ece606: solid state devices lecture 20 ...ee606/downloads/ece606_f...ece606: solid state devices...

Klimeck – ECE606 Fall 2012 – notes adopted from Alam

ECE606: Solid State DevicesLecture 20

Heterojunction Bipolar Transistor

Gerhard [email protected]

1


Outline

2

1. Introduction2. Equilibrium solution for heterojunction3. Types of heterojunctions4. Intermediate Summary5. Abrupt junction HBTs6. Graded junction HBTs7. Graded base HBTs8. Double heterojunction HBTs9. Conclusions

“Heterostructure Fundamentals,” by Mark Lundstrom, Purdue University, 1995.

Herbert Kroemer, “Heterostructure bipolar transistors and integrated circuits,” Proc. IEEE , 70, pp. 13-25, 1982.


How to make a better Transistor

3

( ),

,

,

,

2,

, ,

,

2

,

,g B

gg E

g E

BC B V B

C

EEi B

EE Vi E E

EN N ee

N Nn

n

e

ββ

β

−−

−= ≈

Heterojunction bipolar transistor

Graded Base transport

Polysilicon Emitter

Emitter bandgap > Base Bandgap

2,

, 2,

i B

i

E thpoly ballis ic

sEt

B

Nn

Nn

υβυ

→ × ×


Heterojunction Bipolar Transistors

4

nemitter

ncollector n+

EG1>EG2

p+base

EG2 EG2

i) Wide gap Emitter HBT

ii) Double Heterojunction Bipolar Transistor

nemitter

ncollector n+

EG1>EG2

p+base

EG2 EG3>EG2


Mesa HBTs

5

nemitter

ncollector n+

EG1>EG2

p+base

EG2 EG3>EG2

p+ basen-collector

n+

semi-insulating substrate

nMesa HBT

Intentional traps, Fermi level pinnedLow conductanceLow capacitanceHigh speed


Applications

6

1) Optical fiber communications-40Gb/s…….160Gb/s

2) Wideband, high-resolution DA/AD convertersand digital frequency synthesizers

-military radar and communications

3) Monolithic, millimeter-wave IC’s (MMIC’s)

-front ends for receivers and transmitters

future need for transistors with 1 THz power-gain cutoff freq.


Background

7

A heterojunction bipolar transistor

Schokley realized that HBT is possible, but Kroemer really provided the foundation of the field and worked out the details.

Kroemer


Outline

8





Topic Map

9

Equi libr ium DC Small signal

Large Signal

Circuits

Diode

Schottky

BJT/HBT

MOS


Bandgaps and Lattice Matching

10


Band Diagram at Equilibrium

11

1P P P

pr g

t q

∂ = ∇ • − +∂

J

( )D AD q p n N N+ −∇ • = − + −

P P Pqp E qD pµ= − ∇J

1N N N

nr g

t q

∂ = ∇ • − +∂

J

J µ= + ∇N N Nqn E qD n

Equilibrium

DC dn/dt=0Small signal dn/dt ~ jωtn

Transient --- Charge control model


N-Al0.3Ga0.7As: p-GaAs(Type-I Heterojunction)

12

Abrupt junction HBT

Vacuum Level

EC

EV

EF

χ2

χ1

EG≈ 1.80 eV

EG≈ 1.42 eV

ND NA


Built-in Potential: Boundary Condition @Infinity

13

qVbi

∆1 ∆2

Eg,2

EC

EV

EF

χ2

χ1

21 2 21 gi ,bqV E+ + = − ∆∆ + χχ

2 2 1 2 1bi g ,qV E ∆ ∆= − − + −χ χ

( )2

2 1

12g , B

A DB E / k T

V , C ,

N Nk T ln

N N e−= + −χ χ


Interface Boundary Conditions

14

( ) ( )0 0D D− +=

Position

E-field

xn xp

( ) ( )0 0

0

2

2

1

00

10

0 0E E

dV dV

dx dx

κ

κ

ε

ε

κ

κ

ε

ε− +

− +=

=

D is not continuous if there is a surface charge

Displacement is continuous


Analytical Solution for Heterojunctions

15

Charge

xpxn x

Potential

x

E-field

x

( ) ( )2

,

2

0,0

0 0

2 2

2 2

n p

bi

pn

s B

D A

E s

E x E xV

q xq N

kk

xN

ε ε

− +

= +

= +

( )

( ), 0

, 0

0

0 A

s B

D

s E

D

n

n A

p

p

N

q xE

q xE

x x

N

k

k

NN

ε

ε

−

+

=

=

⇒ =Charge continuity (independent of k)


Base Emitter Depletion Region

16

ND NA

,

22,,

0 0,2 2s E

B p BEE n BEbi

s B

qN xqN xV

ε κ εκ= +

, ,E n BE B p BEN x N x=( )

,

,,

0 ,2 s BBn bi

E B

s E

sE EBs

Nx V

q N N N

εκ κ

κκ=

+

xn

( ),

,,

0 ,2 s EBp bi

B B

s E

sE EBs

Nx V

q N N N

εκ κ

κκ=

+

xp


Outline

17





N-Al0.3Ga0.7As: p-GaAs(Type-I Heterojunction)

18

Abrupt junction HBT

Vacuum Level

EC

EV

EF

χ2

χ1

EG≈ 1.80 eV

EG≈ 1.42 eV

ND NA


P-Al0.3Ga0.7As : n-GaAs (Type I junctions)

Alam ECE-606 S09 19

E0Vacuum level

χ1

EC

EV

EC

EV

EG≈ 1.80 eV

EG≈ 1.42 eV

χ2

V

jP

V

jn

∆EV

qV (x)

El

qVBI

∆EC

EF

Depletion layerDepletion layer


N-p vs P-n of Type I Heterostructure

20


(AlInAs/InP) Type II Junctions

21

ND NA

Vacuum level

EC

EV

EF

χ2

χ1


N-Al0.3Ga0.7As : n-GaAs Junctions

22

EC

EV

EC

EV

EG≈ 1.80 eV

EG≈ 1.42 eV

‘Isotype Heterojunction’

EV

Accumulation LayerDepletion Layer

Metal-Metal junctions have similar features … different workfunctions => different band lineups

thermoelectric coolers , electrons traveling from one side to the other

can gain and lose energy


P-GaSb : n-InAs (Type III)

23

EC

EV

EC

EV

E0 Field-free vacuum level

χ1

χ2

EFP

EG≈ 0.72 eV

EFn

EG≈ 0.36 eV


P-GaSb : n-InAs (Type III)

24

EC

EV

EC

EV

∆EC

= 0.87 eV

EF EG

≈ 0.72 eV

EG≈ 0.36 eV

Accumulation Layer!Accumulation Layer!


Conclusion

25

1. Heterojunction transistors offer a solution to the limitations of poly-Si bipolar transistors.

2. Equilibrium solutions for HBTs are very similar to those of normal BJTs.

3. Depending on the alignment, there could be different types of heterojuctions. Each has different usage.

4. We will discuss current transport in HBTs in the next section.


Outline

26





Abrupt Junction HBTs

27

EC EF

EV

Jn

Jp

/2

BE Bp qV k

E E

iEp

TnJ

Dq e

N W

=

( )2

2/C B

i

Rp E k TE

B p E

iB

E

Ne

n

n

N D W

υβ −∆

=

2/ /C B BE BE k T qV k T

RpnB

iBqJN

ne eυ −∆

=

β =NDE

NAE

υRp

Dp WE( )e∆EV /kBT

∆EV

Gain in abrupt npn BJT defined only by valence band

discontinuity!

2/

, ( 0)C BE k TR

iBnn B E BEp

B

q eN

nJJ Vυ −∆

→

= =

=

∆EC

( ),

,

,

,

2,

, ,

,

2

,

,g B

gg E

g E

BC B V B

C

EEi B

EE Vi E E

EN N ee

N Nn

n

e

ββ

β

−−

−= ≈


… but we are hoping for even better gain

28

( )2,

2,

~/ /

gEi B E th E th

i E B p E B p E

n N Ne

n N D W N D W

∆→ × × × βυ υβ

( ), /V BR p E k TE

B p E

Ne

N D W∆=

υβ Abrupt junction HBT

For full gain, we need graded junction HBT


Outline

29





Abrupt Junction

30

EC

EC

EV

EG≈ 1.80 eV

V

jN V

jp

∆EV

EG≈ 1.42 eV

EV

V (x)

x xP −x

N

χ(x)

x xP −x

N E

C(x) = E

0− χ(x)−qV x( )

EV(x) = E

C(x)− E

G(x)


Graded Base-Emitter Junction

31

EC

EC

EV

EG≈ 1.80 eV ∆E

V

EG≈ 1.42 eV

EV

V (x)

x xP −x

N

χ(x)

x xP −x

N E

C(x) = E

0− χ(x)−qV x( )

EV(x) = E

C(x)− E

G(x)


Current Gain

32

EC

EF

EV

x

Jn

Jp

2/BE Bp qV k Ti

Dp

E

E E

Dn

N WJ q e

=

EGE > EGB

β =NDE

NAE

Dn

Dp

WE

WB

niB2

niE2

ni = NCNV e−EG /2kBT

β ≈NDE

NAE

Dn

Dp

WE

WB

e∆EG /kBT

2/BE BqV kn

ABn

TiB

B

n D

N WJ q e

=

No exponential Suppression!


Advantages of HBT: Inverted Base Doping

33

/G BE k TDE n EDC

AB p B

N D We

N D Wβ ∆≈

1) Thin Base for high speed

2) Very heavily doped Base to prevent Punch Through,reduce Early effect, and to lower Rex

3) Moderately doped Emitter (lower Cj,BE)

“inverted base doping” NAB >> NDE


Outline

34





How to make a better Transistor

35

( ),

, ,

,

2, , ,

2, , ,

g B

g E g B

g E

EE Ei B C B V B

Ei E C E V E

n N N ee

n N N e

−−

−= ≈β

ββ

Heterojunction bipolar transistor

Graded Base transport

Polysilicon Emitter

2,

, 2,

i B E n Bpoly ballistic

i E B s

n N D W

n Nβ

υ→ × ×


Graded Bases

36

EC

EV

χ(x)

EVAC

EG(x)

Intrinsiccompositionally graded

EC

EV

EG(x)EF

Uniformly p-dopedcompositionally graded


Graded Base HBTs

37

EC

EV

x

Jp

EF2

/BE Bp qV k TiE

E EpJ

Dnq e

N W

=

2

2E n E

DCB

i

p B iE

BN D W n

N D W nβ =

2

2eff

B Bb

n n

W W

Dτ

µ= <<

E

Geff

B

E q

W

∆=E

Jn2

/BE BqV k Tn

Bn

B

B

iJD

q eW

n

N

=


Outline

38





Double HBJT

39

EC

EF

EV

x

Jn

Jp

EGE > EGBEGC > EGE

• Symmetrical operation(simplify circuiots)

• No charge storage whenthe b-c junction is forwardbiased(ni is smaller)

• Reduced collector offsetvoltage

• Higher collectorbreakdown voltage(higher gap)


Offset Voltage

40

IC

IB

VCE

does I C = 0 at VCE = 0?

C

E

B ICVB

I B


Offset Voltage

41

EC

EF

EV

x

J1( )

2

1/B E Bq V V k TiB n

AB B

Jn D

q eN W

− =

J2

( )2

2/B C Bq V V k TiB n

AB B

Jn D

q eN W

− =

J3

( )2

3/B C Bq V V k TpiC

DC C

JDn

q eN W

− =

1 2 3C J JJ J= − −

set JC = 0, assume VE = 0, solve for VC=VOS


Offset Voltage Result

42

VOS =kBT

qln 1+ 1 γ R( )

( )( )( )( )

2

2

23

iB AB n B

R

iC DC p C

n N D

J

W

n N W

J

Dγ = = (Reverse Emitter injection efficiency)

Want a large γR for small Vos. Wide bandgap collector helps.


Outline

43

1. Introduction2. Equilibrium solution for heterojunction3. Types of heterojunctions4. Intermediate Summary5. Abrupt junction HBTs6. Graded junction HBTs7. Graded base HBTs8. Double heterojunction HBTs9. Conclusions – modern design




Putting the Terms Together

44

2

, ,

1

2 2 2B BC

T n sat

Bj BC j BE

C

W W

f D

k TC C

qI

= + +

+

π υ

10log Tf

10log CI

Kirk Current

Quations: Why does HBTs have such high performance ?

KI

Base transit time

Collector transit time

junction charging time …


Epitaxial Layer Design (II)

45

DHBT: Abrupt InP emitter, InGaAs base, InAlGaAs C/B grades


Epitaxial Layer Design (III)

46


Summary

47

1) The use of a wide bandgap emitter has two benefits:-allows heavy base doping-allows moderate emitter doping

2) The use of a wide bandgap collector has benefits:

-symmetrical device-reduced charge storage in saturation-reduced collector offset voltage-higher collector breakdown voltage

3) Bandgap engineering has potential benefits:

-heterojunction launching ramps-compositionally graded bases-elimination of band spikes

4) HBTs have the potential for THz cutoff frequencies. However, it has yield issues and heating and contact R problems.


Outline from previous lecture

48

1) Current gain in BJTs

2) Considerations for base doping

3) Considerations for collector doping

4) Intermediate Summary

5) Problems of classical transistor

6) Poly-Si emitter

7) Short base transport

8) High frequency response

9) Conclusions

REF: SDF, Chapter 10


Topic Map

49

Equilibrium DC Small signal

Large Signal

Circuits

Diode

Schottky

BJT/HBT

MOS


Small Signal Response

50

P+

N

P

C

E E

B

VEB(in)

VEC(out)

ICIB

log10 β

log10 f

fT

1

fβ

2

, ,

1

2 2 2B BC B

j BC j BET n sat C

W W k TC C

f D qI

= + + +

π υ

β ω( ) ≈ βDC

ωβ

ω

Desire high fT⇒High IC⇒Low capacitances⇒Low widths


Frequency Response

51

� The gain of an amplifier is affected by the capacitance associated with its circuit.� This capacitance reduces the gain in both the low and high frequency ranges of

operation.� The reduction of gain in the low frequency band is due to the coupling

and bypass capacitors selected. They are essentially short circuits in themid and high bands.

� The reduction of gain in the high frequency band is due to the internalcapacitance of the amplifying device, e.g., BJT, FET, etc.

� This capacitance is represented by capacitors in the small signal equivalent circuit for these devices. They are essentially open circuits in the low and mid bands.


Small Signal Response (Common Emitter)From Ebers Moll Model

52

Cµ

( )1 F FIα−

Cπ

IRF F R RI Iα α−

( )/0 1BEqV kT

F FI I e= −

( )11 F F

BE

B B

BE B

Idd q

r dV dV T

I

k

I

π

α =−

= =

( )F C

BE Bm

Fd qI

dV Tg

I

k

α= =

( )F F B BEmm Eg VI gδ δα υ= =

P+

N

P

C

E E

B

VEB(in)

VEC(out)

ICIB

IB

1 C

DC B

qI

k Tβ=

DC Circuit => AC small signal CircuitBC In reverse bias, IR=0

Cµ

Cπrππππ gmVBE

IBIC

IE


Short Circuit Current Gain

53

( )( )1

1T

T mT

T

m j gf

j C Cj C j

g

Cr

C

π µπ µ

π

µωβ

ω ω ω−

≡ = ≈ +

+ +

( )1

m BE CB

BE BE BCB

Cj

f

j C j

C

Ci

r

gi

π µπ

µω υβ

υ ω υ ω

υ

υ

+= =

+ +

Cπrπ

m BEg υ

Cµ

( ) ( ), , , ,

1 1

2 j BC jT

BBE d B

B

m C CC d BE

T

C k T k T

g

CC C C C

f qI qIπ µ

πω+

≡ = = + + +

,,

d BCB Bd BC

C C BE C

Ck T dQC

qI dI dV dI= =

E

CB



54

( )( )1

1T

T mT

T

m j gf

j C Cj C j

g

Cr

C

π µπ µ

π

µωβ

ω ω ω−

≡ = ≈ +

+ +

Cπrπ

m BEg υ

Cµ

( ) ( ), , , ,

1 1

2 j BC j

T

BBE d B

B

m C C

C d BE

T

C k T k T

g

CC C C C

f qI qIπ µ

πω+

≡ = = + + +

,,

d BCB Bd BC

C C BE C

Ck T dQC

qI dI dV dI= =

E

CB



55

Cπrπ

m BEg υ

Cµ

( ) ( ), , , ,

1 1

2 j BC j

T

BBE d B

B

m C C

C d BE

T

C k T k T

g

CC C C C

f qI qIπ µ

πω+

≡ = = + + +

,,

d BCB Bd BC

C C BE C

Ck T dQC

qI dI dV dI= =

E

CB


Base Transit Time

56

B B

C C

dQ Q

dI I=

N+ P

∆n n1

21

1

12

2

BB

n

B

q nW Wn Dq

W

= =

Ref. Charge control model

Base transit time


Collector Transit Time

57

?BC

sat

W=τυ

N+ P

1

2qi τ× × =

, 2 2BC

eff BCsat

q W

i

ττυ

= = =

t

i τ

Electrons injected into collector depletion region – very high fields more than diffusion => drift => acceleration of carriersCharge imaged in collector


Putting the Terms Together

58

2

, ,

1

2 2 2B BC

T n sat

Bj BC j BE

C

W W

f D

k TC C

qI

= + +

+

π υ

Base transit time

Collector transit time

Junction charging time

10log Tf

10log CI

Kirk Current

Do you see the motivation to reduce WB and WBC as much as possible? What problem would you face if you push this too far ?

KI

Increasing IC too high reduces WBC and increases the overall capacitance=> frequency rolls off….


High Frequency Metrics

59

( ) ( )2

, ,

1

2 2 2B BC B

j BE j BCT n sat C

ex c cb

W W k T qC C R

f DR C

Iτ

π υ= = + + ++ +

(current-gain cutoff frequency, fT)

(power-gain cutoff frequency, fmax)

fmax =fT

8π RbbCcbi


Summary

60

We have discussed various modifications of the classical

BJTs and explained why improvement of performance

has become so difficult in recent years.

The small signal analysis illustrates the importance of

reduced junction capacitance, resistances, and transit

times.

Classical homojunctions BJTs can only go so far, further

improvement is possible with heterojunction bipolar

transistors.

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