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ELECTRICAL BEHAVIOUR AND MODELLING OF A N-DOPED a-Si : H EMITTER BIPOLAR
TRANSISTOR
O. Bonnaud, A. El Gharib, M. Sahnoune
To cite this version:
O. Bonnaud, A. El Gharib, M. Sahnoune. ELECTRICAL BEHAVIOUR AND MODELLING OF A
N-DOPED a-Si : H EMITTER BIPOLAR TRANSISTOR. Journal de Physique Colloques, 1988, 49
(C4), pp.C4-383-C4-386. �10.1051/jphyscol:1988480�. �jpa-00227978�
0.
BONNAUD,
A.EL GHARIB and M. SAHNOUNE
Groupe de MicroBlectronique, UniversitB Rennes I, Campus de Beaulieu.
F-35042 Rennes Cedex. France
In recent years, heterojunction bipolar transistors (HBT's) have attracted much attention because it is possible to get a high current gain with heavily base doping. Furthermore, heterojunction structures mainly employing the conventional silicon technology are particularly attractive, especially hydrogenated amorphous silicon/singlel crystalline silicon (a-Si:H/c-Si) heterojunctions 11 -41. The use of n-doped a-Si:H/p- doped c-Si as emitter-base junction of a HBT allows to create an extra barrier to holes that minimizes the minority carrier injection in the emitter and thus allows to get a high current gain bipolar transistor in silicon technology. This paper is devoted to the study of the electrical behaviour of a n-doped a-Si:H emitter HBT, to the understanding and the modelling of the electrical mechanisms in order ta improve the features of these devices.
I1
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FABRICATION OF THE DEVICESThe bipolar transistors are fabricated on an epitaxial substrate as follows. The n+-doped substrate constitutes the collector layer. The base is fabricated from ion implantation of boron in the n--epilayer. The emitter is made of a-Si:H deposited on the base layer from a silane decomposition at low temperature (250-275°C) with PH3 as doping gas (glow discharge technique). The a-Si:H layer is very thin in order to minimize the series resistance of the emitter and is 50 nm thick. Following this deposition, an emitter contact is made of chromium that plays the role of a diffusion barrier to the aluminum atoms of the contact overlayer (fig.1). The choice of a chromium layer instead of titane layer is due to a better ohmic contact obtained with this element after checking the contact resistance on test structures (chromium or titane and a-Si:H layers deposited on glass substrates and n+ dege- nerated silicon substrates). The finished devices are annealed in forming gas at low temperature temperature (230°C) ligthly lower than the a-Si:H deposition temperature. This annealing step improves the electrical characteristics of the transistor
EMITTER
P M S E
N COLLECTOR
Fig. I : Crosssection of the structure
possibly because the aluminum contacts on silico and chromium are improved. The emitter area varie in the range 60-3500 um2. Figure 2 shows a fina structure.
Fig. 2 : Photograph of the final transistor
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988480
JOURNAL DE PHYSIQUE
I11
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DEVICE CHARACTERISTICSThe electrical behaviour of the devices is deduced from the electrical characteristics.
F i g . 3 : ICiVcE, I J c h a r a c t e r i s t i c s
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I = 2 m A / d i v I , = S u d / d i v V = ? V / d i v l o s t e p s o f ICE
Figure 3 shows the variation of the collector current versus collector emitter voltage with base current as parameter. In this case, the dynamic current gain equals to 800, but depending on the Gumnel number of the base, Gg, and the quality of the amorphous layer, more especially the conductivity, this current gain varies in the range 80-1200.
1 0
--
1 COLLECTOR CURRENT 1 0 IC (MA) 1 0 2 F i g . 4 : Dynamic c u r r e n t g a i n v e r s u s c o l l e c t o rc u r r e n t .
Figure 4 shows the variation of the dynamic curcent gain versus cc.llector current for two types of amorphous silicon layers. In the case of curve a), the saturation of the current gain is not observed and furthermore the law 0 proportionnal to 1 ~ 1 1 2 is verified. Therefore, the current appears to be strongly infuenced by the recombination phenomenon in the space charge layer of the emitter-base junction. Curve b) corresponds to a poor conductivity (10-4 ohm-lcm-l) of the a-Si:H layer ; the doping of the emitter is low enough to limit the maximum current gain.
GUMMEL NUMBER G B ( s / c m 4 ) F i g . 5 : Dynamic c u r r e n t g a i n v e r s u s Gummel number
Figure 5 provides the current gain results versus the Gummel number of the base deduced from the ion implantation dose. The law 6 proportionnal to l/GB is approximately verified.
I
1 0 1
bz
COLLECTOR CURRENT IC (mA)
5
103d
h
z
W a a
u3
1 0 2F i g . 6 : C o m p a r i s o n o f t h e s t a t i c a n d d!ynamjc c u r r e n t g a i n v e r s u s c o l l e c t o r c u r r e n t .
-
d y n a m i c -6 ++
+
' 2
- L ' -
*static,*C/,
+
2
/'
-
series resistm-
sl;c
EMITTER-BASE VOLTAGE VEB(V) Fig. 7 : Total emitter-base junction current versus
base-emitter voltage when collector-base junction is shortened.
magnitude. At high level, the conduction is governed by the series resistance of the emitter layer ; at low level, a leakage current occurs. Photograph on figure 8 shows the experimental base-emitter characteristics. The reverse breakdown voltage of the emitter-base junction is close to 7 V , similar to its monocrystalline conterpart.This can be explained
Fig. : 8 : Photograph o f the basr-emitter characte- ristics. I = Zm/ydiv V = 2 V/div
BE
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We propose a modelling of the collector current versus the base emitter voltage (fig. 8, simulated curve) and of the current gain versus the collector current taking into account the following parame- ters :
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surface recombination rate at the a-Si:H/c-Si interface (fig. 9 )- S : surface recornhination rate (Ws)
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lo6CURRENT DENSITY Jn (A/cm2) rlq. : 9 Slng~~l.ti<d curreni g . l l n vt.r.;u<; r o l l r < . f < , r L 8,r
rent wlth surfacc reconrl~lnntlon r . l t < , .IS p.+r.I- mPtPr
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carrier lifetime in the space charge layer of the heterojunction (fig. 10)-
minority carrier density in the a-Si:H emitter (fig. 11)-
series resistance of the n-doped a-Si:ll layer-
access resistance of the extrinsic base-
crowding effect in the case of large emitter areas.V
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DISCUSSIONThe comparison between the modelled curves and the experimental results allows the following interpretations :
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for values as high as 1000, the current gain is not limited by the interface recombination rate,JOURNAL DE PHYSIQUE
1c-6 10-4 10-2 1 102 1 0 4 CURRENT D E N S I T Y J, (A/cm2)
Fig. 10 : Simulated current gain versus collector cur- rent with carrier lifetime in the space char- ge layer a s pardmeter
that means that the a-Si:H acts as a passivant layer For the c-Si surface. In a contrary case, the current gain rould saturate at low level
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the minority carrier density in the a-Si:H is low enough to permit a high current gain even when the n-doped a-Si:H conduct:ivity is as low as 1 0 - ~ 0 h m - ~ c m - ~ ; thus, the extra barrier For the hole deduced from the energy band diagram is effective and close to 0.4 eV-
the current gain essentially depends on the recombination in the space charge layer of the emitter-base heterojunction because the 1 ~ 1 1 2 variation law of 0-
the emitter electron current density injected in the base is governed by the diffusion process (the ideality factor equals 1)-
at high level of the current density, we have to take into account of the emitter series resistance which is strongly Function of the a-Si:H deposition conditions.References :
/
1I
0. BONNAUD,P. VIKTOROVITCH, IEE Proceedings, vo1.132, Part I, nol, Feb. 198512) R. MERTENS, J. NIJS, J. SYMONS, K. BAERT, ESSDERC 87, BOLOGNE (It.), Sept. 1987
131 H. FUJIOKA, S. RI,K. TAKASAKI, K. FUJINO, Y.
BAN, IEDM 1987, pp 190-93
141 J. SYMONS, M. GHANNAM, A. NEUGROSHEL, J. NIJS, R. MERTENS, Solid State Elec., vol. 30, nO1l, Dec.
1987, pp 1143-45
This work is partially supported by GCIS CNRS France. The authors thank M. MORIN, J.L. FAVENNEC and Y. CHOUAN with CNET LANNION Eor a-Si:H deposition, and wish to thank all the team working in Technology Laboratory of C.C.M.O. (Centre Comnun de Micro6lectronique de llOuest).
1 V I I I I I I I I I I
10-6 10-4 :1o-2 1 - 1 0 2 104 CURRENT DENSITY J, (A/cmZ) Fig. I ! : Simulated current gain versus collector
current with minority carrier density in a-Si:Il as parameter.
