AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistor with oxidized Ni as a gate insulator

Article abstract of DOI:10.1063/1.1811793

We fabricated the AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistor (MOSHFET) using the oxidized Ni(NiO) as a gate oxide and compared electrical properties of this device with those of a conventional AlGaN/GaN heterostructure field-effect transistor (HFET). NiO was prepared by oxidation of Ni metal of 100 A at 600°C for 5 min in air ambient. For HFET and MOSHFET with a gate length of 1.2 μm, the maximum drain currents were about 800 mA/mm and the maximum transconductances were 136 and 105 mS/mm, respectively. As the oxidation temperature of Ni increased from 300 to 600°C the gate leakage current decreased dramatically due to the formation of insulating NiO. The gate leakage current for the MOSHFET with the oxidized NiO at 600°C was about four orders of magnitude smaller than that of the HFET. Based on the dc characteristics, NiO as a gate oxide is comparable with other gate oxides.

Full text of DOI:10.1063/1.1811793

AlGaN ∕ GaN metal-oxide-semiconductor heterostructure field-effect transistor with
oxidized Ni as a gate insulator
C. S. Oh, C. J. Youn, G. M. Yang, K. Y. Lim, and J. W. Yang
Citation: Applied Physics Letters 85, 4214 (2004); doi: 10.1063/1.1811793
View online: http://dx.doi.org/10.1063/1.1811793
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 18
1 NOVEMBER 2004
AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect
transistor with oxidized Ni as a gate insulator
C. S. Oh,a) C. J. Youn, G. M. Yang, K. Y. Lim, and J. W. Yang
Department of Semiconductor Science and Technology and Semiconductor Physics Research Center,
Chonbuk National University, Chonju 561-756, Korea
(Received 12 April 2004; accepted 31 August 2004)
We fabricated the AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistor
(MOSHFET) using the oxidized Ni͑NiO͒ as a gate oxide and compared electrical properties of this
device with those of a conventional AlGaN/GaN heterostructure field-effect transistor (HFET). NiO
was prepared by oxidation of Ni metal of 100 Å at 600 °C for 5 min in air ambient. For HFET and
MOSHFET with a gate length of 1.2 ␮m, the maximum drain currents were about 800 mA/mm and
the maximum transconductances were 136 and 105 mS/mm, respectively. As the oxidation
temperature of Ni increased from 300 to 600 °C the gate leakage current decreased dramatically due
to the formation of insulating NiO. The gate leakage current for the MOSHFET with the oxidized
NiO at 600 °C was about four orders of magnitude smaller than that of the HFET. Based on the dc
characteristics, NiO as a gate oxide is comparable with other gate oxides. © 2004 American
Institute of Physics. [DOI: 10.1063/1.1811793]
Recent advancements in AlGaN/GaN based heterostruc-
ture field-effect transistors (HFETs) have opened the way for
their practical applications to high power and high frequency
electronic devices.1–3 However, several significant problems
still remain hindering wide range applications of these de-
vices. First, the gate leakage current limiting the operation of
HFET is too high.4 Second, HFETs exhibit current collapse
with a high rf input drive on the gate resulting in signifi-
cantly reducing the rf powers below the values expected
from dc.5,6 AlGaN/GaN metal-oxide-semiconductor HFETs
(MOSHFETs) using the gate oxide such as Ga2O3͑Gd2O3͒,7
about 10−7 Torr. However, there is no work about NiO gate
insulator for MOSHFET.
In this letter, we report on the fabrication of the
AlGaN/GaN MOSHFET with NiO insulating layer and
present dc characteristics of the devices. NiO was prepared
by oxidation of Ni metal of 100 Å at 600 °C for 5 min in air
ambient. For a gate length of 1.2 ␮m, the maximum drain
currents were about 800 mA/mm for both devices. The gate
leakage current for the MOSHFET with the oxidized NiO at
600 °C was about four orders of magnitude smaller than that
of the HFET. Based on the dc characteristics, NiO as a gate
oxide is comparable with other gate oxides.8,12,14
The AlGaN/GaN HFET structures were grown by met-
alorganic chemical vapor deposition on c-plane Al2O3 sub-
strates. The layer structure had a 2-␮m-thick semi-insulating
GaN buffer followed by a 500-Å-thick unintentionally doped
Al0.28Ga0.72N layer. The Hall mobility and sheet carrier den-
sity were 720 cm2/V s and 7.4ϫ1012 cm−2, respectively.
Two device structures with and without NiO gate insulator
layer were fabricated. The device dimensions are gate length
͑Lg͒ of 1.2 ␮m, gate width ͑Wg͒ of 20 ␮m, and source-drain
separation of 2.7 ␮m. Figure 1 shows a cross-sectional sche-
matic diagram of the MOSHFET with NiO gate insulator.
The device fabrication started with device isolation. A BCl3
based inductively coupled plasma system was used to define
the mesa. Ohmic metals consisted of thermal deposited
SiO2,8–10 MgO,11 Sc2O3,12 and insulators such as AlN13 and
14,15
Si3N4
offer lower gate leakage current and greater volt-
age swings than conventional HFETs. In addition, the gate
oxide can also be used for surface passivation, mitigating the
current collapse that occurs in unpassivated devices due to
traps between the gate and drain regions.
Recently, Ho et al.16,17 successfully developed a contact
to p-type GaN with a specific contact resistance lower than
1ϫ10−4 ⍀ cm2 using an oxidizing Ni/Au͑100/50 Å͒. They
attributed the low resistance ohmic contact to the formation
of p-NiO. However, it is estimated that the hole concentra-
tion of oxidized Ni is less than 1ϫ1016 cm−3. In addition,
the perfect NiO crystal is an insulator of which band gap and
dielectric constant are 4.0 eV and 11.9, respectively.17 The
dielectric constant of NiO is much larger than that of
SiO2͑␧=3.9͒, Si3N4͑␧=7.5͒, and MgO͑␧=9.8͒ and compa-
rable to Sc2O3͑␧=14.5͒. Therefore, NiO can act as a gate
insulator for MOSHFET. In view of process point, moreover,
using oxidized NiO has two advantages compared to other
gate oxides. The one is free from the plasma exposure that
can damage the device property due to the direct exposure of
plasma to active region during the oxide deposition.18–21 The
other is a relative clean interface between AlGaN and gate
oxide because Ni metal is deposited at very high vacuum of
a)Author to whom correspondence should be addressed; electronic mail:
stones_oh@hotmail.com
FIG. 1. Schematic diagram of NiO/AlGaN/GaN MOSHFET structure.
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0003-6951/2004/85(18)/4214/3/$22.00 4214 © 2004 American Institute of Physics
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Appl. Phys. Lett., Vol. 85, No. 18, 1 November 2004
Oh et al.
4215
FIG. 2. Drain-source current ͑Ids͒ characteristics of the (a) NiO/AlGaN/GaN MOSHFET and (b) AlGaN/GaN HFET with Lg =1.2 ␮m and Wg =20 ␮m as a
function of drain-source voltage ͑Vds͒. The gate-source voltage ͑Vgs͒ was varied from 4 to −6 V in steps of −2 V.
Ti/Al/Ni/Au͑250/1100/300/1200 Å͒ and annealed at
800 °C for 30 s using rapid thermal annealing in a N2 am-
bient. The ohmic contact characteristics of the Ti/Al/Ni/Au
were investigated by transmission line method . For the oxi-
dized Ni layer Ni metal of 100 Å was deposited on AlGaN
layer using e-beam evaporator at about 10−7 Torr between
ohmic contacts and then standard lift-off procedures were
used to remove the metal’s outside patterns. Ni oxidation
was performed with a thermal treatment from 300 to 600 °C
for 5 min in an air ambient. As the annealing temperature
increased the oxidized Ni layer changed into transparency
from dark color, which revealed the transformation of Ni
layer into NiO. Thermal evaporated Ni/Au gate contacts
were employed on both the HEFTs and MOSHFETs.
Current–voltage measurements were performed using a pa-
rameter analyzer (HP 4155A).
Figure 2 shows the drain current ͑Ids͒ versus drain-
source voltage ͑Vds͒ characteristics of MOSHFET and HFET
for different values of gate-source voltage ͑Vgs͒ at room tem-
perature. As shown in Fig. 2, for both devices the maximum
drain current of about 800 mA/mm was comparable and the
negative resistance characteristics due to self-heating were
shown for higher gate biases. Good pinch off properties were
observed at gate biases of about −5.9 and −5.0 V for
MOSHFET and HFET, respectively. The higher pinch off
voltage in MOSHFET compared with that of HFET is con-
sidered to be due to the addition of NiO insulating layer. For
a higher gate bias, however, MOSHFET is well modulated
more than HFET due to NiO.
Figure 3 shows extrinsic transconductance ͑Gm͒ and Ids
as a function of Vgs for MOSHFET and HFET. The maxi-
mum transconductances ͑Gmmax͒ of MOSHFET were
105 mS/mm which was smaller than 136 mS/mm of HFET
in the saturation region of Vds=8 V due to the addition of
NiO insulating layer. As can be seen from Fig. 3, the
MOSHFET using NiO gate oxide has an advantage of having
a larger gate voltage swing and a higher linearity than the
HFET. This should lead to smaller intermodulation distor-
tion, to a smaller phase noise, and to a larger dynamic range
compared to the HFET.
The gate leakage current properties for MOSHFET at
different Ni oxidized temperatures at 5 min in air ambient
and HFET are shown in Fig. 4. Above 500 °C the
MOSHFETs were observed to have better gate properties
than those of HFET. The color of the as-deposited Ni was
dark. However, as the oxidation temperature increased Ni
metal changed into transparency, which was related to the
electrical property of oxidized Ni layer. The inset of Fig. 4
shows the two dominant gate leakage current passes in
MOSHFET. The gate leakage current for the MOSHFET at
600°C oxidation temperature was shown to be about four
orders of magnitude smaller than that for the HFET at Vgs
FIG. 4. Gate current comparisons of MOSHFET at different Ni oxidized
temperatures and HFET as a function of gate voltage. The device width is
20 ␮m. The inset shows the gate leakage current passes through the oxi-
FIG. 3. Extrinsic transconductance ͑Gm͒ and Ids characteristics of the
NiO/AlGaN/GaN MOSHFET and AlGaN/GaN HFET as a function of
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gate-source voltage ͑Vgs͒ at Vds=8 V.
dized Ni layer in the MOSHFET.
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4216
Appl. Phys. Lett., Vol. 85, No. 18, 1 November 2004
Oh et al.
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characteristics of the MOSHFET showed a good potential of
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