BJT Intro and Large Signal Model - Penn Engineeringese319/Lecture_Notes/Lec_3...ESE319 Introduction...
Transcript of BJT Intro and Large Signal Model - Penn Engineeringese319/Lecture_Notes/Lec_3...ESE319 Introduction...
ESE319 Introduction to Microelectronics
1by Kenneth R. Laker, update 03Sep14 KRL
BJT Intro and Large Signal Model
ESE319 Introduction to Microelectronics
2by Kenneth R. Laker, update 03Sep14 KRL
VLSI Chip Manufacturing Process
ESE319 Introduction to Microelectronics
3by Kenneth R. Laker, update 03Sep14 KRL
Why BJT?What's the competition to BJT and bipolar technologies?
What advantages does the competition have over BJT?
What advantages does BJT and bipolar technologies have over their competition?
What circuit applications benefit from BJT and bipolar techno-logies?
ESE319 Introduction to Microelectronics
4by Kenneth R. Laker, update 03Sep14 KRL
Why BJT?What's the competition to BJT and bipolar technologies?
MOSFET, in particular CMOS is the leading competitor
What advantages does the competition have over BJT?Small size (die area), low cost and low power dissipation
What advantages does BJT and bipolar technologies have over their competition?
High frequency operation, low noise, high current drive, high reliability in severe environmental conditions.
What circuit applications benefit from BJT & bipolar technolo-gies?
RF analog & digital circuits, power electronics, wireless communications, automobile electronics, rad hard electronics.
ESE319 Introduction to Microelectronics
5by Kenneth R. Laker, update 03Sep14 KRL
Why BJT?What's the competition to BJT and bipolar technologies?
MOSFET, in particular CMOS is the leading competitor
What advantages does the competition have over BJT?Small size (die area), low cost and low power dissipation
What advantages does BJT and bipolar technologies have over their competition?
High frequency operation, low noise, high current drive, high reliability in severe environmental conditions.
What circuit applications benefit from BJT & bipolar technolo-gies?
RF analog & digital circuits, power electronics, wireless communications, automobile electronics, rad hard electronics.
ESE319 Introduction to Microelectronics
6by Kenneth R. Laker, update 03Sep14 KRL
Why BJT?What's the competition to BJT and bipolar technologies?
MOSFET, in particular CMOS is the leading competitor
What advantages does the competition have over BJT?Small size (die area), low cost and low power dissipation
What advantages does BJT and bipolar technologies have over their competition?
High frequency operation, low noise, high current drive, high reliability in severe environmental conditions.
What circuit applications benefit from BJT & bipolar technolo-gies?
RF analog & digital circuits, power electronics, wireless communications, automobile electronics, rad hard electronics.
ESE319 Introduction to Microelectronics
7by Kenneth R. Laker, update 03Sep14 KRL
Why BJT?What's the competition to BJT and bipolar technologies?
MOSFET, in particular CMOS is the leading competitor
What advantages does the competition have over BJT?Small size (die area), low cost and low power dissipation
What advantages does BJT and bipolar technologies have over their competition?
High frequency operation, low noise, high current drive, high reliability in severe environmental conditions.
What circuit applications benefit from BJT & bipolar technolo-gies?
RF analog & digital circuits, power electronics, wireless communications, automobile electronics, rad hard electronics.
ESE319 Introduction to Microelectronics
8by Kenneth R. Laker, update 03Sep14 KRL
Moore's Law
2014
Future of Moore's Law?
ESE319 Introduction to Microelectronics
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Moore's Law and Moore
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0.35 µm SiGe BiCMOS Layout for RF (3.5 GHz) Two-Stage Power Amplifier
Each transistor above is realized as a net of four parallel HBTs
“Design of a SiGe BiCMOS Power Amplifier for WiMAX Application” by Cheng-Chi Yu, Yao-Tien Chang, Meng-Hsiang Huang, Luen-Kang Lin, and Hsiao-Hua Yeh, 2009
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0.35 µm SiGe BiCMOS Layout for RF (3.5 GHz) Two-Stage Power Amplifier
Two stage power amplifier
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0.35 µm SiGe BiCMOS Layout for RF (3.5 GHz) Two-Stage Power Amplifier
ESE319 Introduction to Microelectronics
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BJT Physical Configuration – Big Picture
Each transistor looks like two back-to-back diodes, but each behaves much dif-ferently!
NPN PNP
Closer to actual lay-out
ESE319 Introduction to Microelectronics
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BJT Symbols and ConventionsNPN PNP
Note reversal in current directions and voltage signs for PNP vs. NPN!
IE
IE
Metal contactIC
IB
IC
IBIE = IC + IB
forward active forward active
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NPN BJT Modes of Operation
ModeForward-ActiveReverse-ActiveCutoffSaturation
VBE
VBC
> 0 ≤ 0≤ 0 > 0≤ 0 ≤ 0> 0 > 0
Forward-Active ModeEBJ forward bias (V
BE > 0)
CBJ reverse bias (VBC
≤ 0)
Not Useful!
VBC
= -VCB
vCE
= vCB
+ vBE
iE = i
C + i
B
VBE
VCB
vXY = VXY + vxylarge signal
dc bias
ac signal
ESE319 Introduction to Microelectronics
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The PNP Transistor Modes of Operationnpnpnp
ModeForward-ActiveReverse-ActiveCutoffSaturation
VEB
VCB
> 0 < 0< 0 > 0< 0 < 0
> 0
Not Useful!
VCB
= -VBC
> 0
ESE319 Introduction to Microelectronics
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nMOSNPN BJT
Substrate (S)
p-type substrate
B
C
E S
QP
VS < 0 VB < 0
Q V EB QP=V BCQ0
V CB QP=V S−V C Q0
E
B
CWhen Q FwdAct & S<0 => QP Cutoff
ESE319 Introduction to Microelectronics
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VBE
VCE
iC
iC
vCE
Saturation region
NPN BJT Modes of Operation
0
-VA
vBE1
vBE2
vBE3
vBE4
Forward-Active region
IE = IC + IB
VCE = VCB + VBE
iB
iE
iC
ESE319 Introduction to Microelectronics
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iD
vDS
Saturation region
Triode region
vGS1
vGS2
vGS3
vGS4
ESE319 Introduction to Microelectronics
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iE
iE
iE i
CiC
iC
iB v
CBvBE
VBE V
CB
iCi
E
n n
Recombined electrons (i
B2)i
B
p
Injected holes (iB1
)
Collected electrons
Injected electrons
Injected electrons
Forward-biased Reverse-biased
B
CE
NPN BJT Forward-Active Current Flow - Details
i B=iB1iB2
iE=iCiB iB=iE−iC=iC
0 Wn p0=n p0 evBE
V T n p W =0x
ESE319 Introduction to Microelectronics
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NPN BJT Forward-Active Mode Basic ModelCollector-base diode is reverse biased
V CB≥ 0
Base-emitter diode is forward biased
V BE≈0.7
iC≈ I S evBE
V T
iB=iC
iE=iBiC=1iB
V T=kTq≈25mV @ 25o CI S=
AE q Dn ni2
N AWArea of base-emitter junction {m2}Width of base region {m}Doping concentration in base {m-3}Electron diffusion constant {m2/s}Intrinsic carrier concentration = f(T) {m-3} =common−emitter current gain
( or VBC
≤ 0)
saturation or scale current
AE
WN A
ni
Dn
1
ESE319 Introduction to Microelectronics
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=1
D p
Dn
N A
N D
WL p
12
W 2
Dnb
AE -> Area of base-emitter junction {m2}W -> Width of base region {m}NA -> Doping concentration in base {m-3}ND -> Doping concentration in emitter {m-3}Dn -> Electron diffusion constant {m2/s}Dp -> Hole diffusion constant {m2/s}Lp -> Hole diffusion length in emitter {m}τb -> Minority-carrier lifetime {s}ni -> Intrinsic carrier concentration = f(T) {m-3}
Large β =>NA -> smallND -> largeW -> small
NPN BJT Forward-Active Beta (β)
ESE319 Introduction to Microelectronics
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Using Eqs. iE = iB + iC and iC = βiB we can derive iE in terms of iC
, i.e.
iE=iBiC=11 iC=
1
iC
and write:
iE=1
I S ev BE
V T −1= 1
I S evBE
V T −1=iC
=common−emitter current gain=common−base current gain
=1
where
NPN BJT Forward-Active Alpha (α)
ESE319 Introduction to Microelectronics
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The basic BJT Fwd. Act. equations (model) are:
iC=I S ev BE
V T −1= iE
iE=iC
=
I S
e
v BE
V T −1
iB=iC
=
1
Where:
50200
10−18 I S10−12 A.
Typically:
Is is strongly temperature-
dependent, doubling for a 5 degree Celsius increase in ambient temperature!
50200
I s=AE q Dn ni
2
N A W
0.9800.995=>
ESE319 Introduction to Microelectronics
25by Kenneth R. Laker, update 03Sep14 KRL
Two equivalent large signal circuit models for the forward-active mode NPN BJT:
Nonlinear VCCSNonlinear CCCS
iC≈ I S evBE
V T =F iE
iE≈I S
Fe
vBE
V T
Key Eqs.
=F
iC= I S evBE
V T −1≈ I S ev BE
V T
I SEi B=iE−iC=
1
iC
=F
ESE319 Introduction to Microelectronics
26by Kenneth R. Laker, update 03Sep14 KRL
Yet another NPN BJT large signal model
iC= iB≈ I S evBE
V T ⇒ iB≈I S
e
vBE
V T
Note that in this model, the diode current is represented in terms of the base current. In the previous ones, it was rep-resented in terms of the emitter current.
iB i
C
iE
iB
ESE319 Introduction to Microelectronics
27by Kenneth R. Laker, update 03Sep14 KRL
iC≈ I S ev BE
V T = iE
iE≈I S
e
vBE
V T
iB=iE−iC=1
iC
≫1⇒≈1Note
Sometimes used approximation:=∞⇒=1
Hence:iE=iC⇒ iB=0
1. At this limit the BJT resembles an MOS.2. Be careful when using this approximation – important details are lost.
iC=I S evBE
V T −1
ESE319 Introduction to Microelectronics
28by Kenneth R. Laker, update 03Sep14 KRL
iC=I S evBE
V T −1
iC
vBE
Tangent point (i
C vs. v
BE)
0.6≤V BE ≤0.8V
V BE≈0.7V Fwd. Act. BJTAnother approximation:
ESE319 Introduction to Microelectronics
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I S evBE /V T
DEvBE
E
CBI S evBE /V T
DEvBE
B
E
C
Forward Active BJT Model Configuration Options
“T” Configuration “Π” Configuration
iCiB
iE
ESE319 Introduction to Microelectronics
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V CE
iB iC
-
<=>
ESE319 Introduction to Microelectronics
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NPN BJT Operating in the Reverse-Active Mode
● Weak transistor action if we:● Forward bias the base-collector junction and● Reverse bias the base-emitter junction● Collector and emitter reverse roles
● The physical construction of the transistor results ● Weak reverse-active performance
● Small values of β on the order of 0.01 to 1 ● Correspondingly smaller values of α, e. g.
R=R
R1≈0.11.1
≈0.091 for R=0.1
Recall for NPN Reverse-Active Mode VBE
≤ 0 & VBC
> 0
Due to BJT being non-symmetricalR≪F R≪F
ESE319 Introduction to Microelectronics
32by Kenneth R. Laker, update 03Sep14 KRL
The equivalent large signal circuit model for the reverse-active mode NPN BJT:
iC≈−I S
Re
vBC
V T =−I SC evBC
V T
iE≈−I S evBC
V T =R iC
iB
iC
iE
RVRS Active
FWD Active
Key Eqs.BJT is non-symmetricalR≪F R≪FNote that the directions of
the reverse-active currents are the reverse of the for-ward-active currents; hence the “minus” signs.
iC
iEi
B
collector and emitter reverse roles
iC≈ I S evBE
V T =F iE
iB
iC
iE
ESE319 Introduction to Microelectronics
33by Kenneth R. Laker, update 03Sep14 KRL
The Ebers-Moll Large Signal ModelThe E-M model combines the FWD & RVRS Active equivalent circuits:
Note that the lower left di-ode and the upper right con-trolled current source form the forward-active mode model, while the upper left diode and the lower right source represent the re-verse-active mode model.
iC=F iDE−i DC
iE=iDE−R i DC
i B=1−F iDE1−RiDC
i DE=I S
Fe
vBE
V T −1
iDC=I S
Re
vBC
V T −1
ESE319 Introduction to Microelectronics
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iB
βiB
BJT Model Summary
iC
iE
ESE319 Introduction to Microelectronics
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Operation in the Saturation Mode
Consider the E-M model for collector current. iC=F iDE−iDC
F iDE= I S evBE
V T −1The second is the reverse mode collector current:
iDC=I S
Re
vBC
V T −1
Recall for Saturation Mode vBE
> 0 & vBC
> 0 (or vCB
< 0)
iC= I S evBE
V T −1−I S
Re
v BC
V T −1
The first term is the forward mode collector current:
ESE319 Introduction to Microelectronics
36by Kenneth R. Laker, update 03Sep14 KRL
Combining terms:
iC=[e vBE
V T −1−R1R
ev BC
V T −1] I S
Using typical values:
R=0.1
I S=10−14 A
V T=0.025V
iC=[e40v BE−1−11 e40 vBC−1 ]10−14We obtain:
1R
=R1R
iC= I S evBE
V T −1−I S
Re
v BC
V T −1
Let's plot iC vs. v
BC (or v
CB) with v
BE = 0.7 V
ESE319 Introduction to Microelectronics
37by Kenneth R. Laker, update 03Sep14 KRL
Scilab Saturation Mode Calculation//Calculate and plot npn BJT collector//current in saturation modevBE=0.7;VsubT=0.025;VTinv=1/VsubT;betaR=0.1;IsubS=1E-14;alphaR=betaR/(betaR+1);alphaInv=1/alphaR;ForwardExp=exp(VTinv*vBE)-1;vCB=-0.7:0.001:-0.1;vBC=-vCB;ReverseExp=alphaInv*(exp(VTinv*vBC)-1);iC=(ForwardExp-ReverseExp)*IsubS;signiC=sign(iC);iCplus=(iC+signiC.*iC)/2; //Zero negative valuesplot(vCB,1000*iCplus); //Current in mA.
iC=I S ev BE
V T −1−I S
Re
vBC
V T −1
ESE319 Introduction to Microelectronics
38by Kenneth R. Laker, update 03Sep14 KRL
Saturation Mode Plot
Forward-activeSaturation
Recall for Sat. Mode v
BE > 0
& v
BC > 0
iC(ma)
vBC(V)vBE = 0.7V
Note: forward-active NPN opera-tion continues for positive vBC up to about 0.5V.
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
iC=[e40v BE−1−11e40v BC−1]10−14 A
ESE319 Introduction to Microelectronics
39by Kenneth R. Laker, update 03Sep14 KRL
Scilab Plot of NPN Characteristic (iC vs. v
CE and v
BE)
//Calculate and plot npn BJT collector//characteristic using Ebers-Moll modelVsubT=0.025;VTinv=1/VsubT;betaR=0.1;alphaInv=(betaR+1)/betaR;IsubS=1E-14;for vBE=0.6:0.02:0.68 ForwardExp=exp(VTinv*vBE)-1; vCE=-0:0.001:10; vBC=vBE-vCE; ReverseExp=alphaInv*(exp(VTinv*vBC)-1); iC=(ForwardExp-ReverseExp)*IsubS; signiC=sign(iC); iCplus=(iC+signiC.*iC)/2; //Zero negative vals plot(vCE,1000*iCplus); //Current in mA.end
iC=I S ev BE
V T −1−I S
Re
vBC
V T −1
ESE319 Introduction to Microelectronics
40by Kenneth R. Laker, update 03Sep14 KRL
Plot Output
V BE=0.68V
V BE=0.66V
V BE=0.64V
Early effect not included.
forward-active modesaturation mode
VCE (V)
iC (ma)
VCE
= - VBC
+ VBE
V BE=0.62V
V CEsat=V CE=−V BCV BE≈−0.5V0.7V≈0.2V@ start of saturation
ESE319 Introduction to Microelectronics
41by Kenneth R. Laker, update 03Sep14 KRL
More on NPN Saturation● The base-collector diode has much larger
area than the base-emitter one.● Therefore, with the same applied voltage, it will
conduct a much larger forward current than will the base-emitter diode.
● When vCE
drops below vBE
, the base-collector diode is forward biased (v
BC > 0)and conducts heavily.
vBC=vBE−vCE≈0.5V
iC= I S evBE
V T −1−I S
Re
v BC
V T −1 where αR << 1
Let VBE
= 0.7 V & VCE
= 0.2 V =>
ESE319 Introduction to Microelectronics
42by Kenneth R. Laker, update 03Sep14 KRL
More on NPN Saturation - cont.● In saturation the forward-biased current through
the collector-base junction increases iB and de-
creases iC as V
BC increases.
● Test for saturation mode operation● V
CE = V
CEsat = 0.2 to 0.3 V => collector-base junction is
forward biased
● Current ratio => collector-base junction is forward biased
iC
iB
sat≤
or
sat= forced=iC
iB
sat≤
ESE319 Introduction to Microelectronics
43by Kenneth R. Laker, update 03Sep14 KRL
More on NPN Saturation - cont.
Does iC = 0 A => v
CE = 0 V ?
ESE319 Introduction to Microelectronics
44by Kenneth R. Laker, update 03Sep14 KRL
Voltage at Zero Collector Current
iC = 0 =>I S
Re
vBC
V T −1= I S ev BE
V T −1
R ev BE
V T −1=ev BC
V T −1
R ev BE
V T −1=evBE−vCE
V T −1
R 1−e−
vBE
V T =e−
vCE
V T −e−
vBE
V T
V BE
V T≈40∗0.7⇒ e
−vBE
V T ≪1
iC= I S evBE
V T −1−I S
Re
v BC
V T −1
For βR = 0.1 => αR = 0.09 => vCE = 0.06 V
R=e−vCE
V T ⇒ vCE=−V T ln R
Multiply by e−vBE
V T
ESE319 Introduction to Microelectronics
45by Kenneth R. Laker, update 03Sep14 KRL
The PNP Transistor npnpnp
ModeForward-ActiveReverse-ActiveCutoffSaturation
VEB
VCB
> 0 < 0< 0 > 0< 0 < 0
> 0
Not Useful!
VCB
= -VBC
> 0
ESE319 Introduction to Microelectronics
46by Kenneth R. Laker, update 03Sep14 KRL
PNP BJT Forward-Active Mode Basic Model
Collector-base diode is reverse biased
V CB0
Emitter-base diode is forward biased
V EB≈0.7 iC=I S ev EB
V T −1
iB=iC
iE=iBiC=1iB
Note reversal in voltage polarity and in current directions!
ESE319 Introduction to Microelectronics
47by Kenneth R. Laker, update 03Sep14 KRL
PNP BJT Large Signal Model FWD. Active
Note reversal in all current directions!
iC= I S evEB
V T −1
iB=iC
iE=I S
e
vEB
V T −1
NPN
iE=iBiC=1iB
iE
PNP
iC= I S evEB
V T −1 iC= I S evBE
V T −1
vBE
vEB
ESE319 Introduction to Microelectronics
48by Kenneth R. Laker, update 03Sep14 KRL
Yet another PNP BJT large signal model
Again, in this model, the diode carries only base current, not emitter current.
iB
iC
iC= iB= I S evEB
V T −1⇒ iB=I S
e
v EB
V T −1≈I S
e
vEB
V T
NPN
iE
PNP