
2
)(1
1ln)(1
1ln
Bc
nnBS
nON IIIQt
)(1
1ln)(1
1ln
Bc
nnBS
nON IIIQt
)]exp(1[)(n
nBBtItQ
Bipolar Transistor: a switch ?
![Page 40: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/40.jpg)
• Remark: the charge can increasesover QB(ON) to over saturate the transistor
• Turn off time : input with a « 0 »:• Evacuation of the stored charge
• This is the storage time ts
• after, this is the samemechanism than for PN Junction
40
S
nBnS Q
It
ln
S
nBnS Q
It
ln
Final value:
Bn I
Bipolar Transistor: a switch ?
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• Storage time (desaturation) limits the switching time
• 2 ways to reduce it:• Add impurities which
decrease strongly the lifetimeinto the base
• Schottky Diode in // with CB junction: avoid saturation of the transistor
41
Bipolar Transistor: a switch ?
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AC signal : equivalent circuitAC signal : equivalent circuit
42
CµIB Ib Ic
![Page 43: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/43.jpg)
Transistor and ac signal: equivalent circuit
• Transconductance :links the variation of the collector current to the base – emiter voltage:
• Input resistance : links the variation of Base current to the base – emitervoltage:
• Output resistance:
43
kTeI
VI
g C
BE
Cm
11
1
hgeI
kTVIr
mBBE
B
11
1
hgeI
kTVIr
mBBE
B
22
11
hIV
VIr
C
A
CE
Co
22
11
hIV
VIr
C
A
CE
Co
![Page 44: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/44.jpg)
• Capacitance :
• Storage capacitance
• transit time
• Capacitance : CB junction capacitance reverse biased
• Collector –substrate (depletion layer) capacitance:
44
EBTSE CCC
CBTCC
dCSC
mFSE gC
BCBEB tttEF tttt
C
C
Transistor and ac signal: equivalent circuit
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• Cut off frequency (current gain =1)• We neglect r0
• Current gain is given by:
45
beµb
beµbemc
vCjCjr
i
vCjvgi
1
21)()/1()( h
CCjrCjg
ii
µ
µm
b
c
21)()/1()( h
CCjrCjg
ii
µ
µm
b
c
Ib Ic
x
Transistor and ac signal: equivalent circuit
![Page 46: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/46.jpg)
• At low frequency:• In modern devices , in general,
• At high frequecy, the imaginary term dominates and :
46
mµ gC )(1
)(µ
m
b
c
CCrjrg
ii
)(1)(
µ
m
b
c
CCrjrg
ii
)()(
µ
m
CCjg
)(
)(µ
m
CCjg
Transistor and ac signal: equivalent circuit
![Page 47: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/47.jpg)
Transistor en ac: schéma équivalent
• On obtient alors la fréquence de coupure (« cutoff frequency ») en faisant iC/iB=1
• Soit encore
47
µ
mT CC
gf
2
)()(2
1ceTTSE
CF
T
rrCCCeIkT
f BCBC
Temps de transit en direct
![Page 48: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/48.jpg)
Transistor en ac: schéma équivalent
• Maximun oscillation frequencyPower gain=1• Laborious and tedious calculation/ we have to take into account the
base resistance
48
dBCb
T
Crff
8max dBCb
T
Crff
8max
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Heterojunction Bipolar Transistor
• Current gain:
• In the case of narrow base:
49
2
2
2
2
2111
1BEB
B
B
E
E
E
n
Beff
p
Beff
i
A
n
p
D
i
E
E
LW
LW
nN
DD
Nn
2
2
2
2
2111
1BEB
B
B
E
E
E
n
Beff
p
Beff
i
A
n
p
D
i
E
E
LW
LW
nN
DD
Nn
EB
B
B
E
E
E
p
Beff
i
A
n
p
D
i
E
E
LW
nN
DD
Nn
2
2
111
EB
B
B
E
E
E
p
Beff
i
A
n
p
D
i
E
E
LW
nN
DD
Nn
2
2
111
![Page 50: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/50.jpg)
• If we want a gain with a high value, we need close to 1. It means: • Lowering base doping• Lowering base width (careful with punchthrough!)
50
Increase of the base resistance fmax decreases
EB
B
B
E
E
E
p
Beff
i
A
n
p
D
i
E
E
LW
nN
DD
Nn
2
2
111
EB
B
B
E
E
E
p
Beff
i
A
n
p
D
i
E
E
LW
nN
DD
Nn
2
2
111
Heterojunction Bipolar Transistor
![Page 51: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/51.jpg)
• Other solution:• Increase emitter doping
• Improve emitter efficiency• Problem: « gap shrinking »
• This « problem » can be transform in opportunity !! (next twoslides)
51
EB
B
B
E
E
E
p
Beff
i
A
n
p
D
i
E
E
LW
nN
DD
Nn
2
2
111
EB
B
B
E
E
E
p
Beff
i
A
n
p
D
i
E
E
LW
nN
DD
Nn
2
2
111
)exp()()( 22
kTE
Basenémetteurn gii
.0)()( émetteurgbasegg EEE
Heterojunction Bipolar Transistor
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52
kTE
LW
nN
DD
Nn g
p
Beff
i
A
n
p
D
i
EB
B
B
E
E
e exp1 2
2
kTE
LW
nN
DD
Nn g
p
Beff
i
A
n
p
D
i
EB
B
B
E
E
e exp1 2
2
1)exp(
1 kTE
WL
DD
NN g
Beff
p
p
n
A
D E
E
B
B
E
1)exp(
1 kTE
WL
DD
NN g
Beff
p
p
n
A
D E
E
B
B
E
We see that it is difficult to have at the same time a large emitter doping, a lightly doped and thin base and a large gain
Heterojunction Bipolar Transistor
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• We design a structure with a negative energy gap difference: this is an HBT
53
eb EgEgEg
Heterojunction Bipolar Transistor
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54
Requirements for a bipolar device
•High gain
•High emitter efficiency
•High speed
Demands and Problems for a BJT
Demands Problems
Heavy emitter doping Bandgap shrinkage causingbase injection
Low base dopingNarrow base width High base resistance
Solution: Heterojunction Bipolar Transistors
•Emitter can be heavely doped using a SC with Eg larger than the base semiconductor•Base can be heavely doped and beheavely doped and be made narrow without increasingbase resistance•Collector can be chosen from a material to increase breakdown voltage
(From Singh)
![Page 55: Transistor Bipolaire eng - Polytech Niceusers.polytech.unice.fr/~lorenz/Bipolar_Transistor_eng.pdf · • Ebers-Moll Equations ... • Transistor in HF domain • Hetero-junction](https://reader031.vdocuments.pub/reader031/viewer/2022020114/5b4aca7f7f8b9a2d2f8c77e0/html5/thumbnails/55.jpg)
55
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Transistor with a Silicon –Germanium base
56
Electrons density
kTeV
IQQDnAe
I BESpE
SB
nBiE exp)(
22
kTeV
QQDnAeI BE
SB
nBiC exp)(
22
emettir collector
base
)(2 Sini
)(2 SiGeni
Improve gain gainImprove transist timeIncrease Early voltage