Novel of Normally-off GaN HEMT Device Structure by ... - CS Mantech

Novel of Normally-off GaN HEMT Device Structure by Using Nano-rods Technology
Chwan-Ying Lee, Young-Shying Chen , Lurng-Shehng Lee, Chien-Chung Hung, Cheng-Tyng Yen, Suh-
Fang Lin, Rong Xuan, Wei-Hung Kuo, Tzu-Kun Ku, and Ming-Jinn Tsai
Electronics and OptoElectronics Research Laboratories (EOL), ITRI
No.195, Sec.4, Chung Hsing Rd., Chutung, Hsinchu, 31040, Taiwan, R.O.C.
*Email: cyleei@itri.org.tw; Phone: +886-3-591-3490
Keywords: GaN on Si, Normally-off, nano-rod
Abstract
This paper reports a novel process by introducing
nano-rods technique into AlGaN/GaN high-electronmobility-transistor
(HEMT) device. This process adapts
nickel to form the nano-rod hard mask pattern and then
transfers to the electron supply AlGaN layer of the twodimensional
electron gas (2DEG) device, followed by SF 6
irradiation and p-GaN layer encapsulation. This device
with the novel gate structure exhibits normally-off
characteristic. The threshold voltage of this nano-rod
device is higher than 0.5V and the breakdown voltage is
higher than 1500V.
INTRODUCTION
The High-Electron-Mobility Transistor (HEMT) device
based on AlGaN/GaN hetero structure has low resistance
characteristic by taking advantage of two-dimensional
electron gas (2DEG) induced by piezoelectric polarization
mechanism, so this device has been attracting considerable
attention and intensively studied as for the next-generation
power electronic devices. However, the conventional
HEMT device typically has a negative threshold voltage
because of the inherent existence of the 2DEG channel, and
thus it becomes a normally-on device. This is rather
inconvenient of use for some safety-concerned applications.
Several techniques to achieve the normally-off property of
the AlGaN/GaN devices have been reported, such as the
fluoride-based plasma treatment method[1][2], the use of p-
type layers underneath the gate region[3]-[5] to lift up the
conduction band, or thin down the electron supply AlGaN
layer to form a recessed gate structure[6][7]. This recess
would weaken the electric field contributed by a lower
piezoelectric polarization, and generate a lower carrier
concentraion in the 2DEG layer for Vt improvement.
However, this approach is difficult to control the remaining
AlGaN thickness and therefore exhibits poor device
uniformity. Although many papers adopted various methods
to achieve normally-off property, it sacrificed device turn-on
performance in some cases. The purpose of this study is to
develop a normally-off device with minimum impact on the
drain current by proposing a novel imprint method to form
the nano-rod or nano-strip gate structures.
PROCESS PROCEDURE
A nano-rod hard mask was formed by thermally annealed
Ni particles around the gate region, followed by partially
AlGaN etching and then implanting F-ion by SF 6 plasma
irradiation. And then the P-type GaN is selectively grown
on the AlGaN to help to raise the potential of the 2DEG
channel region. This device combines all the benifits of F-
ion implantation, p-GaN gate and recessed gate devices to
exhibit the normally-off property. Owing to the AlGaN
layer under the gate is not fully removed, the piezoelectric
polarization can still be high enough so that the drain current
can be almost comparable to the conventional HEMT device.
Figure 1 illustrates the schematic structure of the proposed
device structure.
u-AlGaN
u-GaN
Buffer
Substrate
S G D
p-AlGaN
F-ion
Figure 1. Schematic representation of device cross-sectional structre in this
investigation.
Some key process steps of this device are shown in Fig.
2. Figure 2a shows the typical HEMT device structure (u-
AlGaN/u-GaN/buffer/substrate) as the starting material and
CS MANTECH Conference, April 23rd - 26th, 2012, Boston, Massachusetts, USA

then deposits the oxide material as for the etching hard
mask layer. Figure 2b shows the hard mask opening and the
oxide
u-AlGaN
u-GaN
Buffer
Substrate
(a)
Nano-rod structure formed in the oxide layer by using
imprint method. The nano-rod pattern was transferred into
AlGaN layer and then the SiO 2 layer was removed as shown
in Fig. 2c. Then, the F-ion was implanted by SF 6 plasma
irradiation as shown in Fig. 2d, the thick oxide layer was
used as hard mask. Last, the p-GaN layer was selectively
grown on the gate region as shown in Fig. 2e. After that, the
standard process of ohmic contact on the source and drain
region (Fig. 2f) and Schottky contact on the gate region (Fig.
2g) was performed.
Substrate
(b)
Figure 3 shows the schematic representation of process
steps of the imprint method. The Nickel material was
deposited on SiO2 layer and then thermally annealed to form
the nano-rod pattern around the gate region and then the
oxide hard mark was formed by the ICP reactive ion etching.
Substrate
(c)
SF 6
oxide
Ni
SiO 2
SiO 2
1.6 mm GaN thick GaN
1.6 mm GaN thick GaN
GaN
GaN
Substrate
(d)
Sapphire substrate
substrate
Sapphire substrate
substrate
substrate
substrate
(a) GaN grown followed (b) Ni deposition. (c) Ni particle formed (d) SiO2 Nanorodsformed nano by
by SiO2 deposition by thermal annealing formed by ICP RIE.
Figure 3. Schematic representation of Nano-rod formation.
p-AlGaN
EXPERIMENTAL RESULTS
Substrate
S
(e)
D
Figure 4 shows the tilt SEM view of one Ni nano-rod
structure around gate region. The area ratio occupied by
these nano rods is about 38% . The threshold voltage and
the output characteristic of this device can be adjusted by
designing different area ratio of the nano-rod structure. We
believe the partially etched AlGaN layer, i.e. nano-rod
structure, can has better tradeoff characteristic as compared
to the fully etched AlGaN layer, i.e. gate-recessed structure.
And this result proves the feasibility of the proposed nanorod
process.
Substrate
S
(f)
G
D
Substrate
(g)
Figure 2. Key process steps for the novel device.
Figure 4. SEM view of Ni nano-rod structure.
CS MANTECH Conference, April 23rd - 26th, 2012, Boston, Massachusetts, USA

Figure 5 shows a cross-sectional SEM image of the
nano-rod structure after oxide hard mask opening. This
nano-rod process is unlike the recessed-gate process, in
which the whole area of AlGaN layer underneath the gate
electrode is removed. The piezoelectric polarization may
decrease too much from fully etched the AlGaN layer of the
recessed-gate structure. So, the drain current is not easy to
maintain a higher level. The etch process of the recessedgate
process usually not easy to control and sometimes
damages the 2DEG layer, and therefore contributes to nonuniformity
of threshold voltage and decrease of drain current.
As to the nano-rod structure, certain ratio of the 2DEG layer
remains underneath the gate region, the device can have
higher drain current as compared to the typical recessed-gate
structure.
The threshold voltage Vth can be extracted from the
linear extrapolation of the Id-Vgs plot in Fig. 6. A positive
Vth value of 1V was achieved from the nano-rod HEMT
device, while a negative value of -2V was presented from
the standard AlGaN/GaN HEMT device.
Figure 7 shows the reverse blocking characteristic of
the proposed nano-rod HEMT device. The breakdown
voltage of this device is higher than 1500V. This is a good
result that exhibits high current, normally off, and large
breakdown voltage at the same time by the novel nano-rod
structure.
Ni
5000Å
SiO 2
rods
GaN Template
Figure 5. Crossectional SEM view of SiO2 nano-rod structure.
Figure 6 shows the comparison of the DC transfer
characteristics on different HEMT devices. The nano-rod
device exhibits a peak drain current Id of 170 mA/mm at
gate to source voltage Vgs of 5V. This value does not
degrade too much as compared to peak Id of 220 mA/mm at
Vgs of 4V from the standard HEMT device.
Figure 7. Reverse blocking characteristic of the novel device.
This is a preliminary result that shows good device
properties on the on-state current, off-state voltage, and gate
control capability. However, we still need to optimize
device performance and also simplify the nano-rod process.
Figure 8. shows alternative approach by nano-imprint
process, which can easily achieve different pattern density of
the nano-rods. The tradeoff characteristics of on-state
current and threshold voltage can therefore be optimized
according to experiment by changing the nano-rod area ratio.
Figure 8. Proposed a simplified nano-imprint process flow for nano-rod
GaN device
Figure 6. Comparison of DC transfer characteristics on different GaN-on-Si
HEMT devices. The nano-rod device shows normally-off property.
From the nano-rod device, the on-state drain current can
be improved and modulated by increasing the area ratio of
the nano-rods underneath the gate region. In addition, the
threshold voltage can also be increased by adding the F-ion
implantation and the P-GaN expitaxy layer to exhibit a
normally-off characteristic. The breakdown voltage can still
CS MANTECH Conference, April 23rd - 26th, 2012, Boston, Massachusetts, USA

maintain high enough value due to the discontinued 2DEG
layers formation by the nano-rod hard mask patterning. We
will continue to develop and optimize the device property
and target to achieve a nano-rod device with threshold
voltage higher than 1.5V and drain current larger than 300
mA/mm.
CONCLUSIONS
In conclusion, we have demonstrated a nano-rod
AlGaN/GaN HEMT device with normally-off, high on-state
current and high breakdown voltage performances. The
positive threshold voltage and high drain current
performance is attributed to the nano-rod structure
underneath the gate region because of the discontinued
2DEG layers formed so that the high current maintained and
the threshold voltage can be modulated by F- irradiation and
adopt P-GaN layer. This device also exhibits high
breakdown voltage and low leakage current. The nano-rod
AlGaN/GaN device is very promising for the high current
and high voltage motor drive applications.
REFERENCES
[1] H. Mizuno, et al., Phys. Stat. Sol. (c ), vol.4, no. 7, p.2732 , July, 2007.
[2] Y.Cai et al., IEEE Electron Device vol. 53, no. 9, p. 2207, 2006.
[3] N. Tsuyukuchi, et al., Jpn. J. Appl. Phys., vol.45, no.11, p.L-319, 2006.
[4] Y.Uemoto, et al., IEEE Electron Device vol. 54, no.12, p.3393, 2007.
[5] X.Hu, et al., Electron. Lett., vol.36, no.8, p.753, 2000.
[6] C. Chen, et al., IEEE Electron Device Letters, vol. 32, no. 10, p. 373,
2011.
[7] R. Chu, et al., IEEE Electron Device Letters, vol. 32, no. 5, p. 632, 2011.
ACRONYMS
HEMT: High Electron Mobility Transistor
CS MANTECH Conference, April 23rd - 26th, 2012, Boston, Massachusetts, USA