BJT Intro and Large Signal Model - Penn Engineeringese319/Lecture_Notes/Lec_3...ESE319 Introduction...

48
ESE319 Introduction to Microelectronics 1 by Kenneth R. Laker, update 03Sep14 KRL BJT Intro and Large Signal Model

Transcript of BJT Intro and Large Signal Model - Penn Engineeringese319/Lecture_Notes/Lec_3...ESE319 Introduction...

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ESE319 Introduction to Microelectronics

1by Kenneth R. Laker, update 03Sep14 KRL

BJT Intro and Large Signal Model

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VLSI Chip Manufacturing Process

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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?

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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.

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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.

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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.

Page 7: BJT Intro and Large Signal Model - Penn Engineeringese319/Lecture_Notes/Lec_3...ESE319 Introduction to Microelectronics by Kenneth R. Laker, update 03Sep14 KRL 4 Why BJT? What's the

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.

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ESE319 Introduction to Microelectronics

8by Kenneth R. Laker, update 03Sep14 KRL

Moore's Law

2014

Future of Moore's Law?

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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

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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

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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

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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

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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

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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

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iD

vDS

Saturation region

Triode region

vGS1

vGS2

vGS3

vGS4

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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

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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

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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)

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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)

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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=>

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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

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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

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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

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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:

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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

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V CE

iB iC

-

<=>

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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

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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

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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

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iB

βiB

BJT Model Summary

iC

iE

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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:

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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

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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

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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

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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

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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

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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 =>

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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≤

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ESE319 Introduction to Microelectronics

43by Kenneth R. Laker, update 03Sep14 KRL

More on NPN Saturation - cont.

Does iC = 0 A => v

CE = 0 V ?

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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

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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

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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!

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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

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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