NASA Technical Reports Server (NTRS) 20000062854: Device Physics Analysis of Parasitic Conduction Band Barrier Formation in SiGe HBTs

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75
This isapreprint or reprint of a paper intended for presentation at a conference.
Because changes may be m_d,ebefore formal publication, this ismade available with the
understanding that itwill not be cited or reproduced without the permission of the author.
h .
Device Physics Analysis of
Parasitic Conduction Band Barrier Formation in SiGe HBTs
K. P. Roenker* and S. A. Alterovitz**
* Department of Electrical and Computer Engineering and Computer Science
University of Cincinnati, Cincinnati, Ohio 45221-0030
Tel: [phone] Fax: [phone] Email: kroenker_,ececs.uc.edu_
NASA Glenn Research Center, Cleveland, Ohio 44135 Email: salterovitz(_grc.nasa.lov
ABSTRACT from theirnumerical simulator. Due to device oporation
at high current density, the peak field-is pushed to the
subcollector interface corresponding to the onset of base
This paper presents a physics-based model
describing the current-induced formation of a pushout. However, for SiGe HBTs, the valence band
discontinuity at the base-collector junction blocks hole
parasitic barrier in the conduction band at the base-
flow out of the base. As a result, a current-induced,
collector heterojunction in upn SiGe heterojunetion
parasitic barrierforms nearthe junction corresponding to
bipolar transistors (HBTs). Due to the valence band
a positive field region. This barrier inhibits electron
discontinuity AEv, hole injection into the collector at
injection into the collector from the base producing
the onset of base pushout is impeded, which gives rise
electron buildup at the end of the base, increa._.=dbase
to formation of a barrier to electron transport which
recombination and degradation in the current gain.
degrades the device's high frequency performance. In
this paper, we present results from an analytical
model for the height of the barrier calculated from the Electric Field (Vlcm)
device's structure as a function of the collector
junction bias and collector current density.
Wp
We+
INTRODUCTION Eb (x)_l_" _
x
In recent years SiGe heterojunction bipolar transistors
(HBTs) have been reported with high gain and impressive
Wpb E,(X)__ E,+(x)
high fIequency performance [1,2]. These devices employ
Ge in the base and compositional base grading so that a •4 -'_
Wc
hetemjunction is formed at the collector junction. While
the conduction band discontinuity is small, the presence
Figure i Electric field profile at B-C junction athigh current density
of a valence band discontinuity AEv at the junction
during formation of parasiticbarrier.
modifies the physics of electron transport across the
collector junction at high current densities. Dynamic
formation of a parasitic barrier in the conduction band Fordevelopment of thisdevice model, we note thatthe
occurs, which degrades device performance [3-5]. The electron concentration th is nearly constant in the high
effect is important for device design since transistor electric field in the base-collector space charge region
operation at high collector current densities (Jc (BC-SCR) near the subcollector due tovelocity saturation
-lmA/pm 2) is essential to achieve high gain at %. As a result, the electron concentration is given by th=
microwave frequencies. Previously, Joseph et al. [6] Jc/qv,, where Jc is the collector current density and the
term arises fi'om the need for a finite electron concen-
employed a numerical simulator and showed that a
parasitic barrier as large as 34 meV forms at current tration sufficient to carry the collector current. Substitu-
densities of---4 mA/pm 2. Song and Yuan[7] andMazhari ring this th in Poisson's equation, we integrate to get the
electric field in the depleted collector E,.(x), which varies
and Morkoc [8] have reported simple, physics-based
models to describe the formation of this parasitic barrier linearly as shown in Figure 1. On the base side of the
predicting similar and much larger barrier heights, heterojunction, we note that we have majority carrierhole
accumulation due to the valence band discontinuity AEv,
respectively. The motivation for this study is to develop
which we take into account in determining the electric
an enhanced description of the physics of this barrier
formation foruse indevice design. field in the base Eb(x). The location where the electric
field iszero in the collector coneslmnds tothe peak of the
ANALYTICAL MODEL parasiticpotential barrier.
To analyze the formation of the barrier, we initially
match the fields at the base-collector and embcollector
Shown in Figure I is aschematic profile of the electric
junctions. Subsequently, we piecewise integrate the
field assumed at the base-collector junction during
electric field across thejunction to relate ourresults to the
formation of the parasitic barrier. This profile is similar
collector doping and width and the applied junction
to that reported by Joseph et al. [6], which they derived
• b
reverse bias Vcs. Combining these results, we get a :.--s 10 3]
single equation for the potential at the base-collector
junction _(x_:), where ourzero for the potential was taken
to be in the quasi-neutral base. After simplification, the
solution is expressed intermsof the function f(¥(x_:)) as
m
t_ 1i°
f(w) =2jb[j:j¢ _jt] (1) tt__
cq 10
where Jt = qNcv,, Jl+=qNc÷v,, Jb= qNsv,, and ff_(x_)) ._o
isgiven by .*mUdq
¢U
t-
10 , '
eL
VqN Wd[ J 1.6 1.7 1.8 1.9 2.0
Collector Current Density (mA/um 2)
where Jk isgiven by
j, =ljc(Jc+J _2/l)_jl(j _j,)__c_[_+Vc___xj,_ (3) Figure 2 Parasitic barrier Might K as afunction ofooilcctor current
density.
From (1)-(3) it isapparent that ¥(x_:) is a function of
the device structure, current density Jc and Vce. From current densities is desirable. Increasing the collector
(3), we see that J_is nearly independent of ¥(x_) so we junction reverse bias and the collector doping help in this
can neglect it in calculating Jb find f(w) fi_m (1), and regard.
then determine ¥(x_) from (2). We can then determine
the height of the parasitic potential barrier/_, from the CONCLUSIONS
expression below, which corresponds to the magnitude of
the potential at the point where the electric field in the In summary, we have developed an improved
depleted collector Ec(x_+Wp)is equal to zero. description of the physics associated with the onset of the
j".:, (,x),).
dynamic formation of the parasitic barrier at the base-
,'(sr_c,...... (4) collector junction at high collector current densities. The
2z v, dc - J, model will provide a useful tool for device engineers in
the design of the base-collector junction for optimizing
SIMULATION RESULTS the device's performance athigh currentdensities near the
onset of basepushout.
The above described device model was used to
investigate the extent of the formation of the parasitic
barrier /Faand base pushout W_ for a typical device
ACKNOWLEDGEMENTS
structure similar to that of Joseph et al. [6]. Linear
compositional grading from zero at the emitter to 10%Ge
This work was supportebdy aNASA Summer Faculty
at the collector end of the base was assumed correspon-
Fellowship from the Glenn Research Center.
ding to AEv = 75 meV at the collector junction. A base
width of 90 nm was assumed with a doping of
Ixl0tB/cm 3. A collector width Wc of 0.5 lan and doping
REFERENCES
of lxl0tT/cm _was used. The current density constants Ji,
Jl+ and Jb WCre calculated to be 1.6, 160, and 16mA/ttm2, !. J.D. Cre_ler, "SiGe HBT Technology:. ANew Contender for Si-
respectively. A builtin potential of 0.75V and ajunction Based RF and Microwave Circuit Applications," IF..EE Tram.
reverse bias of 1V were assumed. M/ermc, at_ 2r_or7 rec/t, 46, 572, May 1998.
Shown in Figure 2isthe parasitic barrier _ plotted as 2. U. Konig. "SiCm & GaAs as Compe/iflve Technologies for RF-
Applkstio_" Proe. IEF_ B0_tar/B/CMOS C/r_its T_chnot
a function of the collector current density. The onset of
Meetly, 1998, 87.
formation of the parasitic barrier _ occurs at a current 3. S. Tiwsri, "A New Effect st High Cumm_ inH_
densityof1.75 mA/lan2,whichisslightllyargerthanJtffi BipolerTnmsistx_'*/EF..EFJe¢o'onDcvtes/aft 9. 142,Mar. 1988.
4. I.W. Slotboom, {3.Streutker, A. PmUmboom and D. I. Gravesteijn,
!.6 mA/ttm=. The parasitic barriershows asharp increase "Parasitic Enerl_ Bmk_ inSiOe HBI",." _ £_--wan Devk-e
with increasingJc, which is comparable to that described /au.12,486,Sept.1991.
by Mazhari and Morkoc [8], but larger than that reported 5. P.E. Cottreil sad Z. Yu, "Velocity Satu_ion htthe Collector of
Si/GeSi/Si EIBT't," IF.F_ _bct,_m Dok_/at 10, 431, Oct. 1990.
by Joseph et al. [6]and Song and Yuan [7]•
6. A.J. Joseph, L D. Cltstd_, D. M. ltJclmy tnd G. lfftu, *'Optimization
Since the formation of this parasitic barrier leads to ofSlOeI_'r's ferOperattoaatHighOarreatDemities,"/£EE
excess electron buildup at the collector end of the quasi- Tr_e.El_rnmDrt4_ 46,1347,_ 1999.
7. I..gongsadJ.S.Ymm,"Minimal IksDme-Collectm"
neutral base, itproduces asaturation effect inthe collector
jencUeBnx_ 1_ stHighCw_e_Dmsitie_"_/d,-f_atr
current and an increase in the quasi-neutral base Ek¢lro_ 43,437,1999.
recombination, with a corresponding falloff in the current 8. B.Ma,dmrllindH.Modm_"EI_ d CoB_:_'-Ba._V_ Bamt
gain. This also degrades the base transit time and the Disce_inui_yo_KirkEff_t inDm/_ _ju_zfion Bipolar
cutoff frequency so that delay of the phenomena to higher Tramistms,',4/_ P/r_/a_ _, 2162, Oct. 1991.
This is a preprint or reprint of apaper intended for presentation at a conference.
Because changes may be mad_ before" formal publication, this is made available with the
• . • _| understanding that itwill not be cited or reproduced without the permission otthe author.
Device Physics Analysis of
Parasitic Conduction Band Barrier Formation in SiGe HBTs
K. P. Roenker* and S. A. Alterovitz**
* Department of Electrical and Computer Engineering and Computer Science
University of Cincinnati, Cincinnati, Ohio 45221-0030
Tel: [phone] Fax: [phone] Email: kroenker_ececs.uc.edu_
NASA Glenn Research Center, Cleveland, Ohio 44135 Email: salterovitz(-_grc.nasa.iov
from theirnumerical simulator. Due to device operation
ABSTRACT
at high-current density, the peak field-is pushed to the
subcollector interface corresponding to the onset of base
This paper presents a physics-based model
pushout. However, for SiGe HBTs, the valence band
describing the current-induced formation of a
discontinuity at the base-collector junction blocks hole
parasitic barrier in the conduction band at the base-
flowout ofthe base. As a resulta, current-induced,
collector heterojunction in npn SiGe heterojunction
parasiticbarrier forms near the junction corresponding to
bipolar transistors 08[BTs). Due to the valence band
a positive field region. This barrier inhibits electron
discontinuity AEv, hole injection into the collector at
injection into the collector from the base producing
the onset of base pushout is impeded, which gives rise
electron buildup at the end of the base, increa:_'d base
to formation of a barrier to electron transport which
recombination and degadation inthe current gain.
degrades the device's high frequency performance. In
this paper, we present results from an analytical
model for the height of the barrier calculated from the Electric Field (Vlcm)
device's structure as a function of the collector
junction bias and collector current density.
Wp
Wc.
INTRODUCTION Eb (x) _',.q ,j
x
In recent years SiGe heterojunction bipolar transistors
(HBTs) have been reported with high gain and impressive
W,b Ec(x)__ E,+(x)
high frequency performance [1,2]. These devices employ
Ge in the base and compositional base grading so that a
Wc
heterojunction is formed at the collector junction. While
the conduction band discontinuity is small, the presence
of a valence band discontinuity AEv at the junction Figure 1 Electric field profile at B-C junction at high current density
during formation of parasitic barrier.
modifies the physics of electron transport across the
collector junction at high current densities. Dynamic
formation of a parasitic barrier in the conduction band For development of this device model, we note thatthe
occurs, which degrades device performance [3-5]. The electron concentration n_ is nearly constant in the high
effect is important for device design since transistor electric field in the base-collector space charge region
operation at high collector current densities (Jc (BC-SCR) nearthe subcollector due m velocity saturation
~lmA/tun 2) is essential to achieve high gain at v,. As a result, the electron concentration isgiven by n,=
microwave frequencies. Previously, Joseph et al. [6] Jc/qv,, where Jc is the collector current density and the
term arises from the need for a finite electron concen-
employed a numerical simulator and showed that a
trationsufficient to carry the collector current. Substitu-
parasitic barrier as large as 34 meV forms at current
ring this n, in Poisson's equation, we integrate to get the
densities of.-4 mA/pm 2. Song and Yuan [7] and Mazhari
electric field in the depleted collector E_x), which varies
and Morkoc [8] have reported simple, physics-based
models to describe the formation of this parasitic barrier linearly as shown in Figure 1. On the base side of the
heterojunction, we note thatwe have majority carrierhole
predicting similar and much larger barrier heights,
accumulation due to the valence band discontinuity AEv,
respectively. The motivation for this study is to develop
which we take into account in determining the electric
an enhanced description of the physics of this barrier
field in the base Eb(x). The location where the electric
formation for use indevice design.
field iszero inthe collector corresponds to the peak of the
parasiticpotential barrier.
ANALYTICAL MODEL
To analyze the formation of the barrier, we initially
match the fields at the base-collector and subcollector
Shown in Figure 1is aschematic profile of the electric
junctions. Subsequently, we piecewise integrate the
field assumed at the base-collector junction during
electric field across the junction to relate our results to the
formation of the parasitic barrier. This profile is similar
collector doping and width and the applied junction
to that reported by Joseph et al. [6], which they derived
reversebiasVcm Combining these results, we get a 103
single equation for the potential at the base-collector E
junction V(x_:),where ourzero for the potential was taken
to be in the quasi-neutral base. After simplification, the
10 2
solution isexpressed interms of the function f (_/(xj¢)) as q_
t..
f(_t)= __gb[j_+jc_jj (1) [Lm
e_ 101
where Ji= qNcvs, JI. =qNc.v,, Jb= qNav,, and f(_(x_)) gJ
isgiven by
L
f,, 10
1.6 1.7 1.8 1.9 2.0
Collector Current Density (mA/um 2)
where Jkis given by
J, =JJc(J¢+J.-2J,)-JJJ_ 2sv;,., _ - _(3)
Figure 2 Parasitic barrier height g as a function of collector c_t
! density.
From (1)-(3) itisapparent that ¥(x_) is afunction of
the device structure, current density Jc and Vce. From current densities is desirable. Increasing the collector
(3), we see that Jkis nearly independent of ¥(x_:) so we junction reverse bias and the collector doping help in this
can neglect it in calculating Jk, find f(w) from (I), and regard.
then determine ¥(x_) from (2). We can then determine
the height of the parasitic potential barrier _, from the CONCLUSIONS
expression below, whichcorresponds to the magnitude of
the potential at the point where the electric field in the In summary, we have developed an improved
depleted collector E_(x_+Wp)is equal to zero. description of the physics associated with the onset of the
dynamic formation of the parasitic barrier at the base-
,_it_ W_ J_fZ(_(xu)) (4)
collector junction at high collector current densities. The
v.b_C j = _----- --/-----_-
Z£ Vs J(, -- J! model will provide a useful tool for device engineers in
the design of the base-collector junction for optimizing
SIMULATION RESULTS the device's performance athigh currentdensities near the
onset ofbase pushout.
The above described device model was used to
investigate the extent of the formation of the parasitic
barrier _'s and base pashout W=b for a typical device
ACKNOWLEDGEMENTS
structure similar to that of Joseph et al. [6]. Linear
compositional grading fromzero at the emitter to 10%Ge
This work was supported by aNASA Summer Faculty
at the collector end of the base was assumed correspon-
Fellowship from the Glenn Research Center.
ding to AEv = 75 meV at the collector junction. A base
width of 90 nm was assumed with a doping of
Ixl01S/cm 3. A collector width Wc of 0.5 pm and doping REFERENCES
of lx1017/cm 3was used. The current density constants Jb
J,+ and Jbwere calculated to be 1.6, 160, and 16mA/pm 2,
!. J.D. Cressler, "SiGe I-1BTTechnology: ANew Contender for Si-
respectively. A builtin potential of 0.75V and ajunction Based RF and Microwave CircuR Applications," IEEE Trans.
reverse bias of IV were assumed. M'_rowave Theory Tcch., 46,_72,May 1998.
Shown inFigure 2isthe parasitic barrier/_ plotted as 2. U. Konig, "SiGe & GaAs as Competitive Technologies fix RF-
Applications," Pro¢. IF_.F_Bipolar/BICMOS Circu_ T¢chnoi.
a function of the collector current density. The onset of
Meeting, 1998, 87.
formation of the parasitic barrier _ occurs at a current 3. S. Tiwarl, =A New Effi_ at High Currents inHe_
density of 1.75 mA/pm2,which is sfightly larger thanJm= Bipolar Tramistot$," IEF-.EElectron Dcv/¢¢/_u. 9, 142, Mar. 1988.
4. I.W. Siothoom, O. Streutker, A. Pruijmboom and D. J.Gravesteijn,
1.6 mA/pm 2. The parasiticbarriershows asharp increase "Parasitic Energy Barrie_ inSiGe HBT's," lEEK E/ccwon Dev/ce
with increasing Jc, which is comparable to that described Lctt 12, 486, Sept. 1991.
by Mazhari and Morko¢ [8], but larger than that reported 5. P.E. Cottrell and Z. Yth =Velocity Saturation inthe Collector of
Si/GeSi/Si HBT's," IEEE E/¢ctron Dev_ Leg 1_, 431, Oct. 1990.
by Joseph et al. [6] and Songand Yuan [7].
6. A.j. Joseph, J. D. Ct.e_r, D. M. Richey ItndG. Nitt, _
Since the formation of this parasitic barrier leads to
of SiGe HBT's for Operation at High Current Densities,"/FEE
excess electron buildup at the collector end of the quasi- Trans. El#¢tron Devtc_ 445,1347, July 1999.
neutral base, itproduces asaturation effect inthe collector 7. J. Song and J. S.Yuan, =Modeling the BL_-.Colloct_ Hetem-
current and an increase in the quasi-neutral base junefio_a Bmier Effect at High _t Densities,," ,go//d State
E/ectron. 43, 457,1999.
recombination, with acorresponding falloff in the current 8. B. Mazlu_ and H. Mmkoe,, "Effect of Collector-Base Valance Band
gain. This also degrades the base transit time and the Discontinuity on Kirk Effect in Double Heterojunction Bipolar
cutoff frequency so thatdelay of the phenomena to higher Transistors," AppL Phys.Left59,2162,Oct 1991.