Recent revolutionary progress of the internet and wireless technologies has created a concept of the ubiquitous network society for the new century. Evolution of all these technologies is producing new off-roadmap trends for semiconductor device research in addition to the main-stream Si CMOS technology. The new device trends include trends toward the quantum nanotechnology, toward use of new materials, toward realization of new functions sensors and actuators and use of new system architectures and toward formation of new wireless networks.
Recently, the concept of Intelligent Quantum (IQ) chip introduced by Hasegawa
(2003) using III-V material as a base material where nanometer scale quantum
processors and memories are integrated on chip with capabilities of wireless
power supply, wireless communication circuit and various sensing functions,
has been demonstrated. III-V materials are the most promising for high-frequency
devices because of the high electron mobility and other unique features such
as the formation of two-dimensional electron gas (2DEG). The devices switch
faster as collisions are less frequent. Rectenna (combination of a rectifying
circuit and an antenna) is one of the most promising devices to be integrated
on the IQ chip to form the wireless power supply. This device can capture microwave
power and convert to the DC power to generate the other on-chip nanoelectronic
devices or circuits. Schottky diodes are known for their fast rectifying features
and hence are ideal for applications as rectenna (Sharma,
Almost all past rectennas were designed using various material for over 100
mW rectifying and the RF-DC power conversion efficiency is less than 20% at
the 1 mW microwave input (Suh and Chang, 2002). However,
the design and fabrication of ultra-low power n-AlGaAs/GaAs high-electron-mobility-transistor
(HEMT) Schottky diode for on-chip rectenna does not appear in the published
literature. As a by-product of our groups investigations (Hashim
et al., 2007a, b, 2008),
THz wave detectors, plasma-wave THz amplifiers, RF power detectors utilizing
the same AlGaAs/GaAs HEMT structure and several other unique features have been
In this study, we present the possible direct integration of Schottky diode
to planar dipole antenna via coplanar waveguide (CPW) without insertion of any
matching circuit. The design and fabrication of Schottky diode is directed towards
fast conversion of RF signals in nanocircuits and nanosystems to supply ultra
low DC power. The DC and RF characterizations of Schottky diodes are presented.
POSSIBLE DIRECT INTEGRATION OF SCHOTTKY DIODE WITH DIPOLE ANTENNA
The possible direct connection between Schottky diode and planar dipole antenna is illustrated in Fig. 1. This proposed configuration is designed on the same substrate with components directly connected to each other. This is purposely done to model, characterize and observe the simultaneous behavior of the Schottky diode and planar dipole antenna around the operating frequency.
A planar integrated fabrication of this nature can guarantee excellent mechanical
tolerances for a wide variety of tuning features. The results show excellent
usefulness of the proposed Schottky diode configuration and the effectiveness
of uniplanar technology with high performance-to-cost ratio. Next section will
present the details of Schottky diodes design and characterization.
||Schematic of direct integration between Schottky diode and
However, the design and characterization of a dipole antenna are presented
elsewhere (Mustafa et al., 2010).
DESIGN AND FABRICATION OF SCHOTTKY DIODE
Schottky diode was fabricated on the AlGaAs/GaAs layered structure grown by
molecular beam epitaxy. The higher electron mobility in 2DEG layer exists because
of modulation doping in which the scattering by impurities is considerably suppressed.
AlGaAs/GaAs heterostructures confine electrons so itinerant electron motion
is confined in two dimensions only. It has emerged to be suitable nanostructure
for the development of the so-called IQ chip which has been considered as the
most promising chip structure for future ubiquitous network society (Hasegawa,
2003). The characteristics of the layered nanostructure are as follows:
625 μm semi-insulated GaAs substrate with 500 nm GaAs buffer layer on top;
10 nm AlGaAs buffer (spacer) layer; 20 nm undoped GaAs layer; 10 nm AlGaAs spacer
layer; n-doped AlGaAs (Si δ doping) barrier layer; terminated with 10 nm
GaAs undoped cap layer. The devices were designed and fabricated using photolithography
and a standard lift-off technique. The carrier mobility and the carrier sheet
density obtained by Hall measurements at room temperature were 6040 cm2/Vsec
and 8.34x1011 cm-2, respectively.
The Schottky electrode was formed by Ni/Au and ohmic electrode was formed by
alloyed Ge/Au/Ni/Au. As shown in Fig. 2a, the fabricated device
has a CPW configuration at both sides of Schottky and ohmic contacts possessing
GSG pad structure. The dimension of the gap a and width b for CPW obtained from
Wheelers equation (Wen, 1969) were chosen to be
60 and 90 μm, respectively in order to produce the characteristic impedance
Zo = 50 Ω.
|| (a) Schematic and (b) fabricated Schottky diode (top view)
The Schottky contact area, A is 20x20 μm. The length of CPWs is 100 μm.
The distance d between Schottky-ohmic contacts is 40 μm. The fabricated
Schottky diode is shown in Fig. 2b. The choice is compatible
with the antenna characteristics without insertion of matching circuit. This
CPW structure permits direct injection of RF signal through Cascade GSG Infinity-150
MEASURED RESULT OF SCHOTTKY DIODE AND DISCUSSION
Current-voltage (I-V) measurement: After fabricating the Schottky diode, the DC I-V characteristics were measured using Keithley semiconductor characterization system Model 4200 and micromanipulator probe station. As shown in Fig. 3, the DC I-V curve of fabricated Schottky diode shows a diode I-V curve with series resistance, Rseries of 909.1 Ω. The series resistance is defined as the inverse slope between 2.0 V and 3.0 V. The threshold voltage, VTH, for the devices is estimated to be 1.1 V as shown in Fig. 3 inset.
The Schottky barrier height (SBH), φB of the device is extracted
from the reverse saturation current, Is given by the Richardson-Dushman
equation for the thermionic emission (Sharma, 1984):
where, Vt is the thermal voltage, A* is the effective Richardson
constant, A is the area of the metal-semiconductor contact and T is the absolute
temperature. The reverse leakage current is 3.97 nA and extracted φB
is found to be 0.5289 eV. The SBH value is almost three times smaller than the
ideal value of 1.443 eV. This decrease in the barrier height is attributed to
the smaller contact area as this parameter is included in Eq.
1, consistent with Jeon et al. (2004). The
reduction of the Schottky barrier height is also due to the fabrication process,
i.e. the annealing process, which can result in a decrease in barrier height
as suggested by Zhang (1999). They have reported Schottky
contacts of different metals to n-type AlGaAs/GaAs structures and proposed a
model which involves the quality of the contact and defect formation at the
semiconductor surface due to interdiffusion and/or penetration of metal into
|| DC I-V characteristics of fabricated Schottky diode
This model can qualitatively explain the difference in barrier heights and
degradation of the barrier due to a certain process. In addition, this was also
reported by Milanovic et al. (1996), where the
work functions of the metal and the semiconductor are determined by the process.
The actual nature of the metal-semiconductor contact is not controllable and
in fact may vary substantially from one process to another. This reduced barrier
height is beneficial for improved RF response and rectification as it requires
a lower turn-on voltage (Jeon et al., 2004).
RF-DC conversion measurement: To achieve a high cut-off frequency, the rectifying metal-to-semiconductor contact area must be reduced. However, too small a contact area limits the delivery of the maximum power before the diode burns out. Therefore, the area of the diode A is the major design parameter since most of the other parameters such as the work function of the metal and the semiconductor are determined by the fabrication process and interface properties. A simple measurement setup, as shown in Fig. 2a, was assembled.
Figure 4 shows the rectified output power as a function of
frequency at an input power level of 5, 15 and 18 dBm. The maximum output power
is achieve around 5 to 10 GHz at input power level of 18 dBm. The input power
is limited to 18 dBm due to the equipment capability. It can be seen that, the
output power decrease to low value at high frequency. The cut-off frequency
for this device is estimated to be around 25 GHz. Figure 5a
and b show the rectified output power as a function of input
power in dBm and mW, respectively. A quadratic rise of output power as a function
of input power as a result of power sweep from -10 dBm to 25 dBm at 1, 10, 15
and 25 GHz can be seen in Fig. 5a.
|| Rectified output power as a function of frequency
The output power starts to rise at the input power level of 5 to 18 dBm for
all tested frequencies where at this level, the input voltage is confirmed at
the same level with the turn-on voltage of a diode. Figure 5b
also shows that the maximum output power is obtainable at certain frequency.
In this measurement, the device seems to show a maximum power at 10 GHz. Such
characteristics also suggest a potential application as a frequency-tunable
As indicated in previous section, the study on ultra-low power n-AlGaAs/GaAs
HEMT Schottky diode for on-chip rectenna does not appear in the published literature.
||Rectified output power as a function of injection power in
(a) dBm and (b) mW
||Rectified output power as a function of series resistance
To project ultra-low power rectenna for operation in milliwatt (mW) range,
the RF-to-DC power conversion of Schottky diode has also been measured for the
other samples which are fabricated on the same wafer and have same values of
turn-on voltages but different dc series resistance, Rseries. Figure
6 shows the rectified output power as a function of series resistance of
devices at input power of 18 dBm. A linear characteristics of output power as
a function of series resistance for 1 and 10 GHz can be projected in Fig.
6. It can be assumed that the output power will shift towards milliwatt
range with the decreasing of device series resistance. The reduction series
resistance can be achieved by removing cap layer and forming ohmic contact directly
on the n-AlGaAs/GaAs barrier layer. The rectifying response also can be improved
by lowering the SBH. In this study, RF-DC power conversion efficiency is not
calculated. This is because of the actual reflected and transmitted signal is
not determined due to a constraint in equipment availability. The determination
of power conversion efficiency will be carried out in the next work. The improvement
of material and device structure should improve the power conversion efficiency.
The proper measurement circuitry should also permit maximum power conversion.
We believe that if the ground lines of the system are connected to the same
point, maximum power conversion can be extracted.
The Schottky diode on AlGaAs/GaAs HEMT structure has been analyzed for on-chip
rectenna device application. The cut-off frequencies of the fabricated Schottky
diodes have been shown to be around 25 GHz estimated in direct injection experiments.
The mW output power can be achieved by optimizing the material and ohmic metals
so that lower series resistance is produced. The feasibility for direct integration
of the planar dipole antenna to the Schottky diode without matching impedance
circuit has been demonstrated. These results will provide new breakthrough ideas
for the direct on-chip integration technology towards realization of ultra-low
power on-chip rectenna technology to be integrated in nanosystems.
The authors wish to extend their thanks for the support provided by the Ibnu Sina Institute, Universiti Teknologi Malaysia, Malaysia and Nano-Optoelectronics Laboratory, Universiti Sains Malaysia, Malaysia. This work was supported by the Ministry of Science, Technology and Innovation under Science-Fund Grant 03-01-06-SF0277, Malaysia government. We wish to thank our colleagues for useful discussions, particularly, Assoc. Prof. Dr Azlan Abdul Aziz, Assoc. Prof. Dr Md. Roslan Hashim at Universiti Sains Malaysia, Malaysia, Assoc. Prof. Dr Zulkafli Othaman at Universiti Teknologi Malaysia, Malaysia and Assoc. Prof. Dr Zhang Dao Hua at Nanyang Technological University, Singapore.