A COMPARATIVE STUDY OF ALUMINUM AND
TUNGSTEN SILICON SCHOTTKY DIODES
PABLO R. DE SOUZA, JACOBUS W. SWART
AND
JOSÉ A. DINIZ.
Centro de Componentes Semicondutores (CCS) – and
Faculdade de Engenharia Elétrica e de Computação (FEEC) – UNICAMP,
C. P. 6061, CEP 13083-970, Campinas - SP.
e-mail: pablo@led.unicamp.br
ABSTRACT
Power semiconductor devices have gained special attention in modern
electronic systems, as the power required has been increased. An
increase in efficiency in use of energy is crucial in the modern world,
because energy, most party provided from natural resourses, is not
unlimited for the human beings. Schottky diodes are important devices
for these applications. This work presents a comparison between
Schottky diodes fabricated with tungsten and aluminum as Schottky
metal contact, to better understand and help to choose between
materials that fulfill power electronics applications requirements. The
results indicate superior characteristics for tungsten Schottky diodes
compared to aluminum Schottky diodes.
1. INTRODUCTION
Power semiconductor devices play a fundamental role in electronic systems.
Besides controlling high power values, it is necessary that there is a good yield of these
devices, increasing the power efficiency of the equipment for the best use of electric
energy, because that is also a limited resource for the human beings. For voltages below
100V, the main applications are: power supplies for computers and telecommunications,
automotive electronics, and so on (1). The volume of data processed by microprocessed
systems, has been growing largely in last times, with these systems operating at high
frequency. With the increase on frequency operation, current is drained more and more at
each clock cycle, with the current levels rising at the same speed of frequency. To reduce
power dissipation on devices, it is necessary to reduce the operating voltage, because as
the power is proportional to the square of voltage, reducing this value to the half, the
power dissipated is reduced to a quarter. Presently, it is common systems working with
voltages below 2V, controlling loads of some hundreds of amperes, with tolerance of just
some dozens of milivolts, like the last generation of Intel processors operating with more
than 100 amperes of current in full load (2).
With the proliferation of personal computers, the increase of power supplies
efficiency is essential to reduce losses of energy, resulting on reduction of size and
weight of the equipment, which is attractive to consumers. These sources operate in high
frequencies due to the efficiency improvements and reduction in size of some
components, as inductors and transformers for switched power supplies. Therefore,
power semiconductor devices with low forward drop voltage and ability to operate at
high frequency applications, is necessary to fulfill the requirements of these electronic
systems. A Schottky rectifier has a low forward voltage drop, around 0.55V, reducing the
power dissipated in on state operation. Besides, the Schottky diode has the advantage of
high frequency operation, because no minority carriers are involved, and so does not
present current transient delay, as in the case of a pn diode. For low voltage applications,
below 100V, silicon Schottky diode is the most appropriate, due to the low cost of the
manufacturing process robustness of silicon compared to other semiconductors.
The forward voltage drop and leakage current of Schottky diodes, can be
controlled with the right choice of the Schottky contact metal. Schottky barrier height is
an important parameter of this device. Ideally, or in a first attempt, it can be determined
by the diference of metal work function and the silicon electronic affinity, for n-type
silicon substrate. The forward voltage drop can be reduced by choosing a low work
function metal to form the Schottky contact. However, leakage current increases for low
Schottky barrier height. Thus, it is necessary to establish a trade-off between direct and
reverse IxV characteristics in the appropriated choice of the Schottky barrier metal. This
work presents a study of static and dynamic characteristics comparing Schottky diodes
made with aluminum and tungsten, as Schottky barrier metal, to determine what is the
best material to use for power Schottky rectifiers.
2. EXPERIMENTS
The devices were fabricated on n-type silicon substrates (orientation (100) and
resistivity ranging from 0.008 – 0.020 Ω.cm) with an n-type epitaxial layer (thickness
3.6µm – 4.4µm and resistivity ranging from 1.09 – 1.11 Ω.cm). The samples were
cleaned with H2SO4/H2O2 (1:5 at 85ºC), 10% HF solution, NH4OH/H2O2/H2O (1:5:5 at
85ºC) and HCl/H2O2/H2O(1:5:5 at 85ºC). For tungsten Schottky diodes, a tungsten film
of 200nm thick was deposited by magnetron sputtering, and in addition, an aluminum
layer of 1µm thick was deposited on top of the tungsten layer, by e-beam evaporation
process. For aluminum Schottky diodes, an aluminum layer of 1µm thickness was
deposited by e-beam evaporation process. Ohmic contact to the substrate was obtained
depositing aluminum of 1µm thick on the backside of the wafers. The samples were
sintered in forming gas by conventional thermal process at 450ºC during 30 minutes.
Figure 1 illustrates the structure of the fabricated devices.
The fabricated devices are squares with 4600µm of side, with rounded corner, to
minimize the point effect. The square shape was chosen to maximize the use of the
silicon wafer area.
(a)
(b)
Figure 1 – Schematic of Schottky diode (a) with aluminum as metal Schottky contact and
(b) with aluminum as metal Schottky contact.
3. RESULTS
Figure 2 shows direct and reverse IxV characteristics of the fabricated Schottky
diodes.
Table I shows the parameters extracted from the curves of figure 2.
Table I – Schottky diodes parameters extracted from Log(I)xV and reverse IxV curves of the Schottky
diodes with W and Al as metal Schottky contact.
Metal Schottky
Rs
VF(I=100mA)
η
Jr(V=-5V)
Is
φB
contact
(ohm)
(V)
(µA/cm2) (µA)
W
Al
2.70
2.64
0.50
0.70
1.05
1.04
63
121
2.96
2.30
(eV)
0.70
0.89
Both devices presented good characteristics, considering that they are
conventional Schottky diodes that generally present poor reverse characteristics. The
ideality factor (η) was close of the unit, indicating that the main process of current
transport is due to the termionic emission. The aluminum Schottky diode presented a
lower forward voltage drop, VF, (0.50V) and reverse current density, Jr, (63µA/cm2)
comparing with the values obtained for aluminum Schottky diode (0.70V and
121µA/cm2). The lower forward voltage drop obtained with tungsten Schottky diodes is
due the Schottky barrier height value (φB), that is lower than the Schottky barrier height
for aluminum Schottky diode (0.70 eV for W/nSi and 0.89 eV for Al/nSi). However, the
higher reverse current density (Jr) obtained for the aluminum Schottky diodes can be
associated with aluminum spikes formed at the Schottky contact. These aluminum spikes
concentrate high electric field, resulting in high leakage current. The barrier height of
aluminum Schottky diodes, formed on n-type silicon wafers, can not be explained by the
work function difference between aluminum and Silicon, and it is attributed to a thin p
doped silicon layer formed at the Schottky interface. During the sintering process, for
ohmic contact formation, some aluminum atoms diffuse into the Silicon surface and act
as p-type dopants, forming this thin p type silicon layer (3). Thus, a pn junction is formed
resulting in the rectifier behavior of aluminum on n-type silicon Schottky contact devices,
resulting on higher breakdown voltage for aluminum Schottky diode. The low breakdown
voltage observed for tungsten Schottky diodes can be due the lowering barrier height
effects (4). The reverse characteristic of conventional Schottky diodes can be improved
introducing a p guard ring to the device structure (5).
-1
10
W
Al
-3
Log(I) (A)
10
-5
10
-7
10
-9
10
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-10
0
V(V)
(a)
V(V)
-70
0.0
-60
-50
-40
-30
-20
I(A)
-5.0m
-10.0m
-15.0m
W
Al
-20.0m
(b)
Figure 2 - (a) Log(I)xV and (b) reverse IxV characteristics of tungsten and
aluminum Schottky diodes.
Table I shows the parameters extracted from the curves of figure 2.
Table I – Schottky diodes parameters extracted from Log(I)xV and reverse IxV curves of the Schottky
diodes with W and Al as metal Schottky contact.
Metal Schottky
Rs
VF(I=100mA)
η
Jr(V=-5V)
Is
φB
contact
(ohm)
(V)
(µA/cm2) (µA)
W
Al
2.70
2.64
0.50
0.70
1.05
1.04
63
121
(eV)
0.70
0.89
2.96
2.30
Table II shows the extracted parameters of voltage and current transient curves in
figures 3 and 4 respectively. The input signal used for voltage transient measurements
was a square wave with voltage levels +5V to –5V and frequency of 200kHz. For the
current transient measurements the square wave frequency was changed to 500Hz.
Table II – Schottky diodes parameters extracted from current and voltage transient curves of Schottky
diodes with W and Al as metal Schottky contact.
Metal Schottky
VF (V)
IF (mA)
IR (µA)
tr (ns)
tf (µs)
contact
W
Al
0.26
0.55
7.39
7.04
65.58
167
360
450
1
1
Figures 3 and 4 shows that both devices presented very similar switching
characteristics, with similar values obtained for rise (tr) and fall (tf) times. It is important
to observe that for aluminum Schottky diode, even with the presence of the p-type silicon
layer in the Schottky junction, the amount of injected holes is very small, not interfering
on switching characteristics of this device. Comparing the on state voltage (VF) and the
leakage current (IR) for both devices, we can conclude that tungsten is the proper choice
for power Schottky diodes, due the smaller forward voltage drop and lower leakage
current. The peaks in the voltage curve are due to the resonant circuit formed by the
series resistance of the diode, junction capacitance and parasitic inductance of the circuit
used for the measurements.
W
Al
1
0
V(V)
-1
-2
-3
-4
-5
-4.0µ
-2.0µ
0.0
2.0µ
4.0µ
t(s)
Figure 3 – Curves of transient of voltage for the devices with tungsten and
aluminum.
40.0m
W
Al
30.0m
20.0m
I(A)
10.0m
0.0
-10.0m
-20.0m
-30.0m
-40.0m
1.016m
1.018m
1.020m
1.022m
t(s)
Figure 4 – Curves of transient of current for the devices with tungsten and
aluminum.
4. CONCLUSION
Comparing the characteristics obtained of the fabricated devices, with tungsten
and aluminum as metal Schottky contact, tungsten schottky diodes presented better
parameters for power electronic applications, mainly for its lower forward voltage drop.
However this device presented a low breakdown voltage (-20V). This parameter can be
improved by introducing a guard ring in the device structure.
The obtained results support the model of the formation of a pseudo pn junction at
the Al/Si contact, explaining the larger barrier height than expected when considering
work function differences only. In addition, the Al/Si contact is known to produce spikes
in the Si surface. This explains the larger leakage currents observed for these diodes.
ACKNOWLEDGEMENTS
The authors would like to thank support of the staff of CCS for device fabrication.
This work received financial support from FAPESP (Fundação de Amparo à Pesquisa do
Estado de São Paulo).
REFERENCES
1 – B. J. Baliga, “Trends in power semiconductor devices”, IEEE Transactions on
Electron Devices, Vol. 43, nº10, pp. 1717-1730, october 1996.
2 – A. Lidow, D. Kinzer, G. Sheridan and D. Tam, “The semiconductor roadmap for
power management in the new millennium”, Proceedings of the IEEE, pp. 803-812, june
2001.
3 – T.M. Reith, J. D. Schick, “The electrical effect on Schottky barrier diodes of Si
crystalization from Al-Si metal films”, Applied Physics Letters, Vol.25, nº. 9, pp.524526, 1974.
4 – J. M. Andrews, M.P. Lepselter, “Reverse current-voltage characteristics of metalsilicide Schottky diodes”, Solid State Eletronics, Vol. 13, p.p. 1011-1023, 1970.
5 – B. W. Liou, C. L. Lee, “Characteristics of high breakdown voltage Schottky barrier
diodes using p+ polycristaline silicon diffused guard ring”, Solid State Electronics, 44,
pp.631-638, 2000.