Principles of Semiconductor Devices-L29

8/8/2019 Principles of Semiconductor Devices-L29

1/26

www.nanohub.org

NCN

Lecture29:BJTDesign(II)[email protected]

Alam ECE606S09 1


2/26

Outline

1) Problemsof

classical

transistor

2) PolySiemitter

3) Shortbasetransport

4 Hi hfre uenc

res onse

5) Conclusions

REF:SDF,Chapter11and12

Alam ECE606S09 2


3/26

TopicMap

signal

Signal

Diode

Schottky

BJT/HBT

MOS

Alam ECE606S09 3


4/26

DopingforGain

,2

,

i Bn E Edc

B p i E B

n D W N

W D n N

m tter op ng: s g asposs ew t outbandgapnarrowing

NE

,currentcrowding,Earlyeffect

NB

withoutKirkEffect

BaseWidth:AsthinaspossiblewithoutNC punchthrough

Alam ECE606S09 4


5/26

HowtomakebetterTransistor

2

,

2,

i Bn E E

B p i E BW D n N

GradedBasetransport

ClassicalShockleyTransistorPolysiliconEmitterHeterojunction

Bipolar

Transistor

Alam ECE606S09 5


6/26

PolysiliconEmitter

CollectorEmitterBase

N+

N+P+

N

N

Dielectric trenchP

N+

P N

Pol silicon

Alam ECE606S09 6


7/26

PolysiliconEmitter

( )2

,

, 1 1 1BEi B qVn

n E

nqD I n n e

W N=

N+

P

, , 2 1

p E

p E p

p

o y

s

s

sE

lID

q p qpW

D W

= =

+

p2p1

vs

1 2p

I Dp

= =

( ), , 1p p si EEI Dq W p=

, , p E poly sI

WE

, , p po s

E

y p

W

2 p ED Wp

=

, , p E si p sEDI W +

Alam ECE606S09 7

1 p E sDp W +

the

hole

current,

not

electron

current?


8/26

GaininPolysiliconTransistor

, , 1 , , p E poly p B pol

s

s

p E

E

y

p

D WI p

Wq I

D

= =

+

( ), , 1p p si EEI Dq W p=

, , ,

, , ,

p B poly B poly

p B s p BE

s

ssi iDI IW

=

+

,

, ,, B si

B siC Cpoly

B poly B polyI

II I

I I

= =

2

,

2

,

i Bn E E

B p i E B s

p sEn D W N

W D n N

D W

+

2

,

2( )

1s

i Bn Ep E

nD ND W


9/26

Outline

1) Problems

of

classical

transistor2) PolySiemitter

3) Shortbasetransport4 Hi h

fre uenc

res onse

5) Conclusions

Alam ECE606S09 9


10/26

HowtomakebetterTransistor

2 2n n, ,

2 2, ,

n E E n E

B p i B B B i E B sW D n N W n N

Graded Base trans ort

PolysiliconEmitter

ClassicalShockleytransistor

Heterojunctionbipolar

transistor

Alam ECE606S09 10


11/26

ShortbaseQuasiballisticTransistor

1 2, 2thE nn

nnI qD nq

= =

B

D Wn n E ballisticI

1 n thBD Wn

=+ , ,n E s n hi tBD WI

=+

nn1

thn2

N+

P

Alam ECE606S09 11


12/26

Gaininshortbase PolysiliconTransistor

, , , p B poly B polysI I , ,n E ballistic thI

, , , p B s p BE ssi iDI IW + , ,n E s thi n B I D W +

, , , ,

,

, , , ,

C ballistic C ballistic C si B si

poly ballistic

B poly C si B si B poly

I I I I

I I I I = =

2

,

2

,

i Bn E E

n B B p i E

th

th

E s

s

p

B

nD W N

D W W D n N

D W

+ +

2

,

2

,

i B E

i E B s

thn N

n N

12Quasi-Ballistic transport in very short base limits the gain


13/26

Outline

1) Problemsofclassicaltransistor

2) PolySiemitter

3 Shortbase

trans ort

4) Highfrequencyresponse

Alam ECE606S09 13


14/26

TopicMap

signal

SignalDiode

Schottky

BJT/HBT

MOS

Alam ECE606S09 14


15/26

SmallSignalResponse

log10

P+

C

BIC

IB N

PVEB(in)

EC

(out)DC

10

fTf

21 B BC BW W k T C C

= + + +

Alam ECE606S09 15

, ,

T n sat C q


16/26

SmallSignalResponse(CommonEmitter)

C

C

C

IRF F R RI I P+

N

B

V

ICIB

B

( )1 F FI gmVBEP

E E

VEB(in)

(out)

( )11 F F

BE

B B

BE B

Idd q

r dV dV T

I

k

I

=

= =1 C

DC B

qI

k T=

( )/0 1BEqV kT

F F I I e= ( )F C

BE B

mFd qI

dV Tg

I

k

= =

Alam ECE606S09 16

( )F F B BEmm Eg VI g = =


17/26

ShortCircuitCurrentGain

m BE CBCj Cgi += =

1 BE BE BC

B

j C j C

i

r

+ +

C CB

( )

( )

1

1

T

T mT

T

m j gf

j C C j C j

g

C

C

=

++ +

r

m BEg

r

1 1 B BC k T k T C +

, , , ,2 j BC jT BE d Bm C C C d BE T gf qI qI

,d BCB BCk T dQ

Alam ECE606S09 17

,d BC

C C BE C qI dI dV dI


18/26

BaseTransitTime

n n1Ref.Chargecontrolmodel

N+

P

B BdQ Q

=21

2B

B

q n WW

= =C C 1 n

B

qW

Alam ECE606S09 18


19/26

CollectorTransitTime

BCW

N+

P

sat

t

1

2qi =

,BC

eff BC

q W = = =

Alam ECE606S09 19

sat


20/26

PuttingtheTermsTogether

Collectortransittime(slide19)

ase rans me(slide18)10

log TfK r urrent

21

2 2 2

B BC

T n sat

W W

f D

= + +

, ,B

j BC j BE

C

k TC C

qI +

Junctionchargingtime(slide17)10log CIKI

Alam ECE606S09 20

oyouseet emot vat ontore uce an BC asmuc asposs e

Whatproblemwouldyoufaceifyoupushthistoofar?


21/26

HighFrequencyMetrics

2

(currentgain

cutoff

frequency,

fT)

( ) ( ), ,2 2 2

B BC B j BE j BC

T n sat C

ex c cbC C R

f DR C

I

= = + + ++ +

powerga ncu o requency, max

fmax =fT

bb cbi

Alam ECE606S09 21


22/26

Summary

WehavediscussedvariousmodificationsoftheclassicalBJTs

andex lainedwh im rovementof erformancehasbecome

sodifficult

in

recent

years.

Thesmallsignalanalysisillustratestheimportanceofreduced

junctioncapacitance,

resistances,

and

transit

times.

Classicalhomojunctions BJTscanonlygosofar,further

transistors.

Alam ECE606S09 22


23/26

Aside:

OnBaseCollector Breakdown

o ages

Alam ECE606S09 23


24/26

EssenceofCurrentGain

P+

C

IC

N

PVEB(in)

VEC(out)

B

VBCVBE

E E

Input Response Input

( )2

,1BE

p i E q

E

B

V

E

qD ne

NI

W ( )

2,

1BEi B

B

V

E

qn

B

nqDe

NI

W

Alam ECE606S09 24Response


25/26

CollectorBreakdown(CommonBase,FixedIE)

+I

VBEP

VEB

(in)

VCB

(out)IB

CommonBase

(IE fixed,IB variable)

Alam ECE606S09 25


26/26

CollectorBreakdown(CommonEmitter) C

P+

N

P

B

VEB

VEC(out)

CIB

BE

E E

n

CommonEmitter

E var a e, B xe

Common emitter breakdown voltage is smaller

Alam ECE606S09 26

an common ase rea own vo age.

Documents

Principles of Semiconductor Devices-L29