Principles of Semiconductor Devices-L31

8/8/2019 Principles of Semiconductor Devices-L31

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Lecture 31: Heterojunction Bipolar Transistor (II)

Muhammad Ashraful Alam/Mark [email protected]

Alam ECE‐606 S09 1



Topic Map

signal

SignalDiode

Schottky

BJT/HBT

MOS




Outline

. rupt unct on s

2. Graded junction HBTs

3. Graded base HBTs

4. Double hetero unction HBTs

5. Conclusions

REF. “Heterostructure Fundamentals,” by Mark Lundstrom,


, .



Abrupt Junction HBTs

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

Rpn B

iBq J N n

e eυ −Δ⎛ ⎞= ⎜ ⎟

/ , ( 0)C B E k T

RiB

nn B E BE p B

q e N n

J J V υ −Δ→ = =⎜ ⎟

⎝ ⎠

=

Jn /

2 BE B p qV k iE

pT n

J D

q e⎛ ⎞= ⎜ ⎟

Δ E C

EC EF

E E

2

2 / C B Rp E k T E iB N en

υ β −Δ⎡ ⎤= ⎢ ⎥

i B p E E

= N DE υ Rp Δ E V / k BT

V

EV Jp N AE D p W E

4

Gain in abrupt npn BJT defined only by valence band discontinuity!



… but we are hoping for even better gain

2n ,

2,

~ / /

gt t

i E B p E B p E

en N D W N D W

→ × × ×

, / V B R p E k T E N eΔ= υ β Abrupt junction HBT B p E

For full gain, we need graded junction HBT




Outline

1. Abrupt junction HBTs

.

3. Graded base

HBTs

4. Double heterojunction HBTs

5. Conclusions






Graded Base ‐Emitter Junction

V ( x)

E C

x xP

− x N

E C

E V

G≈ . e

G . V

E V

χ ( x)

x xP− x N

E C ( x) = E 0 − χ ( x) −qV x(


V x =C x −

G x



Current Gain No exponential Suppression!

2 / V k n T iBn D⎛ ⎞

E F

n

2 / V k T Dn⎛ ⎞

AB

n

B N W

q e=⎝ ⎠

C D

p E E N W

q e=⎝ ⎠

E GE > E GB 2

E V J p β = DE

N AE

n

D p

E

W BiB

niE 2

x ni = N C N V e− E G /2 k BT

≈ N DE Dn W E eΔ E G / k BT


AE D p W B



Advantages of HBT: Inverted Base Doping

/ G B E k T DE n E DC

N D W e β Δ≈

AB p B

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 C j,BE )

“ ”


AB DE



Outline


.

3. Graded base HBTs


5. Conclusions




How to make a better Transistor

Graded Base transport2,

, 2i B E n B

poly ballistici E B s

n N D W n N

β υ

→ × ×

Heterojunction bipolar transistorPolysilicon Emitter

( ),

, ,

2, , ,

2

g B

g E g B

E E E i B C B V B

E

n N N ee

−−

−= ≈ β

β

,

, , ,i E C E V E n e




Graded Bases

E VAC

E χ ( x ) E C

E x

E V

E G( x )

E V

E F

Intrinsic Uniformly p‐doped




Graded Base HBTs

2 / BE BqV k T n Bi Dn⎛ ⎞

2 / V k T Dn⎛ ⎞

n B

n

BW N

⎝ ⎠

C F E E

p q e N W

=⎝ ⎠

2 N D W n

EVJ

2 DC B p B iE N D W n

=

2

x 2eff

B Bbn n Dτ μ = <<E


Geff

BW

=E



Outline


.

3. Graded base HBTs


5. Conclusions






Offset Voltage

IC

IB

B I C

V B B

V CE


does IC = 0 at VCE = 0?



Offset Voltage

E F

1( )

2

1 / B E Bq V V k T iB n

AB B

J n Dq e N W

−⎛ ⎞= ⎜ ⎟⎝ ⎠

2

C ( )2

/ B C Bq V V k T iB n

AB B

J n D

q e N W

−= ⎜ ⎟⎝ ⎠

2

E V

3 / B C Bq V V T piC

DC C

J q e N W

−= ⎜ ⎟⎝ ⎠

x 1 2 3C = − −


C

, E , C OS



Offset Voltage Result

V OS = k BT ln 1 +1 γ R( )2

( )( )2

23

iB AB n B

R

iC DC p C J n N W Dγ = = (Reverse Emitter injection efficiency)

Want a large γ R for small Vos . Wide bandgap collector helps.




Outline


.

3. Graded base HBTs


5. Conclusions: modern design




Putting the Terms Together

Collector transit time

10log T f K r urrent

Base transit

time

21

2 2 2

B BC

T n sat

W W

f D

= + +⎢ ⎥

⎣ ⎦π υ

, , B

j BC j BE C

k T C C

qI ⎡ ⎤+⎣ ⎦

10log C I K junction charging time …


Quations: Why does HBTs have such high performance ?



Epitaxial Layer Design (II)

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




Epitaxial Layer Design (III)




Summary

1) The use of a wide bandgap emitter has two benefits:‐allows heavy base doping‐a ows mo erate emitter oping

2) The use of a wide bandgap collector has benefits:‐‐reduced charge storage in saturation‐reduced collector offset voltage‐hi her collector breakdown volta e

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.

Alam ECE

‐606

S09

24However, it has yield issues and heating and contact R problems.

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Principles of Semiconductor Devices-L31