Investigation of channel width-dependent threshold voltage variation

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Investigation of channel width-dependent threshold voltage variation in a-InGaZnO
thin-film transistors
Kuan-Hsien Liu, Ting-Chang Chang, Ming-Siou Wu, Yi-Syuan Hung, Pei-Hua Hung, Tien-Yu Hsieh, Wu-Ching
Chou, Ann-Kuo Chu, Simon M. Sze, and Bo-Liang Yeh
Citation: Applied Physics Letters 104, 133503 (2014); doi: 10.1063/1.4868430
View online: http://dx.doi.org/10.1063/1.4868430
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/13?ver=pdfcov
Published by the AIP Publishing
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APPLIED PHYSICS LETTERS 104, 133503 (2014)
Investigation of channel width-dependent threshold voltage variation
in a-InGaZnO thin-film transistors
Kuan-Hsien Liu,1 Ting-Chang Chang,2,3,a) Ming-Siou Wu,4 Yi-Syuan Hung,4 Pei-Hua Hung,5
Tien-Yu Hsieh,2 Wu-Ching Chou,1 Ann-Kuo Chu,5 Simon M. Sze,4 and Bo-Liang Yeh6
1
Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
3
Advanced Optoelectronics Technology Center, National Cheng Kung University, Taiwan
4
Department of Electronics Engineering, National Chiao Tung University, Hsinchu, Taiwan
5
Department of Photonics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
6
Advanced Display Technology Research Center, AU Optronics, No. 1, Li-Hsin Rd. 2, Hsinchu Science Park,
Hsinchu 30078, Taiwan
2
(Received 25 November 2013; accepted 28 February 2014; published online 31 March 2014)
This Letter investigates abnormal channel width-dependent threshold voltage variation in
amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors. Unlike drain-induced
source barrier lowering effect, threshold voltage increases with increasing drain voltage.
Furthermore, the wider the channel, the larger the threshold voltage observed. Because of the
surrounding oxide and other thermal insulating material and the low thermal conductivity of the
IGZO layer, the self-heating effect will be pronounced in wider channel devices and those with a
larger operating drain bias. To further clarify the physical mechanism, fast IV measurement is
utilized to demonstrate the self-heating induced anomalous channel width-dependent threshold
C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4868430]
voltage variation. V
Many recent consumer products require the extensive
use of low power consumption IC,1 non-volatile memory,2–7
and thin film transistors (TFTs).8,9 TFTs with active layers
composed of transparent oxide-based semiconductors, such
as ZnO and amorphous InGaZnO (a-IGZO), have attracted
much attention due to their considerable potential application
in flat, flexible, and transparent displays.10–12 In particular,
a-IGZO thin film transistors have been widely investigated
for the next generation of the display industry owing to their
good uniformity, high mobility, excellent transparency to
visible light, and room temperature fabrication.12–15
Therefore, they are very promising alternatives to replace
amorphous silicon TFTs for application in active matrix liquid crystal displays (AMLCD) and organic light-emitting
diode displays (AMOLED) as switching/driving devices.
However, there are some difficulties which are necessary to
overcome for oxide TFTs to be practical in these applications, such as instability under gate bias stress or the surrounding ambiance.16–19 Moreover, a-IGZO TFTs can also
be used for gate driver on array (GOA) technology.
Conventionally, driving ICs have been fabricated through
CMOS technology and mechanically attached to the sides of
the panel. However, GOA technology fabricates gate driver
ICs on the array itself instead of attaching them to the panel
sides. As a result, GOA technology can reduce process steps
and cost as well as achieving thinner panels with narrower
edge.20,21 However, mobility of driving ICs fabricated by
single crystal silicon is about one hundred times that of aIGZO. As a result, in order to achieve the same driving current, it is necessary to increase channel width of a-IGZO
TFTs for GOA operation. Therefore, investigating the
a)
Electronic mail: [email protected]
0003-6951/2014/104(13)/133503/4/$30.00
performance and reliability of a-IGZO TFTs with large channel width is of great importance.
The n-type a-IGZO TFTs in this work were fabricated
with a bottom gate and back-channel-etching structure. The
double-layer Cu/Mo (500/20 nm) gate electrodes films were
deposited and then patterned via photolithography on a glass
substrate. Then 300-nm-thick Si3N4 and 70-nm-thick SiO2
gate dielectric films were sequentially deposited on the patterned gate electrode by plasma enhanced chemical vapor
deposition (PECVD). An active layer of 30-nm-thick
a-IGZO film was deposited by DC magnetron sputtering
using a target of In2O3:Ga2O3:ZnO ¼ 1:1:1 in atomic ratio at
room temperature, and then patterned. The Mo/Cu
(20/500 nm) source/drain electrodes were formed by
DC-sputtering and then patterned. Finally, 160-nm-thick
SiO2 and 50-nm-thick Si3N4 were sequentially deposited as a
passivation layer by PECVD. After that, the device was
annealed in an oven at 300 C for 2 h in a dark environment.
In this Letter, the conventional and fast I-V measurements
were performed by Agilent B1500A and Agilent B1530A
semiconductor analyzers, respectively. The device dimensions of channel width/length (W/L) were 100, 500, 1000,
5000, and 10 000 lm/5.5 lm. The threshold voltage is
defined as the gate voltage when the normalized drain current (NID ¼ ID L/W) reaches 1 nA, where L and W are
channel length and width, respectively. All measurements
were performed in a dark environment.
Figures 1(a) and 1(b) show the normalized ID-VG curve
at VD ¼ 1, 5, 10, and 20 V, with W/L ¼ 100/5.5 lm for
Figure 1(a) and W/L ¼ 10 000/5.5 lm for Figure 1(b).
Obviously, threshold voltage increases with increasing drain
voltage. Furthermore, the larger the channel width, the larger
threshold voltage that can be observed. Conventionally,
channel width and drain voltage do not affect threshold
104, 133503-1
C 2014 AIP Publishing LLC
V
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133503-2
Liu et al.
FIG. 1. ID-VG transfer characteristic of a-IGZO TFT operated at VD ¼ 1, 5,
10, and 20 V for (a) W/L ¼ 100/5.5 lm. (b) W/L ¼ 10 000/5.5 lm.
voltage in long channel devices. However, this anomalous
threshold voltage variation depends on channel width and
drain voltage that are observed in Figures 1(a) and 1(b).
In order to inspect this phenomenon further, Figure 2
illustrates the threshold voltage shift versus various channel
width and drain voltages, with threshold voltage shift defined
as Vth(measurement) Vth(@VD ¼ 1 V). Note that at low
drain voltage (VD ¼ 5 V), threshold voltage shift is negligible
and unapparent, with the same being true for relatively small
channel widths (W ¼ 100 lm) at all drain voltages. As
W ⭌ 500 lm and VD ⭌ 10 V, a significant threshold voltage
shift can be observed with increasing channel width and
increasing drain voltage. Accordingly, the abnormal channel
width-dependent threshold voltage variation may in fact be
induced by the self-heating effect.22 It is well known that the
self-heating effect arises in silicon-on-insulator (SOI)
MOSFETs and low-temperature-polycrystalline silicon
(LTPS) TFTs, a situation quite similar to that in the IGZO
channel layer.23 In addition, the thermal conductivity of
IGZO is much lower than Si and is comparable to SiO2.
Therefore, heat dissipation in IGZO TFTs is relatively more
difficult than in Si-based TFTs.24 Because the larger drain
voltage will form a higher drain current, resulting in higher
power (P ¼ IV), the heat in channel will be higher, resulting
in a more severe self-heating effect. Furthermore, because
the heat will more likely accumulate at the center of the
channel region and dissipate to the surrounding materials
along the channel width direction, larger channel widths
make heat dissipation in channel more difficult, again resulting in a more pronounced self-heating effect.25 The inset of
Figure 2 shows the energy band diagram. When the large
channel width TFT is operated at high drain voltage, significant self-heating effect will occur, and channel electrons will
FIG. 2. Dependence of threshold voltage shift on the channel width and
drain voltage. The inset illustrates the thermionic-field emission process of
electron trapping.
Appl. Phys. Lett. 104, 133503 (2014)
be trapped at the IGZO/SiO2 interface or in SiO2 bulk
through the thermionic-field emission process, resulting in a
larger observed threshold voltage.22,26,27 In addition, from
Figure 1(b), note that threshold voltage shifts without
obvious variation of the slope in transfer characteristics. This
indicates that no additional trapping states are created at the
IGZO active layer/gate dielectric interface during the trapping process, resulting in unobvious mobility and subthreshold swing degradation.28–30
In order to confirm that the abnormal channel widthdependent threshold voltage variation is indeed induced by
self-heating effect, the ID-VD output characteristic is performed. Figures 3(a) and 3(b) show the ID-VD curve at a
fixed channel length (5.5 lm) but different channel widths
(100 lm and 10 000 lm, respectively). Compared to the
W ¼ 100 lm device, the W ¼ 10 000 lm one exhibits the
anomalous output characteristic. When the measurement
drain voltage exceeds approximately 15 V, drain current
decreases instead of saturating with an increase in drain voltage. The heat dissipation in channel will be rather difficult
for the W ¼ 10 000 lm device because of the considerably
large channel width. As the large channel width device is
operated at high drain voltage conditions (VD ⭌ 15 V here),
a severe self-heating effect-induced charge trapping phenomenon will occur, resulting in a considerable threshold voltage
shift. Because of this large threshold voltage shift, the abnormal drain current decreases as drain voltage increases when
VD ⭌ 15 V.
To further confirm the proposed self-heating effect
induced anomalous channel-width dependent threshold voltage variation, fast IV measurement is performed. The inset of
Figure 4(a) illustrates the waveform of conventional ID-VG
measurement in which drain voltage is fixed with gate voltage performed stepwise. Note that the time scale of each gate
voltage step is on the order of milliseconds (ms). For comparison, the inset of Figure 4(b) shows the waveform of fast
ID-VG measurement in which drain voltage is fixed with gate
voltage performed in a pulse form. Significantly, the time
scale of peak/base time is rather short, approximately on the
order of microseconds (ls). From previous research,21 sufficient heating time is necessary for Joule heating to take place
within the channel, resulting in a pronounced self-heating
effect-induced charge trapping phenomenon. This sufficient
heating time is approximately on the order of ms. Because
the gate pulse peak/base time in fast I-V measurement is on
the order of ls, the short heating time is insufficient for Joule
heating to occur. Therefore, use of the fast I-V measurement
will exclude the self-heating effect induced-charge trapping
FIG. 3. ID-VD output characteristic of a-IGZO TFT operated at VG ¼ 10, 15,
and 20 V for (a) W/L ¼ 100/5.5 lm. (b) W/L ¼ 10 000/5.5 lm.
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133503-3
Liu et al.
Appl. Phys. Lett. 104, 133503 (2014)
FIG. 4. The transfer characteristic of a-IGZO TFT with W/L ¼ 10 000/
5.5 lm before and after measuring at high drain voltage (VD ¼ 10 V) by (a)
conventional ID-VG measurement. (b) Fast ID-VG measurement. The inset of
Figure 4(a) and 4(b) illustrate the waveform of conventional and fast ID-VG
measurement, respectively.
FIG. 5. The transfer characteristic of a-IGZO TFT with W/L ¼ 100/5.5 lm
before and after measuring at high drain voltage (VD ¼ 10 V) by (a) conventional ID-VG measurement. (b) Fast ID-VG measurement. The inset of Figure
5(a) and 5(b) illustrate the waveform of conventional and fast ID-VG measurement, respectively.
phenomenon, and therefore threshold voltage shift can also
be excluded. The measurement sequences of Figures 4(a) and
4(b) are as follows. First, ID-VG curve is measured by conventional ID-VG measurement at low drain voltage
(VD ¼ 1 V) to avoid the self-heating effect and act as the initial state. Second, ID-VG curve is measured at high drain voltage (VD ¼ 10 V) by conventional ID-VG measurement for
Figure 4(a) and by fast ID-VG measurement for Figure 4(b).
Finally, ID-VG curve is measured by conventional ID-VG
measurement at low drain voltage (VD ¼ 1 V) to serve as the
final state. Clearly, there is a positive threshold voltage shift
between initial and final states after conventional ID-VG
measurements at high drain voltage (VD ¼ 10 V), as shown in
Figure 4(a). During the conventional ID-VG measurement
(VD ¼ 10 V), there was sufficient heating time for Joule heating to occur, leading to the self-heating effect-induced charge
trapping phenomenon and resulting in the threshold voltage
shift between initial and final states. Conversely, during the
fast ID-VG measurement (VD ¼ 10 V), the insufficient heating
time required for Joule heating results in no threshold voltage
shift being observed, as shown in Figure 4(b). The fast ID-VG
measurement further corroborates that the abnormal
channel-width dependent threshold voltage variation does in
fact result from the self-heating effect-induced charge trapping phenomenon. In addition, no matter the second measurement step, which is with VD ¼ 10 V, is carried out with
conventional or fast ID-VG measurement, there is no threshold voltage shift between initial and final states, as shown in
Figures 5(a) and 5(b). This indicates that channel width is an
important factor in the self-heating effect-induced charge
trapping phenomenon because larger channel widths make
heat dissipation in channel more difficult.
This paper has investigated the anomalous channel
width-dependent threshold voltage variation in a-IGZO
TFTs. Devices with larger channel widths and which are
operated at higher drain voltages will produce larger threshold voltages, with the effect becoming even more pronounced as channel width or drain voltage increases. This is
due to the surrounding oxide and other thermal insulating
material and the low thermal conductivity of the IGZO layer.
The more pronounced self-heating effect is a product of both
the more difficult heat dissipation in wider channels as well
as the higher drain current in devices operated at higher drain
voltages. Because sufficient heating time (approximately on
the order of ms) is necessary for Joule heating to take place
within the channel, the fast ID-VG measurement is performed
to confirm the proposed mechanism. Because the time scale
of the peak/base time in the fast ID-VG measurement is
shorter, on the order of ls, the heating time is insufficient for
Joule heating, resulting in no observed threshold voltage
shift. The fast ID-VG measurement confirms that the abnormal channel-width dependent threshold voltage variation is
due to the self-heating effect induced-charge trapping
phenomenon.
This work was performed at the National Science
Council Core Facilities Laboratory for Nano-Science and
Nano-Technology in the Kaohsiung-Pingtung area and was
supported by the National Science Council of the Republic
of China under Contract No. NSC 102-2120-M-110-001.
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133503-4
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