Vol. 36, No. 9
Journal of Semiconductors
September 2015
Thin-barrier enhancement-mode AlGaN/GaN MIS-HEMT using ALD Al2 O3 as gate
insulator
Wang Zheli(王哲力)Ž , Zhou Jianjun(周建军), Kong Yuechan(孔月婵), Kong Cen(孔岑),
Dong Xun(董逊), Yang Yang(杨洋), and Chen Tangsheng(陈堂胜)
Science and Technology on Monolithic Integrated Circuits and Modules Laboratory, Nanjing Electronic Devices Institute,
Nanjing 210016, China
Abstract: A high-performance enhancement-mode (E-mode) gallium nitride (GaN)-based metal–insulator–
semiconductor high electron mobility transistor (MIS-HEMT) that employs a 5-nm-thick aluminum gallium nitride (Al0:3 Ga0:7 N) as a barrier layer and relies on silicon nitride (SiN) passivation to control the 2DEG density
is presented. Unlike the SiN passivation, aluminum oxide (Al2 O3 / by atomic layer deposition (ALD) on AlGaN
surface would not increase the 2DEG density in the heterointerface. ALD Al2 O3 was used as gate insulator after
the depletion by etching of the SiN in the gate region. The E-mode MIS-HEMT with gate length (LG / of 1 m
showed a maximum drain current density .IDS / of 657 mA/mm, a maximum extrinsic transconductance (gm / of
187 mS/mm and a threshold voltage (Vth / of 1 V. Comparing with the corresponding E-mode HEMT, the device
performances had been greatly improved due to the insertion of Al2 O3 gate insulator. This provided an excellent
way to realize E-mode AlGaN/GaN MIS-HEMTs with both high Vth and IDS .
Key words: enhancement-mode (E-mode); AlGaN/GaN; metal–insulator–semiconductor high electron mobility
transistor (MIS-HEMT); atomic layer deposition (ALD); Al2 O3
DOI: 10.1088/1674-4926/36/9/094004
EEACC: 2560
1. Introduction
Provided with both high blocking voltage and current
handling capability, GaN-based devices have been expected
to be a viable alternative to GaAs, silicon, and even to
SiC ones in power electronics. The performances, especially
of AlGaN/GaN high electron mobility transistors (HEMTs),
have been tremendously improved over the past two decades.
However, AlGaN/GaN HEMTs reported so far are usually
operated as depletion-mode (D-mode). The development of
enhancement-mode (E-mode) AlGaN/GaN HEMTs has become essential to expand the range of their applications. Emode AlGaN/GaN HEMTs enable a circuit to be simple. This is
important for high-frequency circuits, because all passive components tend to result in large loss when they operate in highfrequency range. E-mode AlGaN/GaN HEMTs have another
benefit whereby various monolithic integrated D-/E-mode digital circuits can be realized. Furthermore, E-mode devices are
particularly desirable for high-voltage and high-power switching applications in which safety is a critical issueŒ1 .
Common approaches to achieve E-mode AlGaN/GaN
HEMTs are based on the conventional D-mode devices (20nm-thick barrier layer with an inherent two dimensional electron gas (2DEG) channel) and include growth of a p-type AlGaN cap layerŒ2 , recessed-gateŒ3 or fluorine ion implantationŒ4 to locally deplete the channel underneath the gate. These
approaches suffer from non-uniformity in device performance,
device instability and low Vth , respectivelyŒ5 . They are gener-
ally faced with the trade-off between the available drain current
density and threshold voltage. An alternative method proposed
by Higashiwaki et al. is the combination of thin-barrier AlGaN/GaN heterostructure and SiN passivationŒ6 . They found
that passivation by SiN can dramatically increase the density of 2DEG in thin-barrier AlGaN/GaN heterostructuresŒ7 .
This effect was utilized to fabricate E-mode GaN HEMTs by
etching the deposited SiN in the gate region to deplete the
2DEG under itŒ1 . As a result of Schottky contact, the gate
voltage swing was restricted to a small value, thus limiting
the available maximum drain current density. In addition, the
reverse gate leakage current was quite large which deteriorated the device performances and long-term reliabilityŒ8 . A
metal–insulator–semiconductor (MIS) structure was preferred
to solve the above problemsŒ9 . Owing to its large bandgap
(9 eV) and high dielectric constant (10), Al2 O3 was often
chosen as the gate insulator and protective passivation layer for
both D- and E-mode HEMTsŒ10; 11 .
In this paper, we studied the passivation effect of atomiclayer-deposited (ALD) Al2 O3 and found that it was a potential gate insulator for thin-barrier E-mode AlGaN/GaN MISHEMT with little influence on the electrical properties of
2DEG. The fabricated MIS-HEMT was compared with the corresponding E-mode HEMT (studied in Reference [1]) using the
same material and process without ALD Al2 O3 gate insulator.
The DC performances had been greatly improved after the insertion of gate insulator. A simple method to realize E-mode
operation with large positive Vth and high source–drain current
* Project supported by the National Natural Science Foundation of China (Nos. 61474101, 61106130) and the Natural Science Foundation
of Jiangsu Province of China (No. BK20131072).
† Corresponding author. Email: zlwang1990@163.com
Received 9 March 2015, revised manuscript received 31 March 2015
© 2015 Chinese Institute of Electronics
094004-1
J. Semicond. 2015, 36(9)
Wang Zheli et al.
Figure 1. Schematic cross-section view of the thin-barrier E-mode AlGaN/GaN MIS-HEMT.
Figure 2. Gate current characteristics of thin-barrier E-mode AlGaN/GaN HEMT and MIS-HEMT.
Table 1. Room-temperature Hall measurement before and after the
passivation of SiN and Al2 O3 on AlGaN/GaN heterostructure.
density was feasible with this structure.
2. Device structure and fabrication
The AlGaN/GaN heterostructure used in this study was
grown on a c-face sapphire substrate by metal-organic chemical vapor deposition (MOCVD) system. The epitaxial wafer
consisted of, from bottom to top, a 100-nm-thick AlN nucleation layer, a 2000-nm-thick GaN buffer and channel layer, and
a 5-nm-thick Al0:3 Ga0:7 N barrier layer. No intentional doping
was performed during the MOCVD growth. Figure 1 shows a
schematic cross-section view of the E-mode AlGaN/GaN MISHEMT. The source–drain spacing was 4 m and the gate width
was 100 m.
The E-mode device was fabricated with the following procedures: (1) passivation. 10-nm SiN was deposited on the
cleaned AlGaN surface by PECVD. A thinner SiN layer was
preferred for better ohmic contactsŒ12 . However, the thickness
error was around ˙1 nm in our PECVD system and 10 nm was
chose due to the equipment limitation. (2) Ohmic contact. After
the definition of source and drain patterns by photolithography,
Ti/Al/Ni/Au metals were deposited directly on SiN, followed
by a lift-off process then annealed at 800 ıC for 30 sŒ13 . Using on-wafer transfer length measurement (TLM) patterns, the
specific contact resistance (sc / was typically measured to be
4.6 10 6 /cm2 . (3) Isolation. Isolation was achieved by BC
implantation. (4) Etching of SiN in gate region. Following the
opening of gate resist pattern by lithography, a two-step reactive ion etching (RIE) based on SF6 was performed to remove
the exposed SiN with little damage to the AlGaN surface. (5)
Gate insulator. 8-nm-thick Al2 O3 was deposited by ALD on
the AlGaN in the gate region. (6) Gate and interconnection.
The Ni/Au metals were evaporated for gate metallization and
the Ti/Au bond-pad metals were evaporated for device characterization. We also fabricated an E-mode AlGaN/GaN HEMT
without gate insulator using the same epi-wafer for comparison.
Parameter
Rsh (//
(cm2 /(Vs))
As-grown
After SiN passivation
After Al2 O3 passivation
1842
546
386
951
Ns (1013
cm 2 /
0.88
1.2
1830
396
0.86
3. Results and discussion
The passivation effects of PECVD SiN and ALD Al2 O3
on AlGaN/GaN heterostructures were first studied. Roomtemperature Hall measurements were conducted to estimate
the electrical properties of 2DEG. Table 1 shows summaries of
sheet resistance (Rsh /, electron mobility (/ and sheet charge
density (Ns / before and after the passivation respectively. Rsh
of the as-grown epitaxial wafers was very high due to the thin
AlGaN barrier layerŒ14 . It decreased from 1842 to 546 / after the SiN passivation, which was consistent with the results in
Reference [7]. However, the passivation of ALD Al2 O3 nearly
had no influence on the properties of 2DEG. This made it a feasible gate insulator for E-mode MIS-HEMTs without increasing the 2DEG density in the gate region.
An Agilent B1500A was employed to measure the DC
characteristics of the MIS-HEMT. The gate leakage characteristics of Schottky-gate HEMT and MIS-HEMT are shown in
Figure 2. During the measurement, both source and drain electrodes were grounded and a varying voltage, from –10 to 10 V,
was applied to the gate electrode. The leakage current was reduced by two orders of magnitude after the insertion of gate
insulator, indicating the excellent insulation characteristics of
ALD Al2 O3 . The fluctuation in leakage current of MIS-HEMT
was due to the accuracy of the measuring equipment. For the
Schottky-gate HEMT, the forward gate current density rapidly
increased with the gate voltage and exceeded 10 mA/mm at a
gate bias of 2 V, while in the MIS-HEMT, the forward gate
current density at a gate bias of 10 V stayed as low as 1 10 6 A/mm. For power switching applications, the larger gate
094004-2
J. Semicond. 2015, 36(9)
Wang Zheli et al.
Figure 4. The transfer characteristics of thin-barrier E-mode AlGaN/GaN HEMT and MIS-HEMT.
Figure 3. DC I –V characteristics of thin-barrier E-mode AlGaN/GaN
(a) HEMT and (b) MIS-HEMT.
input voltage swing may improve the safety and reliability of
the deviceŒ11 .
Figure 3(a) shows typical DC current–voltage (I –V )
curves of the E-mode AlGaN/GaN HEMT without Al2 O3 gate
insulator. The gate was biased from –1 to 2 V in steps of 1 V.
The device exhibited a maximum drain current density (IDS /
of 512 mA/mm at a gate bias of 2 V and a drain bias of 3.2 V.
However, it had a large forward gate current (exceeding 3 mA)
when the gate was biased at 2 V. This problem was solved by
inserting an ALD Al2 O3 layer between the gate and AlGaN
(Figure 2(b)). The gate voltage (VG / could be biased at 6 V
and IDS was increased to 657 mA/mm which nearly reached
the saturation current density. It was obvious that at the same
gate bias, IDS in MIS-HEMT was much smaller than the one in
HEMT. This was because the gate capacitance was decreased
by the insertion of gate insulator. When the same positive voltage was applied to the gate, the density of 2DEG under the gate
was much smaller in the MIS structure. For the same reason,
the pinch-off characteristic (bias at 0 V) of the MIS-HEMT was
much better than the HEMT.
The DC transfer characteristics of the two devices are
shown in Figure 4. For the device without gate insulator, a
peak extrinsic transconductance (gm / of 268 mS/mm was measured at VG D 0.9 V. By defining the threshold voltage .Vth /
as the gate bias intercept of the extrapolation of drain current
Figure 5. Three-terminal breakdown characteristics of thin-barrier Emode AlGaN/GaN HEMT and MIS-HEMT with LGD D 1.5 m and
LDS D 4 m.
at the point of peak gm , Vth of the HEMT was determined to
be 50 mV. After the insertion of gate insulator, gm was decreased to 187 mS/mm (at VG D 2 V) and Vth was increased
to 1 V. The increased distance between gate and channel was
the main cause of the decrease of gm . Two main factors might
contribute to the positive shift of Vth : the Al2 O3 /AlGaN interface statesŒ15 and the fixed negative charges in the dielectric layerŒ16 . Further experiments are needed to determine the
mechanism.
Both devices’ breakdown voltage (Vbr / at pinch-off state
(with gate voltage of 0 V) was measured and shown in Figure
5. The maximum drain–source current .ID;br / was chosen to be
1 mA/mm to avoid permanent degradation of the devices. The
MIS-HEMT showed great improvement against the HEMT in
breakdown characteristics, with Vbr increased from 41 to 94 V.
4. Conclusion
In summary, an E-mode AlGaN/GaN MIS-HEMT with
excellent performances was achieved by combining thin barrier
and passivation technology. Unlike SiN passivation, the ALD
Al2 O3 deposited on the AlGaN surface would not increase
the 2DEG density and was used as the gate insulator for the
094004-3
J. Semicond. 2015, 36(9)
Wang Zheli et al.
thin-barrier AlGaN/GaN heterostructure which featured a high
sheet resistance without SiN passivation. So we etched the SiN
layer only in the gate region to deplete the 2DEG while keeping the 2DEG density of access region high and then deposited
ALD Al2 O3 as the gate insulator to fabricate an E-mode MISHEMT. Compared with the corresponding HEMT without gate
insulator, the DC characteristics, such as maximum drain current density, gate leakage current and threshold voltage, have
been greatly improved. Along with the D-mode AlGaN/GaN
MIS-HEMT, these results demonstrate the great potential of
thin-barrier AlGaN/GaN heterostructures for monolithic integrated E-/D-mode GaN-based circuits.
References
[1] Higashiwaki M, Mimura T, Matsui T. Enhancement-mode
AlN/GaN HFETs using cat-CVD SiN. IEEE Trans Electron Devices, 2007, 54(6): 1566
[2] Hu X, Simin G, Yang J, et al. Enhancement mode AlGaN/GaN
HFET with selectively grown pn junction gate. Electron Lett,
2000, 36(8): 753
[3] Wang C, Zhang J, Quan S, et al. An enhancement-mode AlGaN/GaN HEMT with recessed-gate. Journal of Semiconductors, 2008, 29(9): 1682
[4] Quan S, Hao Y, Ma Y, et al. Enhancement-mode AlGaN/GaN
HEMTs fabricated by fluorine plasma treatment. Journal of Semiconductors, 2009, 30(12): 124002
[5] Brown R, Macfarlane D, Al-Khalidi A, et al. A sub-critical
barrier thickness normally-off AlGaN/GaN MOS-HEMT. IEEE
Electron Device Lett, 2014, 35(9): 906
[6] Higashiwaki M, Mimura T, Matsui T. Enhancement-mode
AlN/GaN HFETs using cat-CVD SiN. IEEE Trans Electron Devices, 2007, 54(6): 1566
[7] Higashiwaki M, Hirose N, Matsui T. Cat-CVD SiN-passivated
AlGaN-GaN HFETs with thin and high Al composition barrier
layers. IEEE Electron Device Lett, 2005, 26(3): 139
[8] Turuvekere S, Karumuri N, Rahman A A, et al. Gate leakage
mechanisms in AlGaN/GaN and AlInN/GaN HEMTs: comparison and modeling. IEEE Trans Electron Devices, 2013, 10(10):
3157
[9] Khan M A, Hu X, Sumin G, et al. AlGaN/GaN metal oxide semiconductor heterostructure field effect transistor. IEEE Electron
Device Lett, 2000, 21(2): 63
[10] Yue Y Z, Hao Y, Feng Q, et al. GaN MOS-HEMT using ultra-thin
Al2 O3 dielectric grown by atomic layer deposition. Chin Phys
Lett, 2007, 24(8): 2419
[11] Dong Z H, Tan S X, Cai Y, et al. 5.3 A/400 V normally-off
AlGaN/GaN-on-Si MOS-HEMT with high threshold voltage and
large gate swing. Electron Lett, 2013, 49(3): 221
[12] Higashiwaki M, Onojima N, Matsui T, et al. Effects of SiN passivation by catalytic chemical vapor deposition on electrical properties of AlGaN/GaN heterostructure field-effect transistors. J
Appl Phys, 2006, 100(3): 033714
[13] Higashiwaki M, Mimura T, Matsui T. AlN/GaN insulted-gate
HFETs using cat-CVD SiN. IEEE Electron Device Lett, 2006,
27(9): 719
[14] Tilak V, Green B, Kaper V, et al. Influence of barrier thickness
on the high-power performance of AlGaN/GaN HEMTs. IEEE
Electron Device Lett, 2001, 22(11): 504
[15] Capriotti M, Alexewicz A, Fleury C, et al. Fixed interface charges
between AlGaN barrier and gate stack composed of in situ grown
SiN and Al2 O3 in AlGaN/GaN high electron mobility transistors with normally off capability. Appl Phys Lett, 2014, 104(11):
113502
[16] Freedsman J J, Kubo T, Egawa T. High drain current density
E-mode Al2 O3 /AlGaN/GaN MOS-HEMT on Si with enhanced
power device figure-of-merit (4 108 V2  1 cm 2 /. IEEE
Trans Electron Devices, 2013, 60(10): 3079
094004-4