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>
HAL Id: jpa-00227978
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JOURNAL DE PHYSIQUE
Colloque C4, suppl6ment au n09, Tome 49, septembre 1988
ELECTRICAL BEHAVIOUR AND MODELLING OF A N-DOPED a-Si:H EMITTER
BIPOLAR TRANSISTOR
0. BONNAUD, A. EL GHARIB and M. SAHNOUNE
Groupe de MicroBlectronique, UniversitB Rennes I, Campus de Beaulieu.
F-35042 Rennes Cedex. France
EMITTER
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/pdoped 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
-
FABRICATION OF THE DEVICES
The 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+ degenerated 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
P
N
Fig. I
:
MSE
COLLECTOR
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
- DEVICE CHARACTERISTICS
The electrical behaviour of the devices
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
I,=Sud/div
V =?V/div
CE
-
is
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 2is
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.
I = 2 mA/div
losteps of I
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.
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.
5
103
10
-1
10
102
COLLECTOR CURRENT IC (MA)
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 r
current.
z
W
a
a
102
-6+ +
dynamic
,*C/,
h
u3
-
-L ' 2 /'
d
+
'
2
+
*static
-
I
10
1 bz
COLLECTOR CURRENT IC(mA)
F i g . 6 : Comparison o f t h e s t a t i c and d!ynamjc c u r r e n t
gain v e r s u s c o l l e c t o r c u r r e n t .
comparison between static and dynamic current
gains versus collector current is given on figure 6.
The difference can be easily explained by the
variation of the total current gain versus the
current density (Fig. 4 ) . Figure 7 shows the total
emitter-base junction current versus the baseemitter bias when
collector-base junction is
Because the slope equals ~ / V T ,
shortened (VCB'O).
the collector current law is well described by the
diffusion model in a three or four orders of
A
from C(V) measurements. Because the doping profile
of the base is directly deduced by tracing 1/c2
versus
V,
the
space
chqrge
layer
of
the
heterojunction mainly extends in the monocrystalline
region. Therefore, the breakdown voltage depends
mainly of the doping profile of the base as in
heavily doped
monocrystalline emitter
bipolar
tansistor. On the other hand, we may predict that
the volumic recombination mainly occurs on the base
side of the space charge layer of the junction.
sl;c
-
IV
series resistm-
MODELLING
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 parameters :
surface recombination rate at the a-Si:H/c-Si
interface (fig. 9 )
-
-
S : surface recornhination
-
rate (Ws)
lo6
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
CURRENT 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~lnntlonr . l t < , .IS p.+r.I-
mPtPr
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
Fig.
:
8 : Photograph of the basr-emitter characteristics. I = Zm/ydiv V = 2 V/div
BE
-
DISCUSSION
The comparison between the modelled curves and the
experimental
results
allows
the
following
interpretations :
for values as high as 1000, the current gain is
not limited by the interface recombination rate,
-
JOURNAL DE PHYSIQUE
-
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 :
I
1c-6
10-4
10-2
1
102
104
CURRENT DENSITY J, (A/cm2)
Fig. 10
1987, pp 1143-45
Simulated current gain versus collector current with carrier lifetime in the space charge 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
the minority carrier density in the a-Si:H is low
enough to permit a high current gain even when the
a-Si:H
conduct:ivity
is
as
low
as
n-doped
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
the
1~112
emitter-base heterojunction because
variation law of 0
:
-
-
1
V
10-6
I
I
10-4
I
I
I
:1o-2
I
1
I
I
I
-102
I
104
CURRENT DENSITY J, (A/cmZ)
Fig. I !
/ 1 0. BONNAUD,P. VIKTOROVITCH, IEE Proceedings,
vo1.132, Part I, nol, Feb. 1985
12) 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.
:
Simulated current gain versus collector
current with minority carrier density in
a-Si:Il as parameter.
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).