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

Transcription

1 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, München 1/21

2 Outline Introduction Barrier high current SiGe and breakdown voltage Retrograde Ge profile Results Discussion and outlook T. Zimmer BipAk, München 2/21

3 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 f T BV CE0, BV CB0 PAE P 1dB T. Zimmer BipAk, München 3/21

4 Introduction (2/4) Figure of merit: f T (I C ) shape ft(ghz) IC(mA) The upper curve is preferable I C =1mA: f T1 =15GHz, f T2 =20GHZ 33% improvement T. Zimmer BipAk, München 4/21

5 Introduction (3/4) f T (I C ) and PAE and power gain 3 different devices 3 Ge content From: Pan et al, BCTM 2004 T. Zimmer BipAk, München 5/21

6 Introduction (4/4) Why does f T 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, München 6/21

7 Conventional Kirk effect Before floating the barrier 1.E+06 Electric Field (V/cm) 5.E+05 0.E+00-5.E+05 E B C Kirk effect 0.75 V 0.9 V 1 V SiGe-Si barrier Valence Band (ev) E B C 0.75 V 0.9 V 1 V x (µm) x (µm) Here: Abrupt SiGe-Si barrier T. Zimmer BipAk, München 7/21

8 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 Electric Field (V/cm) 1.E+06 5.E+05 0.E+00-5.E+05 Valence Band (ev) E B C E B C 0.75 V 0.9 V 1 V SiGe-Si barrier x (µm) 0.75 V 0.9 V 1 V T. Zimmer BipAk, München x (µm) 8/

9 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 f T and f max h Density (cm-3) 1.E+20 1.E+18 1.E+16 Conduction Band (ev) E B C 0.75 V 0.9 V 1 V x (µm) V 0.9 V 0.92 V 1 V x (µm) T. Zimmer BipAk, München 9/21

10 Barrier effect (3/3) Transit time Abrupt SiGe-Si barrier with different barrier distance From Jiang et al, IEEE-ED, 2002 Barrier effect Kirk effect T. Zimmer BipAk, München 10/21

11 Only SiGe collector? SiGe and breakdown voltage E ΔE = ESi ΔE( y) ( y) = 0.74 y ( ev ) SiGe y : Ge content From People et al, Appl. Phys. Lett, 1986 From Sze, Physic of semiconductor devices, John Wiley The SiGe profile has to be optimized: Tradeoff between breakdown and barrier effect Thick SiGe layer: relaxing risk T. Zimmer BipAk, München 11/21

12 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 E r depends on the retrograde distance D r Increasing D r reduces E r and hence M-1 From: G. Zhang et al. / Solid-State Electronics 46 (2002) T. Zimmer BipAk, München 12/21

13 Retrograde profile Ge content and doping concentration Ge content (%) 20% 15% 10% 5% E B C Standard Ge profile 60 nm retrograde Ge profile 120 nm retrograde Ge profile Doping concentration 1.0E E E E E E E+15 Doping concentration (at.cm -3 ) 0% Vertical cut along X (cf Figure 1) from emitter to collector (µm) 1.0E+14 T. Zimmer BipAk, München 13/21

14 TCAD simulation results (1/3) Band energy diagram Band Energy (ev) E B C Valence Band Band discontinuity attenuation Standard Ge profile 60 nm retrograde Ge profile 120 nm retrograde Ge profile Conduction Band J c = 1mA.µm Depth (µm) T. Zimmer BipAk, München 14/21

15 TCAD simulation results (2/3) Hole density Standard Ge profile 5.0E E nm retrograde Ge profile hole Density (cm -3 ) 4.0E E E E E E E nm retrograde Ge profile B C Integrated hole density J c = 1mA.µm E E E E+17 Integrated hole Density 5.0E E E Depth (µm) T. Zimmer BipAk, München 15/21

16 TCAD simulation results (3/3) Transit frequency ft (GHz) A E =0.4*6.4 µm² Simulated f T (J C ) V CE 1.5V Std -13GHz 0.5 ma.µm -² Standard Ge profile 60 nm retrograde Ge profile 120 nm retrograde Ge profile Optimized -8GHz J C (ma.µm -2 ) T. Zimmer BipAk, München 16/21

17 Measurement results (1/2) Transit frequency ft (GH z) f T (J C ) V CE 1.5V Standard Ge profile 60 nm retrograge Ge profile 120 nm retrograde Ge profile High f T operating current density widening J C (ma.µm -2 ) T. Zimmer BipAk, München 17/21

18 Measurement results (2/2) f T -BV CE0 product f T.BV CEO product Deeper extention of retrograge Ge profile into collecteor Collector epitaxy thickness reduction ft (GHz) Plots iso-line 182 GHz.V iso-line 188 iso-line 193 SiGe:C +120 nm SiGe:C +60 nm BV CEO (V) Std T. Zimmer BipAk, München 18/21

19 Conclusion Barrier effect analysis Tradeoff between SiGe and BV CE0 Ge retrograde profile in the collector Device simulation high injection operation f T performance confirmed by measurements T. Zimmer BipAk, München 19/21

20 Outlook (1/2) New figure of merit to characterize the wider current range with suitable f T Half f T current range ΔI = I T Normalized half f T current range ΔI I C C1 C f Tmax ( f ) T max f max 2 = I C 2 ( f ) I ( f 2) max ( f 2) I ( f 2) T 2 C1 max 2 C 2 T T C1 max max 2 T ft (GH z) f T (J C ) V CE 1.5V Standard Ge profile 60 nm retrograge Ge profile 120 nm retrograde Ge profile High f T operating current density widening J C (ma.µm -2 ) I C1 (f Tmax /2) I C2 (f Tmax /2) T. Zimmer BipAk, München 20/21

21 Outlook (2/2) Half f T current range Standard Ge profile 60 nm retrograde Ge profile 120 nm retrograde Ge profile Half f T current range 0.86mA 0.94mA 0.99mA Normalized half f T current range Increase of 10% Increase of 15% The devices with optimized profile exhibit improved f Tmax and wider f T operating current density range Best for PA applications T. Zimmer BipAk, München 21/21