investigation of ge content in the bc transition region ...€¦ · t. zimmer bipak, 18-19.10.07...
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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
T. Zimmer BipAk, 18-19.10.07 München 2/21
Outline
• Introduction• Barrier effect @ high current• SiGe and breakdown voltage• Retrograde Ge profile• Results• Discussion and outlook
T. Zimmer BipAk, 18-19.10.07 München 3/21
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
T. Zimmer BipAk, 18-19.10.07 München 4/21
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)
T. Zimmer BipAk, 18-19.10.07 München 5/21
Introduction (3/4)
• fT(IC) and PAE and power gain– 3 different devices– 3 Ge content
From: Pan et al, BCTM 2004
T. Zimmer BipAk, 18-19.10.07 München 6/21
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
T. Zimmer BipAk, 18-19.10.07 München 7/21
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
T. Zimmer BipAk, 18-19.10.07 München 8/21
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
T. Zimmer BipAk, 18-19.10.07 München 9/21
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
T. Zimmer BipAk, 18-19.10.07 München 10/21
Barrier effect (3/3)
• Transit time
From Jiang et al, IEEE-ED, 2002
Kirk effectBarrier effect
Abrupt SiGe-Si barrier with different barrier distance
T. Zimmer BipAk, 18-19.10.07 München 11/21
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
T. Zimmer BipAk, 18-19.10.07 München 12/21
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
T. Zimmer BipAk, 18-19.10.07 München 13/21
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
T. Zimmer BipAk, 18-19.10.07 München 14/21
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
T. Zimmer BipAk, 18-19.10.07 München 15/21
TCAD simulation results (2/3)
• 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
Standard Ge profile60 nm retrograde Ge profile120 nm retrograde Ge profile
B C
Integrated hole density
Jc = 1mA.µm-2
T. Zimmer BipAk, 18-19.10.07 München 16/21
TCAD simulation results (3/3)
• 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)
Standard Ge profile60 nm retrograde Ge profile120 nm retrograde Ge profile
AE=0.4*6.4 µm²
Std -13GHz Optimized -8GHz
0.5 mA.µm-²
T. Zimmer BipAk, 18-19.10.07 München 17/21
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
T. Zimmer BipAk, 18-19.10.07 München 18/21
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
T. Zimmer BipAk, 18-19.10.07 München 19/21
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
T. Zimmer BipAk, 18-19.10.07 München 20/21
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)
T. Zimmer BipAk, 18-19.10.07 München 21/21
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