APPLIED PHYSICS LETTERS 93, 053504 共2008兲
Inversion-channel GaN metal-oxide-semiconductor field-effect transistor
with atomic-layer-deposited Al2O3 as gate dielectric
Y. C. Chang,1 W. H. Chang,1 H. C. Chiu,1 L. T. Tung,1 C. H. Lee,1 K. H. Shiu,1 M. Hong,1,a兲
J. Kwo,2,a兲 J. M. Hong,3 and C. C. Tsai3
1
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 30013,
Taiwan
2
Department of Physics, National Tsing Hua University, Hsinchu, Taiwan 30013, Taiwan
3
HUGA Optotech, Inc., Taichung, Taiwan 40768, Taiwan
共Received 19 June 2008; accepted 17 July 2008; published online 7 August 2008兲
Inversion n-channel GaN metal-oxide-semiconductor field-effect-transistors 共MOSFETs兲 using
atomic-layer-deposited Al2O3 as a gate dielectric have been fabricated, showing well-behaved drain
I-V characteristics. The drain current was scaled with gate length 共varying from 1 to 16 ␮m兲,
showing a maximum drain current of ⬃10 mA/ mm in a device of 1 ␮m gate length, at a gate
voltage of 8 V and a drain voltage of 10 V. At a drain voltage of 0.1 V, a high Ion / Ioff ratio of
2.5⫻ 105 was achieved with a very low off-state leakage of 4 ⫻ 10−13 A / ␮m. Both MOSFET and
MOS capacitor showed very low leakage current densities of 10−8 A / cm2 at biasing fields of
4 MV/ cm. The interfacial density of states was calculated to be 共4 – 9兲 ⫻ 1011 cm−2 eV−1 near the
midgap. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2969282兴
GaN, with a high saturation velocity at high electrical
fields 共␷sat ⬃ 3 ⫻ 107 cm/ s at 150 kV/ cm兲,1 a high critical
electrical field 共up to 3 MV/ cm兲, good thermal
conductivity,2 and epilayers grown on Si,3,4 has been widely
studied for applications in high-power and high-temperature
devices such as heterojunction field-effect-transistors
共HFETs兲.5–7 Compared to conventional rf AlGaN / GaN
HFETs, GaN metal-oxide-semiconductor field-effecttransistors 共MOSFETs兲 feature a larger voltage sweep range,
lower gate leakage currents, and circuit simplicity, and hence
have attracted much interest lately.8–10
Owing to its wider energy band gap 共3.4 eV兲 that alleviates the adverse affects such as drain-induced barrier lowering and band-to-band tunneling, GaN is now also being
considered as a channel candidate for the next generation
complementary metal-oxide-semiconductor 共CMOS兲 devices
beyond the 22 nm node technology. Furthermore, by taking
into account the short channel effect with the cutoff frequency 共f T兲 given by f T = ␷sat / 2␲L 共where L is the gate
length兲, GaN MOSFETs may outperform its counterparts of
Si and GaAs in further scaled-down devices, despite the fact
that GaN offers no special advantage in electron mobility.
With the discovery of Ga2O3共Gd2O3兲 as a gate
dielectric11 leading to the first depletion-mode GaN
MOSFET,12 much research effort has been paid to alternative
high ␬ dielectrics on GaN, including Ga2O3共Gd2O3兲,11,13
Al2O3,6,10 MgO,14,15 and HfO2.16 For the inversion-channel
GaN MOSFETs with MgO and SiNx as gate dielectrics,
anomalous drain current-voltage 共I-V兲 characteristics such as
high off-state leakage current and nonsaturating behavior
were often observed.14,17 Good device characteristics were
exhibited in inversion-channel GaN MOSFETs based on
SiO2 as a gate dielectric.8,9 However, the CMOS scaling has
demanded alternative high ␬ gate dielectrics to replace conventional SiO2. Al2O3 with a higher dielectric constant of
a兲
Author to whom the correspondence is addressed. Electronic mail:
mhong@mx.nthu.edu.tw and raynien@phys.nthu.edu.tw.
0003-6951/2008/93共5兲/053504/3/$23.00
7–9 is a more suitable gate dielectric than SiO2. Furthermore,
Al2O3, with a considerably larger band gap than other high ␬
dielectrics, gives higher energy band offsets with GaN, resulting in reduced gate leakage currents for the same thickness of other high ␬ dielectrics.
In this work, inversion n-channel GaN MOSFETs based
on atomic-layer-deposited 共ALD兲 high ␬ Al2O3 as a gate
dielectric have achieved an abrupt, smooth interface, and furthermore, have shown electrical characteristics similar to
those with SiO2. Compared to the previously reported
inversion-channel GaN MOSFETs using other high ␬ dielectrics, our device performance has shown excellent dc output
as well as transfer characteristics. Capacitance-voltage 共C-V兲
and leakage-current-density versus gate-electrical-field
共J-Eg兲 measurements of GaN MOS capacitors 共MOSCAPs兲
have been carried out to assess the interfacial quality between ALD-Al2O3 and p-GaN. A low electrical leakage current density of 10−8 A / cm2 at an Eg field of ⫾4 MV/ cm was
obtained along with a low interfacial density of states 共Dit兲 of
共4 – 9兲 ⫻ 1011 eV−1 cm−2 near the midgap.
The GaN epitaxial layers were grown on c-plane sapphire by metal-organic chemical-vapor-deposition, consisting
of a well of a p-GaN epilayer of 0.3 ␮m thickness with Mg
doping of 2 ⫻ 1017 cm−3 grown on an undoped GaN of
1.5 ␮m thickness. The Gd2O3 layers of 20 nm thickness
grown by molecular beam epitaxy18 were deposited on the
samples to protect GaN surface during the high temperature
dopant-activation annealing. The n+ source/drain contact regions were achieved by Si implantation with multiple-energy
conditions 共120 keV/ 1 ⫻ 1015 cm−2, 80 keV/ 6 ⫻ 1014 cm−2,
and 40 keV/ 4 ⫻ 1014 cm−2兲. This implant scheme was designed to produce a uniform Si profile to a depth of
⬃0.1 ␮m. Rapid thermal annealed 共RTA兲 to 1100 ° C for
5 min under He ambient was used for implant activation.
Afterwards, the Gd2O3 layer was removed by HCl etching.
ALD-Al2O3 was then used as a gate dielectric. The Al2O3
deposition includes a substrate temperature of 200 ° C and a
93, 053504-1
© 2008 American Institute of Physics
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053504-2
Chang et al.
FIG. 1. 共a兲 A schematic view of the inversion-channel GaN MOSFET with
ALD-Al2O3 as gate dielectric. 共b兲 Cross-sectional HR-TEM image and low
angle XRR of Al2O3 on GaN, with experimental data 共dots兲 and a theoretical
fit 共line兲.
chamber pressure of 1 Torr using alternating pulses of
Al共CH3兲3 共TMA兲 and H2O as precursors.19
The oxide thickness and interfacial microstructure were
studied using x-ray reflectivity 共XRR兲, and cross-sectional
high-resolution transmission electron microscopy 共HRTEM兲. The XRR measurement was performed using Cu K␣
radiation in a standard Huber four-circle x-ray diffractometer
operated at 50 kV and 200 mA. The reflectivity data were
then fitted with the BEDE REFS MERCURY code.20 The specimen for the cross-sectional HR-TEM studies were prepared
by mechanical polishing, dimpling, and ion milling using a
Gatan precision ion polishing PIPS system operated at
4 keV. The HR-TEM analysis was performed by a TEM of
Philips JEOL 2100F.
After using dilute buffered oxide etch 共BOE兲 solution to
remove Al2O3 in source/drain regions, Ti/ Al/ Pt/ Au
共30/ 120/ 80/ 120 nm兲 was deposited as the Ohmic metal, followed by annealing at 750 ° C for 30 s under He ambient
using RTA. Finally, Pt/ Au 共30/ 120 nm兲 was used as the gate
metal. The fabricated MOSFETs have a gate width of
100 ␮m and gate lengths varying from 1 to 16 ␮m. The
cross-sectional schematic of the fabricated device structure is
shown in Fig. 1共a兲. The Al2O3 / GaN interface remains atomically smooth after high temperature 750 ° C annealing under
He ambient, with the oxide thickness and the roughness determined from XRR to be 12 and 0.5 nm 关Fig. 1共b兲兴, respectively. A cross-sectional HR-TEM image 关Fig. 1共b兲兴 also
showed an abrupt transition from amorphous Al2O3 to crystalline GaN even after high temperature annealing, in agreement with the XRR data.
Well-behaved drain I-V characteristics of a GaN MOSFET are shown in Fig. 2共a兲 with a clean pinch off. For a
device of a 4 ␮m gate length and a 100 ␮m gate width, the
maximum drain current is 3.5 mA/ mm at a gate voltage Vg
of 10 V, and a drain voltage Vds of 15 V. The maximum
drain current was improved to ⬃10 mA/ mm in a 1 ␮m gatelength device, measured at a gate voltage of 8 V and a drain
voltage of 10 V. Our devices also showed normal drain I-V
characteristics and transfer characteristics, as expected for a
typical inversion-channel MOSFET. The measured drain current is scaled with gate length, with the scaling dependence
displayed in Fig. 2共b兲. The overall performances of these
devices are markedly improved over the previously reported
results of the inversion-channel GaN MOSFETs based on
MgO and SiNx high ␬ dielectrics.14,17
Appl. Phys. Lett. 93, 053504 共2008兲
FIG. 2. 共a兲 Drain I-V characteristic for a 4 ␮m gate length GaN MOSFET.
共b兲 The scaling characteristic of drain current vs gate length.
The transfer characteristics 共L / W = 4 / 100 ␮m兲 in Fig. 3
were obtained with Vg sweeping from 0 to 10 V, and Vds set
at 0.1 V. A high Ion / Ioff ratio of 2.5⫻ 105 is achieved with a
very low off-state leakage current of 4 ⫻ 10−13 A / ␮m, as determined with the drain current exhibited in a logarithmic
scale on the left-side y axis. The low gate leakage and the
low n + / p junction leakage current are attributed to the good
device characteristics. The device is normally off with a
threshold voltage of 2.8 V using a linearly scaled drain current shown on the right-side y axis, and a subthreshold slope
of 290 mV/decade. The large threshold voltage is mainly due
to a large potential difference between intrinsic Fermi energy
and Fermi level 共which is caused by the wide band gap property of GaN兲, and a high work function of Pt as the gate
metal. Choosing gate metals with lower work functions and
decreasing acceptor doping concentration in the p epilayer
well are expected to reduce the threshold voltage. Peak intrinsic transconductance and mobility are at least
⬃4.0 mS/ mm and ⬃10 cm2 / V s, respectively, by taking
consideration of the measured high sheet resistance 共6
⫻ 104 ⍀/square兲 and contact resistance 共3 ⫻ 10−3 ⍀ cm2兲 in
the device. The transconductance and mobility will be higher
when a self aligned process is implemented to further reduce
the sheet resistance and a better-quality GaN substrate is employed to increase the electron mobility.
Figure 4 shows the detailed J-Eg and C-V measurements
of MOSCAPs fabricated from the same device wafers, on
which all the device processing steps were applied, including
RTA to 1100 ° C, removal of Gd2O3, cleaning of GaN surface with HCl, deposition of ALD-Al2O3, and annealing at
750 ° C for 30 s under He ambient. Subsequently, the Ohmic
contact with the p-GaN layer was achieved by etching Al2O3
using BOE solution, followed by an e-beam evaporated
metal Pd/ Ni/ Au 共10/ 20/ 120 nm兲 and annealing at 500 ° C
FIG. 3. Transfer characteristics for a 4 ␮m gate length GaN MOSFET at a
drain voltage Vds of 0.1 V.
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053504-3
Appl. Phys. Lett. 93, 053504 共2008兲
Chang et al.
VFB extraction when taking the Pt work function to be
5.7 eV. A small portion of that may come from interfacial
traps, consistent with the Dit calculation. The voltage hysteresis for the MOSCAP is 360 mV at VFB.
In summary, excellent device performance and material
properties have been obtained in the GaN MOSFETs and
MOSCAPs employing high ␬ ALD-Al2O3 dielectrics. The
inversion-channel device has shown a significant breakthrough in dc output and transfer characteristics, compared to
those of previously reported inversion-channel GaN MOSFETs based on other high ␬ dielectrics. The drain current and
transconductance are expected to be further improved with a
self-aligned process and regrown S/D contacts to reduce the
Ohmic contact resistance and other parasitic series resistance.
The work is supported by National Nano projects 共NSC
96-2120-M-007-014 and NSC 96-2628-M-007-003-MY3兲 of
National Science Council in Taiwan, and the Asian Office of
Aerospace Research and Development of the U. S. Air Force.
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FIG. 4. 共a兲 Leakage current density 共J兲 vs gate electrical field 共Eg兲 for
Al2O3 / GaN MOSCAP and MOSFET, with the inset showing schematic
view of the GaN MOSCAP. 共b兲 C-V curves for the MOSCAP under frequencies varying from 1 to 100 kHz, with the inset showing Dit values calculated
by the conductance method.
for 1 min under N2 ambient. Pt/ Au 共30 / 120 nm兲 was still
used as the gate metal defined by photolithography. The
schematic cross-sectional view of the MOSCAPs is shown in
the Fig. 4共a兲 inset. The very low leakage current density of
10−8 A / cm2 at the gate electrical field Eg 艋 4 MV/ cm in
these MOSCAP indicates the highly insulating characteristics of the ALD-Al2O3 dielectrics. The leakage current density of MOSFET is larger than that of MOSCAP at reverse
bias, as caused by damages during device process. Furthermore, the breakdown field of 10 MV/ cm is notably higher
than those of other high ␬ dielectrics deposited on
GaN.11,15,16
C-V characteristics with frequencies varying from
1 to 100 kHz showed clear accumulation, depletion, and
deep depletion 关Fig. 4共b兲兴, displaying a weak frequency dispersion of 7.5% at accumulation. A dielectric constant of 7.5
was obtained at 1 kHz, expected for a good-quality
ALD-Al2O3. The deep depletion occurred due to the low
intrinsic carrier concentration 共ni ⬃ 10−10 cm−3兲 in GaN. The
Dit value was calculated to be 共4 – 9兲 ⫻ 1011 eV−1 cm−2 near
the midgap by the conductance method, shown in the inset of
Fig. 4共b兲. The flatband voltage 共VFB兲 is deduced to be −1.4 V
from the C-V curves. A low fixed charge density on the order
of 5 ⫻ 1011 cm−2 is obtained for the Al2O3 dielectrics from
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