CHIN. PHYS. LETT. Vol. 28, No. 7 (2011) 077202
Enhancement-Mode AlGaN/GaN High Electron Mobility Transistors Using a
Nano-Channel Array Structure *
LIU Sheng-Hou(刘胜厚)1,2 , CAI Yong(蔡勇)1** , GONG Ru-Min(龚孺敏)2 , WANG Jin-Yan(王金延)2 ,
ZENG Chun-Hong(曾春红)1 , SHI Wen-Hua(时文华)1 , FENG Zhi-Hong(冯志红)3 , WANG Jing-Jing(王晶晶)3 ,
YIN Jia-Yun(尹甲运)3 , Cheng P. Wen(文正)2 , QIN Hua(秦华)4 , ZHANG Bao-Shun(张宝顺)1
1
Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215125
2
Institute of Microelectronics, Peking University, Beijing 100871
3
Science and Technology on ASIC Lab, Hebei Semiconductor Research Institute, Shijiazhuang 050051
4
Key Laboratory of Nanodevice and Application, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of
Sciences, Suzhou 215125
(Received 25 March 2011)
A nano-channel array (NCA) structure is applied to realize enhancement-mode (E-mode) AlGaN/GaN highelectron mobility transistors (HEMTs). The fabricated NCA-HEMT, consisting of 1000 channels connected
in parallel with a channel width of 64 nm, shows a threshold voltage of 0.15 V and a subthreshold slope of
78 mV/dec, compared to −3.92 V and 99 mV/dec for a conventional HEMT (C-HEMT), respectively. Both the
NCA-HEMT and C-HEMT show similar gate leakage current, indicating no significant degradation in gate leakage
characteristics for the NCA-HEMT. The surrounding-field effect and relieved polarization contribute to the very
large positive threshold voltage shift, while the work function difference makes it positive.
PACS: 72.80.Ey, 85.30.Tv, 85.30.De
DOI:10.1088/0256-307X/28/7/077202
Field effect transistors (FETs) using an AlGaN/GaN heterojunction structure are widely investigated for high power switching,[1] high temperature digital circuit[2] and rf/microwave power amplifier applications, because of their superior material
properties including wide bandgap, high breakdown
field, high electron saturation velocity and high sheet
carrier concentration.[3] High density two-dimensional
electron gas (2DEG) induced by spontaneous and
piezoelectric polarization effects presents the conventional AlGaN/GaN HEMTs as depletion-mode (Dmode) transistors.[4] However, for the inherent safety
and robustness of a power device configuration,[5] the
simplest circuit configuration of digital circuits[6] and
the single-polarity supply voltage for rf and microwave
circuits require enhancement-mode (E-mode) HEMTs.
Many works about E-mode HEMTs have been undertaken by using a recessed gate structure,[7] fluoride
treatment,[8] the gate injection technique[9] and the
piezo neutralization technique,[10] each with their own
advantages and disadvantages.
Recently, Ohi et al.[11] used multi-mesa-channel
(MMC) structures to realize the positive shift in
threshold voltage, from −4.6 V to −2.6 V for an AlGaN/GaN HEMT with a Si-doping AlGaN barrier
layer, but the E-mode property could not be seen. In
this Letter, we demonstrate a similar technique based
on a nano-channel array (NCA) structure to successfully fabricate E-mode AlGaN/GaN HEMTs on an AlGaN/GaN epi-wafer without Si-doping. As shown in
Fig. 1(a), the channel is not a sole one, but is made
up of multiple nano-channels, which are connected in
parallel to source and drain regions to form an NCA
structure. A gate electrode with a shorter length covers the NCA structure. Compared to C-HEMTs, the
fabricated NCA-HEMTs with the feature size of single channel width of 64 nm show better subthreshold
characteristics and a similar reverse gate leakage current. In addition, the detailed mechanisms of forming
the enhancement mode are discussed.
In experiment, the AlGaN/GaN epi-wafer used
was grown on (0001) sapphire substrates by a metalorganic chemical vapor deposition (MOCVD) system.
The epi-wafer structure consists of a low-temperature
GaN nucleation layer, a 2-µm-thick unintentionally
doped GaN buffer layer, a 1-nm-thick AlN insert layer,
a 18-nm-thick unintentionally doped AlGaN barrier
layer and a GaN cap layer with thickness of 2 nm.
Room temperature Hall measurements of the structure yield an electron sheet density of 0.98×1013 cm−2
and an electron mobility of 1983 cm2 /Vs. Mesaetching isolation with height of 200 nm using chlorinebased inductive coupling plasma (ICP) was performed
followed by the source/drain ohmic contact formation
with Ti/Al/Ni/Au annealed at 900∘C for 30 s. After
that, the NCA structure was patterned by electronbeam lithography. Then, a low etching rate process
was carried out in a chlorine-based ICP system to
form nano-channels. To completely eliminate 2DEG
between nano-channels, the etching depth was ensured
* Supported
by the National Basic Research Program of China under Grant No G2009CB929300.
author. Email: ycai2008@sinano.ac.cn
© 2011 Chinese Physical Society and IOP Publishing Ltd
** Correspondence
077202-1
CHIN. PHYS. LETT. Vol. 28, No. 7 (2011) 077202
to be deeper than the height of the barrier layer. Finally, a Ni/Au gate with 𝐿g = 300 nm was formed
and the sample was annealed at 450∘C for 30 min in
N2 ambient.
Figure 1(b) shows the top view image of the NCA
structure taken by using a scanning electron microscope (SEM). The SEM image confirms that the NCA
structure has a single nano-channel width of 64 nm.
The fabricated NCA-HEMT is made up of 1000 nanochannels and the total effective gate width is about
64 µm.
(a)
Drain
Nano-channel
Array



Gate
Source
AlGaN
2D
E
G
Sapphire
GaN
Defining 𝑉th as the gate bias intercept of the linear
extrapolation of drain current at the point of peak
transconductance,[8] 𝑉th of the E-mode device was determined to be 0.15 V, while 𝑉th of the D-mode device
is −3.92 V. More than 4 V of 𝑉th shift was achieved.
The peak transconductance 𝑔𝑚 is 139 mS/mm for the
C-HEMT and 100 mS/mm for the NCA-HEMT, respectively. By varying the nano-channel width, a systematic positive shift of threshold voltage was also
observed, as shown in Fig. 2(b). The threshold voltage increases faster when nano-channel width is reduced to sub-500 nm. Figure 3 shows a comparison
of subthreshold characteristics for both devices. The
C-HEMT has a subthreshold slope of 99 mV/dec and
a better subthreshold property, 78 mV/dec, was observed for the NCA-HEMT. Figure 4 shows the 𝐼g –𝑉g
curves of these two devices. The similar gate leakage
currents indicate that the NCA structure does not obviously degrade the device gate leakage characteristics.
103
(b)
C
-
T
M
E
H
N
C
A
-
H
E
M
T
64 nm
Gate
101
( m
A
/ m
m
AlGaN
)
(c)
Channel
Fig. 1. (a) Schematic illustration of NCA-HEMT with
𝐿g , 𝐿gs and 𝐿gd of 0.3, 2 and 3 µm, respectively. (b) The
SEM top view image of the NCA structure. (c) Cross
section of the NCA structure.
E
M
T
300
80
60
200
40
20
V
0 . 1 5
- 5
- 4
- 3
- 2

( V
 
0
- 4
- 3
- 2
 
- 1
( V
0
1
2
)
1
0
2
10-
1
10-
3
10-
5
10-
7
10-
9
C
-
N
C
H
E
A
M
-
H
T
E
M
T
3
)

1
- 1
V
( A

0
- 6
3 . 9 2
- 5
/ d e c
120
)
H
- 6
V
m
-
/ m
A
5
m
Fig. 3. Logarithmic plots of drain current as a function
of gate voltage for the C-HEMT and the NCA-HEMT.
140
T
/ d e c
)
C
M
V

m
N
E
10-
S / m
H
m
7 8
( m
-
100
-
3

C
400
100
10-
9 9
 
( a )
 
( m
A
/ m
m
)
500
1

500 nm
10-
 
GaN

( b )
0
)


0 . 1 5
V
f o r
N
C
A
-
H
E
M
T
( V
- 1
=
10-
11
10-
13

 
- 2

- 3
 =
-
3 . 9 2
V
f o r
C
-
H
E
M
T
- 10
- 8
- 6
- 4

- 4
102
103

104

( n m
105

 ( V
- 2
0
2
)
Fig. 4. Gate leakage current characteristics of the CHEMT and the NCA-HEMT.
)
Fig. 2. (a) Transfer characteristics (𝑉ds =8 V) for the CHEMT and NCA-HEMT. (b) Measured relation between
threshold voltages and the width of nano-channel.
The transfer characteristics of both C-HEMT and
NCA-HEMT are shown in Fig. 2(a). The drain current and transconductance of the NCA-HEMT have
been normalized by the total effective gate width.
Three effects are taken into account as the physical mechanism of realizing the enhancement mode in
NCA-HEMTs: (1) an additional lateral field to form
a surrounding-field 2DEG, (2) strain relaxation in the
AlGaN/GaN heterostructure caused by reduction of
electron gas density, (3) the work function difference
077202-2
CHIN. PHYS. LETT. Vol. 28, No. 7 (2011) 077202
between the gate metal and the semiconductor. All
these effects will be discussed in detail in the following.
From Fig. 1(c), we know that, except for top gate
modulation, sidewalls have additional lateral field effects on the edges of 2DEG to form a surroundingfield 2DEG, which results in a more positive threshold voltage shift with reducing nano-channel width.[11]
The most important factor is that in an AlGaN/GaN
heterostructure, strain relaxation causes a reduction
of electron gas density. As we know, high density
2DEG at the AlGaN/GaN interface was induced by
spontaneous and piezoelectric polarization effects.[4]
However, in the channel edge region, strain may be
released, which weakens the piezoelectric polarization
and leads to the reduction of electron gas density. The
reduction of electron gas density at the both sides of
channel, which could be ignored for large scale channel, becomes remarkable in a nano-channel and thus
causes a positive threshold voltage shift, as shown
in Fig. 2(b). Moreover, the difference in work function completely depletes the remaining channel carriers surrounded by gate metal and reverses the CHEMT into E-mode for NCA-HEMT. However, the
quantitative analysis needs to be further investigated.
In the previous report shown by Ohi et al.,[11] the
epi-wafer consisted of a Si-doped AlGaN layer. Both
polarization and the doped barrier layer contribute
to the 2DEG. Even if strain is completely relaxed
in a channel with width of 70 nm, which means that
piezoelectric polarization induced electrons disappear,
there will be still too many remaining electrons that
originated from Si-doping and spontaneous polarization to be depleted by the surrounding gates. Thus
the MMC-HEMT mantains a negative threshold voltage (<−2 V). However, in this work, the AlGaN barrier layer is undoped. The remaining electrons mostly
originate from spontaneous polarization and the density is lower than that reported by Ohi et al.. Thus the
remaining electrons happen to be depleted by the surrounding gate. This is the possible reason why an Emode NCA-HEMT with channel width of 64 nm could
be shown.
In summary, enhancement-mode AlGaN/GaN
HEMTs based on an NCA structure have been
fabricated and characterized. Combination of the
surrounding-field effect, strain relaxation and work
function difference reverses the D-mode into the Emode HEMT by using an NCA structure. The fabricated NCA-HEMTs with a single channel width of
64 nm show a threshold voltage 𝑉th of 0.15 V and a
better subthreshold slope of 78 mV/dec. In addition,
both the C-HEMT and the NCA-HEMT exhibit similar gate leakage currents, indicating that the NCA
structure does not degrade the gate leakage characteristics.
Device fabrication and characterization were fulfilled in Nanofabrication Facility and Platform for
Characterization & Test of Sinano, CAS, respectively.
Authors would like to thank them for their technical
support.
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