Investigation of Ge content in the BC transition region ...€¦ · T. Zimmer BipAk, 18-19.10.07...

T. Zimmer BipAk, 18-19.10.07 München 1/21

Investigation of Ge content in the BC transition region with respect to transit frequency

P.M. Mans (1),(2), S. Jouan (1), A. Pakfar (1),S. Fregonese (2), F. Brossard (1), A. Perrotin (1),

C. Maneux (2), T. Zimmer (2)

(1)ST Microelectronics(2) Laboratoire IMS, Université Bordeaux 1


Outline

• Introduction• Barrier effect @ high current• SiGe and breakdown voltage• Retrograde Ge profile• Results• Discussion and outlook


Introduction (1/4)

• High Power HBT– E.g. for hand-set application– Huge market: 1 Billion of hand-sets sold in 2007– Output power stage in SiGe

• Figure of merits– fT– BVCE0, BVCB0

– PAE– P1dB


Introduction (2/4)

• Figure of merit: fT(IC) shape

• The upper curve is preferable– IC=1mA: fT1=15GHz, fT2=20GHZ– 33% improvement

0

5

10

15

20

25

30

35

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

IC(mA)

f T(G

HZ)


Introduction (3/4)

• fT(IC) and PAE and power gain– 3 different devices– 3 Ge content

From: Pan et al, BCTM 2004


Introduction (4/4)

• Why does fT drop?– the neutral-base charge, which dominates the

transit time at high current density– two dominant high-injection effects

• conventional Kirk effect• heterojunction barrier effect


Conventional Kirk effect

• Before floating the barrier

-1.0

-0.6

-0.2

0.10 0.15 0.20 0.25x (µm)

Val

ence

Ban

d (e

V)

0.75 V0.9 V1 V

E B C

-5.E+05

0.E+00

5.E+05

1.E+06

0.10 0.15 0.20 0.25x (µm)

Ele

ctric

Fie

ld (V

/cm

)

0.75 V0.9 V1 V

E B C

SiGe-Si barrier

Kirk effect

Here: Abrupt SiGe-Si barrier


Barrier effect (1/3)Valence band barrier• The transition from a narrow

gap SiGe base layer to the larger bandgap Si collector layer introduces a valence band offset at the heterointerface.

• Since this barrier is masked by the band bending in the collector-base (CB) depletion region during low-injection operation, it has negligible effect on the device characteristics.

• At high-injection, the collapse of original CB electric field at the heterointerface reveals the barrier which opposes the hole injection into the collector

-1.0

-0.6

-0.2

0.10 0.15 0.20 0.25x (µm)

Val

ence

Ban

d (e

V)

0.75 V0.9 V1 V

E B C

-5.E+05

0.E+00

5.E+05

1.E+06

0.10 0.15 0.20 0.25x (µm)

Ele

ctric

Fie

ld (V

/cm

)

0.75 V0.9 V1 V

E B C

SiGe-Si barrier


Barrier effect (2/3)• The hole pile-up that

occurs at the heterointerface induces a conduction band barrier that opposes the electron flow into the collector

• This causes an increase in the stored base charge that results in the sudden decrease of both fT and fmax

1.E+16

1.E+18

1.E+20

0.10 0.15 0.20 0.25x (µm)

h D

ensi

ty (c

m-3

)

0.75 V0.9 V1 V

E B C

0.0

0.4

0.8

1.2

0.10 0.15 0.20 0.25x (µm)

Con

duct

ion

Band

(eV)

0.75 V0.9 V0.92 V1 V


Barrier effect (3/3)

• Transit time

From Jiang et al, IEEE-ED, 2002

Kirk effectBarrier effect

Abrupt SiGe-Si barrier with different barrier distance


Only SiGe collector ?• SiGe and breakdown voltage

• The SiGe profile has to be optimized:• Tradeoff between breakdown and barrier effect• Thick SiGe layer: relaxing risk

( )( ) ( )

contentGeyeVyyEyEEE SiSiGe

:74.0=ΔΔ−=

From People et al, Appl. Phys. Lett, 1986

From Sze, “Physic of semiconductor devices”, John Wiley


Retrograde Ge profile

• E-field and multiplication factor– Varying Ge content produces an change in the valence band– This change creates a heterojunction-induced electric field

– The Ge retrograde field Er depends on the retrograde distance Dr

– Increasing Dr reduces Er and hence M-1From: G. Zhang et al. / Solid-State Electronics 46 (2002) 655–659


Retrograde profile

• Ge content and doping concentration

0%

5%

10%

15%

20%

0.00 0.05 0.10 0.15 0.20 0.25

Vertical cut along X (cf Figure 1) from emitter to collector (µm)

Ge

cont

ent (

%)

1.0E+14

1.0E+15

1.0E+16

1.0E+17

1.0E+18

1.0E+19

1.0E+20

1.0E+21

Dop

ing

conc

entr

atio

n (a

t.cm

-3)

Standard Ge profile60 nm retrograde Ge profile120 nm retrograde Ge profileDoping concentration

E B C


TCAD simulation results (1/3)

• Band energy diagram

-1

-0.5

0

0.5

1

1.5

0 0.05 0.1 0.15 0.2 0.25Depth (µm)

Ban

d En

ergy

(ev)

Standard Ge profile60 nm retrograde Ge profile120 nm retrograde Ge profile

Conduction Band

Valence Band

E B C

Band discontinuity attenuation

Jc = 1mA.µm-2



• Hole density

1.0E+15

5.0E+17

1.0E+18

1.5E+18

2.0E+18

2.5E+18

3.0E+18

3.5E+18

4.0E+18

4.5E+18

0.05 0.06 0.07

Depth (µm)

hole

Den

sity

(cm

-3)

4.0E+17

4.2E+17

4.4E+17

4.6E+17

4.8E+17

5.0E+17

Inte

grat

ed h

ole

Den

sity


B C

Integrated hole density

Jc = 1mA.µm-2



• Transit frequency

Simulated fT (JC) VCE 1.5V

0

5

10

15

20

25

30

35

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6JC (mA.µm-2)

fT (G

Hz)


AE=0.4*6.4 µm²

Std -13GHz Optimized -8GHz

0.5 mA.µm-²


Measurement results (1/2)

• Transit frequency

fT (JC) VCE 1.5V

0

5

10

15

20

25

30

35

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6JC (mA.µm-2)

fT (G

Hz)

Standard Ge profile60 nm retrograge Ge profile120 nm retrograde Ge profile

High fT operating current density widening


Measurement results (2/2)

• fT-BVCE0 productfT.BVCEO product

27

28

29

30

31

32

33

5 5.5 6 6.5 7

BVCEO (V)

f T (G

Hz)

Plotsiso-line 182 GHz.Viso-line 188iso-line 193

Collector epitaxy thickness reduction

Std

SiGe:C +120 nm

SiGe:C +60 nm

Deeper extention of retrograge Ge profile into collecteor


Conclusion

• Barrier effect analysis• Tradeoff between SiGe and BVCE0

• Ge retrograde profile in the collector – Device simulation – high injection operation– fT performance – confirmed by measurements


Outlook (1/2)

• New figure of merit to characterize the wider current range with suitable fT

• Half fT current range

• Normalized half fT

current range

( ) ( ) ( )222 maxmax

max

max

121

2

TCTCTC

fCfIfI

fI

IT −=

Δ

( ) ( )22maxmaxmax 122 TCTCfC fIfII

T−=Δ

fT (JC) VCE 1.5V

0

5

10

15

20

25

30

35

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6JC (mA.µm-2)

fT (G

Hz)

Standard Ge profile60 nm retrograge Ge profile120 nm retrograde Ge profile

High fT operating current density widening

IC1(fTmax/2) IC2 (fTmax/2)


Outlook (2/2)• Half fT current range

• The devices with optimized profile exhibit improved fTmax

and wider fT operating current density range• Best for PA applications

10.62Increase of ≈15%

10.13Increase of ≈10%

9.25Normalized half fT

current range

0.99mA0.94mA0.86mAHalf fT current range

120 nm retrograde Ge profile

60 nm retrograde Ge profile

Standard Ge profile

Investigation of Ge content in the BC transition region ...€¦ · T. Zimmer BipAk, 18-19.10.07...

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