Lecture 26 OUTLINE The BJT (cont’d) Breakdown mechanisms Non-ideal effects Gummel plot & Gummel numbers Modern BJT structures Base transit time Reading:

Lecture 26

OUTLINE

The BJT (cont’d) • Breakdown mechanisms• Non-ideal effects• Gummel plot & Gummel numbers• Modern BJT structures• Base transit time

Reading: Pierret 11.2-11.3, 12.2.2; Hu 8.4,8.7

BJT Breakdown Mechanisms• In the common-emitter configuration, for high output voltage

VEC, the output current IC will increase rapidly due to one of two mechanisms:– punch-through– avalanche

EE130/230M Spring 2013 Lecture 26, Slide 2

Punch-Through

E-B and E-B depletion regions in the base touch W = 0

As |VCB| increases, the potential barrier to hole injection decreases and therefore IC increases


PNP BJT:

Avalanche Multiplication• Holes are injected into the base [0], then

collected by the B-C junction– Some holes in the B-C depletion region have

enough energy to generate EHP [1]

• Generated electrons are swept into the base [3], then injected into emitter [4]– Each injected electron results in the injection of

IEp/IEn holes from the emitter into the base [0]

For each EHP created in the C-B depletion region by impact ionization, (IEp/IEn)+1 > dc additional holes flow into the collector

i.e. carrier multiplication in the C-B depletion region is internally amplified

mdc

CBCE

VV

/10

0 )1(

where VCB0 = reverse breakdown voltage of the C-B junction

62 m


Non-Ideal Effects at Low VEB

• In the ideal transistor analysis, thermal R-G currents in the emitter and collector junctions were neglected.

• Under active-mode operation with small VEB, the thermal recombination current is likely to be a dominant component of the base current

low emitter efficiency, hence lower gainThis limits the application of the BJT for amplification at low voltages.

GREnEp

Ep

III

I


Non-Ideal Effects at High VEB

• Decrease in dc at high IC is caused by:

– high-level injection

– series resistance

– current crowding

1/2

kTqV

B

BiC

EBeWN

DqAnI


Gummel Plot and dc vs. IC

dc

From top to bottom:VBC = 2V, 1V, 0V


dc

Gummel NumbersFor a uniformly doped base with negligible band-gap narrowing, the base Gummel number is

B

BB D

WNG

(total integrated “dose” (#/cm2) of majority carriers in the base, divided by DB)

E

BWW

NN

DD

nGG

EE

B

B

E

Bi

Ein

1

1

1

1

2

2Emitter efficiency

11 /2

/2

kTqV

B

ikTqV

B

BiC

EBEB eG

qAne

WN

DqAnI

GE is the emitter Gummel numberEE130/230M Spring 2013 Lecture 26, Slide 8

dxxD

xN

n

nG

B

BW

Bi

iB )(

)(0 2

2

B

E

LW

LW

NN

DD

n

dc G

G

BEE

B

B

E

Bi

Ein

2

21

2

2

1Notice that

In practice, NB and NE are not uniform, i.e. they are functions of x

The more general formulas for the Gummel numbers are

dxxD

xN

n

nG

E

EW

Ei

iE )(

)(0 2

2


Modern BJT Structure

Features:•Narrow base •n+ poly-Si emitter•Self-aligned p+ poly-Si base contacts•Lightly-doped collector•Heavily-doped epitaxial subcollector•Shallow trenches and deep trenches filled with SiO2 for electrical isolation


Poly-Si Emitter• dc is larger for a poly-Si emitter

BJT as compared with an all-crystalline emitter BJT, due to reduced dpE(x)/dx at the edge of the emitter depletion region

dx

pd

dx

pd

D

D

dx

pddx

pdqD

dx

pdqD

E

E

EE

E

EE

EE

EE

2

1

22

1

21

22

11

Continuity of hole current in emitter:


(1poly-Si; 2crystalline Si)

Emitter Gummel Number w/ Poly-Si Emitter

pEEi

EEi

E

EW

Ei

i

polyE

EEEi

EEi

E

EW

Ei

iE

SWn

WNndx

D

N

n

n

WDWn

WNnxd

D

N

n

nG

E

E

)(

)(

)(

)(

2

2

0 2

2

,2

2

0 2

2

For a uniformly doped emitter,

pE

E

iE

iEE SD

W

n

nNG

12

2

1/2

kTqV

E

iB

EBeG

AqnI

where Sp DEpoly/WEpoly is the surface recombination velocity


Emitter Band Gap Narrowing

BiE

EiBdc

Nn

Nn2

2

To achieve large dc, NE is typically very large, so that band gap narrowing is significant (ref. Lecture 3, Slide 20).

/)( /2 kTEEvc

kTEvciE

GEGGE eNNeNNn

/22 kTEiiE

GEenn EGE is negligible for NE < 1E18/cm3

N = 1018 cm-3: EG = 35 meV

N = 1019 cm-3: EG = 75 meV


Narrow Band Gap (Si1-xGex) Base

BiE

EiBdc

Nn

Nn2

2

To improve dc, we can increase niB by using a base material (Si1-xGex) that has a smaller band gap

• for x = 0.2, EGB is 0.1 eV

This allows a large dc to be achieved with large NB (even >NE), which is advantageous for

• reducing base resistance• increasing Early voltage (VA)


courtesy of J.D. Cressler (GATech)

Heterojunction Bipolar Transistorsa) Uniform Ge concentration

in base

b) Linearly graded Ge concentration in base

built-in E-field


Example: Emitter Band Gap NarrowingIf DB = 3DE , WE = 3WB , NB = 1018 cm-3, and niB

2 = ni2, find dc for

(a) NE = 1019 cm-3, (b) NE = 1020 cm-3, and (c) NE = 1019 cm-3 and a Si1-xGex base with EGB = 60 meV

(a) For NE = 1019 cm-3, EGE 35 meV

(b) For NE = 1020cm-3, EgE 160 meV:

(c)

226/3522 8.3 imeVmeV

iiE nenn

6.238.310

109

218

219

2

2

i

i

iEB

iE

BE

EBdc n

n

nN

nN

WD

WD

226/16022 470 imeVmeV

iiE nenn

226/602/22 10 imeVmeV

ikTE

iiB nenenn gB 236F

9.147010

109

218

220

2

2

i

i

iEB

iE

BE

EBdc n

n

nN

nN

WD

WD


Charge Control Model

B

BB

B Qi

dt

dQ

Wx

BB tptxp 1),0(),(

W

BBB

tpqAWdxtxpqAQ

0 2

),0(),(

A PNP BJT biased in the forward-active mode has excessminority-carrier charge QB stored in the quasi-neutral base:

In steady state,B

BB

B Qi

dt

dQ

0


Base Transit Time, t

2

),0( tpqAWQ B

B

Bt D

W

2

2

• time required for minority carriers to diffuse across the base • sets the switching speed limit of the transistor

t

BBBC

BB

Wx

BBC

Q

W

QDi

W

tpqAD

x

txpqADi

2

2

),0(),(


Relationship between B and t

• The time required for one minority carrier to recombine in the base is much longer than the time it takes for a minority carrier to cross the quasi-neutral base region.

tdcB

t

BC

Qi

B

BB

Qi


Built-in Base E-Field to Reducet

The base transit time can be reduced by building into the base an electric field that aids the flow of minority carriers.

1. Fixed EGB , NB decreases from emitter to collector:

2. Fixed NB , EGB decreases from emitter to collector:-E B C

-E B C

Ec

Ec

Ev

Ev

Ef

Ef


E

EXAMPLE: Drift Transistor• Given an npn BJT with W=0.1m and NB=1017cm-3

(n=800cm2/Vs), find t and estimate the base electric field required to reduce t

cmkVssVcm

cmW

tW

driftv

W

tn

n

/6102/800

10122

5

ps

sVcmV

cm

D

W

Bt 2

/800026.02

10

2 2

252


Lecture 26 OUTLINE The BJT (cont’d) Breakdown mechanisms Non-ideal effects Gummel plot & Gummel numbers Modern BJT structures Base transit time Reading:

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