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Introduction: Resonant tunnelling through semiconductor double
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Conclusion: A high power and wideband AlGaN/GaN HEMT
feedback amplifier module covering the frequency range of DC to
5 GHz has been developed. Frequency dependent feedback components, inductances modelled from the Au bonding wires in the
drain line and feedback loop, were employed to improve amplifier
performance at high frequency range. From the power module
with a feedback amplifier, a power gain of 9dB, an output power
of 29.5dBm, and a PAE of 20% were achieved in class-A operation.
Acknowledgments: This work was supported by BMDO under
contract No. DASG60-00-1-0001.
2 July 2001
0 IEE 2001
Electronics Letters Online No: 20010787
D 01: IO. IO49/el:200I O 787
Y. Chung, S. Cai, W. Lee, Y. Lin, C.P. Wen, K.L. Wang and T. ltoh
(Department of Electrical Engineering, University of California, Los
Angeles, 405 Hilgard Ave., Los Angeles, CA 90095, USA)
E-mail: ykchung@ee.ucla.edu
References
MISHRA, U.K.,
wu, Y.F.,
KELLER, B.P., KELLER,
s., and
DENBAARS, s.:
barrier structures [l] has been studied due to the physical and
technological interests. To date, in 111-V compound systems such
as GaAdAlGaAs, a number of studies have reported negative differential conductance (NDC) derived from the resonant tunnelling
effect [l, 21. However, with the exception of the SUSiGe system [3],
NDC has not been observed in Si-based systems such as Si/SiN [4]
and Si/Si02 [5, 61, although inflected current-voltage (I-v) curves
were observed and ascribed to resonant tunnelling. The absence of
NDC is probably because the formation of a high quality singlecrystalline Si layer sandwiched between amorphous barriers is difficult in contrast to heteroepitaxial systems.
To observe NDC in the Si/Si02 system, I- V measurements were
performed for SiO2/single-crysta1line-Sidouble barrier diodes fabricated from a silicon-on-insulator (SOI) wafer having an ultrathin
(-2nm) buried Si02 (BOX) layer, which was developed by the
authors [7]. As a result, we observed, for the first time, a clear
NDC due to the resonant tunnelling effect.
I
1
a
b
1599/11
Fig. 1 Schematic diagrams of starting SOZ wafer and Si/SiO2 double
barrier diode
a Starting SO1 wafer with ultrathin BOX layer
b Si02/single-crystalline-Si double barrier diode
Sample preparation: Instead of using deposition [4] and anisotropic etching [5, 61 techniques, a bonded SO1 wafer, as shown
schematically in Fig. l a , was used as the starting substrate. The
most important point is that this wafer has an ultrathin (-2nm)
thermally grown BOX layer, which permits electron tunnelling
between the top Si layer and the n+ base substrate. In the final
structure, this BOX layer works as a lower barrier layer. It was
fabricated in our laboratory by a wafer bonding technique, the
details of which have been described elsewhere [7]. Here, (001)
wafers were used for both the top Si and base substrates, and the
in-plane direction of the top Si layer was caretully aligned to that
of the base substrate to within Bo.
‘GaN microwave electronics’, IEEE Trans. Microw. Theory Tech.,
1998, MTT-46, (6), pp. 756-761
WU, Y.F., KELLER, B.P., KELLER, S., NGUYEN, N.X., LE, M., NGUYEN, C.,
JENKINS, T . J , KEHIAS, L.T., DENBAARS, s.P., and MISHRA, U.K.: ‘Short
channel AlGaNiGaN MODFETs with 50-GHz fr and 1.7 W/mm
output power at lOGHz’, ZEEE Electron. Device Lett., 1997, 18,
(9), pp. 438440
CAI, S.J., LI, R., CHEN, Y.L., WONG, L., WU, W.G., THOMAS, S.G., and
WANG, K.L.:
‘High performance AlGaN/GaN HEMT with
improved ohmic contacts’, Electron. Lett., 1998, 34, (24), pp. 23542356
NICLAS, K.B., WILSER, w.T., GOLD, R.B., and HITCHENS, w.R.: ‘The
matched
feedback
amplifier: ultrawide-band
microwave
amplification with GaAs MESFETs’, IEEE Trans. Microw. Theory
Tech., 1980, MTT-28, (4), pp. 285-294
Negative differential conductance due to
resonant tunnelling through SiO,/singlecrystalline-Si double barrier structure
Y. Ishikawa, T. Ishihara, M. Iwasaki and M. Tabe
The resonant tunnelling effect was studied for Si/Si02 double
barrier structures fabricated from a bonded silicon-on-insulator
wafer with an ultrathin buried SiOz layer. As a result, in Si/Si02
systems, we observed, for the first time, negative differential
conductance due to resonant tunnelling of electrons.
1200
1599/21
Fig. 2 TEM image of SiO~/single-crystalline-Si/SiO~
structure
The double barrier structure was obtained by thinning of the
top Si layer to the nanometre range and by oxidation to form an
upper barrier layer (-2 nm thick SO2). In fact, the transmission
electron microscope (TEM) image in Fig. 2 reveals the ultrathin
(-2 nm f 0.3 nm-rms) single-crystalline-Si layer sandwiched
between Si02 layers.
For the I-V measurements, local oxidation was performed to
isolate the diodes from neighbouring ones (Fig. lb). The top Al
electrodes were formed by conventional vacuum evaporation, and
the sample was annealed in a H2/N2mixture to reduce gap states.
I-‘P characteristics: I- V measurements were performed for diodes
(area IC5
ai2)
with Si well thicknesses of -5 and -2 nm, respectively. As shown in Fig. 3a, positive voltage was applied to the A1
2
ELECTRONICS LETTERS
13th September2001
Vol. 37
No. 19
electrode, corresponding to electron transport from the n+-Si substrate. Application of negative voltage did not lead to NDC, since,
for transport from the AI electrode, the resonance condition is satisfied at any voltages due to the continuous energy distribution of
electrons below the Fermi level.
characteristics, we observed, for the first time, a clear NDC (PVR
of -1.8 at 15K) due to the resonant tunnelling effect.
Acknowledgments: The authors would like to thank T. Tsuchiya of
Shimane University, and Y . Ono and Y. Takahashi of “
Ifor
kind support in experiments. This work was partly supported by a
Grant-in-Aid for Scientific Research and a Grant-in-Aid for
Encouragement of Young Scientists from the Japan Society for
the Promotion of Science, by the Foundation Advanced Technology Institute, and by CREST of Japan Science and Technology.
a
applied voltage, V
b
0 IEE 2001
6 July 2001
Electronics Letters Online No: 2001081 7
DOI: 10.1049/e1:20010817
Y . Ishikawa, T. Ishihara, M. Iwasaki and M. Tabe (Research Institute
of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 4328011, Japan)
E-mail: romtabe@rie.shizuoka.ac.jp
applied voltage, V
C
References
(599/31
CHANG, L.L., ESAKI, L., and TSU, R.: ‘Resonant tunneling in
semiconductor double barriers’, Appl. Phys. Lett., 1974, 24, (12),
pp. 593-595
TSUCHIYA, M., and SAKAKI, H.: ‘Dependence of resonant tunneling
current on AI mole fractions in A1,Ga,,As-GaAs-A1,Ga,,As
double barrier structures’, Appl. Phys. Lett., 1987, 50, (21), pp.
1503-1505
ISMAIL, K., MEYERSON, B.s., and WANG, P.J.: ‘Electron resonant
tunneling in Si/SiGe double barrier diodes’, Appl. Phys. Lett., 1991,
59, (S), pp. 973-975
MIYAZAKI, s., IHARA, Y., and HIROSE, M.: ‘Resonant tunneling
through amorphous silicon-silicon nitride double-barrier
structures’, Phys. Rev. Lett., 1987, 59, (I), pp. 125-127
YUKI, K.,
HIRAI, Y.,
MORIMOTO, K.,
INOUE, K.,
NIWA, M., and
YASUI, I.: ‘Fabrication of novel Si double-barrier structures and
their characteristics’, Jpn. J. Appl. Phys., 1995, 34, (2B), pp. 860863
NAMATSU, H., HORIGUCHI, s., TAKAHASHI, Y.,
NAGASE, M., and
KURIHARA, K.: ‘Fabrication of SiO,/SilSiO, double barrier diodes
using two-dimensional Si structures’, Jpn. J. Appl. Phys., 1997, 36,
(6A), pp. 3669-3674
ISHIKAWA, Y., MAKITA, s., ZHANG, J., TSUCHIYA, T , and TABE, M.:
‘Capacitance-voltage study of silicon-on-insulator structure with an
ultrathin buried SiO, layer fabricated by wafer bonding’, Jpn. J.
Appl. Phys., 1999, 38, (7B), pp. L789-L791
WEINBERG, z.A.: ‘On tunneling in metal-oxide-silicon structures’, J.
Appl. Phys., 1982, 53, (7), pp. 5052-5056
Fig. 3 Schematic band diagram and typical I- V curves
a Schematic band diagram under positive voltage
b I- V curve for diode with -5 nm thick Si layer (taken at 15 K)
c I- V curve for diode with -2 nm thick Si layer (taken at 15 K)
- - _ calculated resonance voltages
Figs. 36 and c show typical I-V curves taken at 15K. As shown
in Fig. 3b, the sample with -5nm thick Si showed a featureless
curve. In contrast, as seen in Fig. 3c, for the sample with -2nm
thick Si, a clear NDC (peak-to-valley ratio (PVR) of -1.8) was
reproducibly observed at 0.28 V together with inflections indicated
by the arrows. We believe that this is the first observation of NDC
in Si/Si02 systems. This NDC feature was observed up to -150K,
as shown in Fig. 4, which also reveals broadening of the peak
width and negative shift of the peak position with increasing temperature. Such features are adequate as the effect of the thermal
energy on the resonant tunnelling current, indicating that the
observed NDC is ascribed to the resonant tunnelling.
Bounds on aperiodic and odd correlations of
spreading sequences with low and zero
correlation zone
0.1
0.2
0.3
applied voltage, V
0.4
X.H. Tang a n d P.Z. Fan
0.5
To eliminate co-channel and multipath interference, a spreading
Fig. 4 Temperature dependence of I-V curves for diode with -2nm thick
Si layer
Resonance voltages were estimated by a conventional method
[6], using the band diagram shown in Fig. 30. Here, the anisotropy of electron mass for the Si (001) plane was taken into
account (m, = 0.984 and m, = 0 . 1 9 ~
(m = electron rest mass)).
The effective mass in Si02 and the band offset at the Si/Si02interface were assumed to be 0 . 5 0 ~and 2.9 eV [8]. The thicknesses of
the lower and upper barriers were assumed to be 3.0 and 2.5nm,
respectively, from the TEM observation.
The dashed lines in Fig. 3c represent the calculated resonance
voltages for Si thickness of 1.6nm. (V; is for mb while V ;is for m,
(i = level index).) These values almost agree with the experiment.
However, further study is necessary to explain the overall features
of the I- V curve.
Conclusion: An Si/SiO, double bamer structure was fabricated
from a bonded SO1 wafer with an ultrathin BOX layer. In the I- V
ELECTRONICS LETTERS
13th September 2001
sequence set with a low or zero correlation zone can be used in an
AS-CDMA system. Here, lower bounds on aperiodic and odd
correlation of the sequences with equal energy and a IOW or zero
correlation zone are presented.
Introduction: To eliminate co-channel and multipath interference,
an approximately synchronised CDMA system (AS-CDMA) was
proposed in [l]. The AS-CDMA system makes use of a new type
of spreading sequences called zero correlation zone (ZCZ) or low
correlation zone (LCZ) sequences [2 - 51, or generalised orthogonal sequences [6], the periodic correlation functions of which take
zero or very low values within a correlation zone. We have derived
the corresponding bounds on the periodic correlation of the LCZ
(ZCZ) sequence set based on the bounds on inner product [7].
However, Pursley showed that the aperiodic correlation property of spreading sequences is more important for system performance compared with the periodic correlation property [8].
Recently, a new class of ZCZ sequences, even with both aperiodic
and odd zero correlation zone, has been constructed [9]. In [lo],
Vol. 37
No. 19
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