Lecture #26

EE130 Lecture 26, Slide 1Spring 2007

Lecture #26

OUTLINE

• Modern BJT Structures– Poly-Si emitter– Heterojunction bipolar transistor (HBT)

• Charge control model

• Base transit time

Reading: Finish Chapter 11, 12.2


• 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

B E C

p+ p+ P base

N collector

N+ subcollector

P substrate

N+ polySi

N+

Deeptrench

Deep trench

Shallowtrench

P+polySiP+polySi

Modern BJT Structure


• 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

Polycrystalline-Silicon (Poly-Si) Emitter

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

Si)2 Si;-poly(1

Continuity of hole current in emitter


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 (Lecture 8, Slide 5) is significant.

/)( /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 (SiGe) 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 0.1eV

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

• reducing base resistance• increasing Early voltage (VA)


If DB = 3DE , WE = 3WB , NB = 1018 cm-3, and niB2 = ni

2, 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) At NE = 1019 cm-3, EgE 35 meV

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

(c)

226/352/22 8.3 imeVmeV

ikTE

iiE nenenn gE

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

EXAMPLE: Emitter Band Gap Narrowing

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


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

Base Transit Time, t

t

BBBC

BB

Wx

BBC

Q

W

QDi

W

tpqAD

x

txpqADi

2

2

),0(),(


Relationship between B and t

tdcB

t

BC

Qi

B

BB

Qi

• 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.


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

• Fixed EgB , NB decreases from emitter end to collector end.

• Fixed NB , EgB decreases from emitter end to collector end.-E B C

-E B C

Ec

dx

dE

qC1E

Ec

Ev

Ev

Ef

Ef

Drift Transistor: Built-in Base Field


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

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Lecture #26