Characteristic Evaluation of Complementary Inverter
using Amorphous Oxide TFT and Polymer Organic TFT
Takashi Nakanishi1, Mariko Sakemi1, Tomoya Okumura1, Yuki Ueda1, Mutsumi
Kimura1, Kenji Nomura2, Toshio Kamiya2, Hideo Hosono2 and Takashi Aoki3
1
2

Ryukoku University, Seta, Otsu, 520-2194, Japan, mutsu@rins.ryukoku.ac.jp
Tokyo Institute of Technology, Nagatsuta, Midori, Yokohama 226-8503, Japan
3
Seiko Epson Corporation, Fujimi, Nagano, 399-0293, Japan

Keywords: Complementary Inverter, Amorphous Oxide TFT, Polymer Organic TFT

Abstract. A complementary inverter is composed by combining an amorphous oxide TFT andpolymer organic TFT, and its characteristic is evaluated. It is found that the complementary
inverter correctly works in DC operation with a proper gain. However, the input-output
characteristic gradually delays at a high sweep rate in transient operation due to the insufficientperformance of the polymer organic TFT and large load capacitance. It is concluded that thedesign parameters such as transistor performances, gate-drain capacitances, load capacitanceand operation frequency should be optimized in order to produce the actual circuit.

Introduction
Recently, oxide TFTs are promising for various applications from flat-panel displays (FPDs)
such as liquid-crystal displays (LCDs), organic light-emitting diode displays (OLEDs) andAu
ITO
α-IGZO
SiO2

SourceChannel

Drain

40nm

Gate insulator+n -Si
Gate

40nm
100nm

F8T2
AuCr
Plastic

400nm

Gate

30nm

Gate insulator
SourceChannelW/L=300/50μm

Drain
W/L=10000/6μm

SourceSourceGate

Channel

Drain
Drain

Amorphous oxide TFT

Polymer organic TFTFig. 1
Device structures of the amorphous oxide TFT and polymer organic TFT.

electronic papers to electronics components because they are transparent and can be fabricatedon flexible substrates at low temperature [1-2]. In particular, since amorphous oxide TFTs canbe fabricated on plastic substrates at room temperature using low-temperature processes such as
sputtering and pulsed laser deposition (PLD) and have high and uniform performances, they
have been extensively studied [3-4]. However, n-type TFTs have been mainly reported for theamorphous oxide TFTs, and p-type TFTs with sufficient performance and reliability have been
hardly reported [5].
On the other hand, organic TFTs are also promising because they can be fabricated on
flexible substrates at low temperature [6]. In particular, since polymer organic TFTs can befabricated on plastic substrates at room temperature with low cost using printing technologies
such as spin coating, offset printing, screen printing and ink jetting, they have been also
extensively studied [7-9]. However, p-type TFTs have been mainly reported for the polymer
organic TFTs, and n-type TFTs with sufficient performance and reliability have been hardly
reported [10].
To date, we have analyzed various TFTs such as poly-Si TFTs, amorphous Si TFTs,
polycrystalline oxide TFTs, amorphous oxide TFTs and polymer organic TFTs. Among theseTFTs, prospective candidates for n-type TFTs are amorphous oxide TFTs while prospectivecandidates for p-type TFTs are polymer organic TFT because both TFTs can be fabricated onplastic substrates at room temperature. In this paper, a complementary inverter is composed bycombining an amorphous oxide TFT and polymer organic TFT, and its characteristic isevaluated. Although the complementary inverter using a polycrystalline oxide TFT andlow-molecule organic TFT was reported [11], the complementary inverter using an amorphousoxide TFT and polymer organic TFT is more attractive because of the reason written above[12].

Device structure and fabrication process

1E-6
1E-7

1E-3
Polymerorganic TFT

Amorphousoxide TFT

1E-4
1E-5

1E-8Ids (A)The device structure of the amorphous oxide
TFT is shown in Fig. 1. The bottom-gate andtop-contact TFT is fabricated as follows. First,
a heavily-doped n+-Si substrate is used as thegate terminal, and a thermally oxidized SiO2
film is used as the gate insulator. After thenecessaryareasarepatternedby
photolithography of photoresist, an amorphous
InGaZnO4 (α-IGZO) film is deposited by PLD
at room temperature in oxygen gas at 6.2 Pa,patterned by lift-off technique and used as thechannel layer. After the necessary areas arepatternedby
photolithography,
anindium-tin-oxide (ITO) film and Au film aresequentially deposited by PLD, patterned bylift-off technique and used as the source anddrain terminals.
The device structure of the polymer organic
TFT is also shown in Fig. 1. The top-gate andbottom-contact TFT is fabricated as follows.
First, a plastic substrate is used. Next, a Crfilm and Au film are sequentially deposited,

1E-9
1E-10
1E-11

1E-6
ti/ts=400/30nm
W/L=10000/6μm
Vds=-5V-4
μ=9.41×10
Vth=-12.0V-1
S=2.41Vdec

ti/ts=100/40nm
W/L=300/50μm
Vds=5V2 -1 -1μ=10.6cm V s
Vth=4.5V-1
S=0.13Vdec

1E-8
1E-9
1E-10
1E-11

1E-12
1E-13-15 -10 -5

1E-7

1E-12
1E-1305Vgs (V)1015Fig. 2Transistorcharacteristics
oftheamorphous oxide TFT and polymer
organic TFT.

patterned by photolithography and etching and used as the source and drain terminals. Next, an
F8T2 film is deposited by spin coating and drying and used as the channel layer, and aninsulator film is also deposited by spin coating and drying and used as the gate insulator.
Finally, a metal film is deposited by mask evaporation and used as the gate terminal.
Since both TFTs are fabricated at room temperature as written above, although each TFT arefabricated on each substrate here, it is possible in principle to be fabricated and integrated on aplastic substrate. Moreover, it is also possible to be formed in a 3-D stacked structure with theamorphous oxide TFT under the polymer organic TFT or vice versa.

Transistor characteristic
The transistor characteristic of the amorphous oxide TFT is shown in Fig. 2. The thickness ofthe gate insulator (ti) is 100 nm, the thickness of the channel layer (ts) is 40 nm, the gate width(W) / gate length (L) is 300 / 50 μm, the gate voltage (Vgs) is -10 ∼ 15 V, and the drain voltage(Vds) is 5 V. The field-effect mobility (μ) is 10.6 cm2V-1s-1, the threshold voltage (Vth) is 4.5 V,and the sub-threshold swing (S) is 0.13 Vdec-1.
The transistor characteristic of the polymer organic TFT is also shown in Fig. 2. ti = 400 nm,
ts = 30 nm, W / L = 10000 / 6 μm, Vgs = 10 ∼ -15 V, and Vds = -5 V. μ = 9.41×10-4 cm2V-1s-1,
Vth = -12.0 V, and S = 2.41 Vdec-1.
Thenecessaryconditionfor
complementary circuits in consideration of10V
Vdd
the higher supplied voltage (Vdd) and lower
Polymer
supplied voltage (Vss) is that the on current
organic TFTof the n-type TFT at Vgs = Vdd - Vss is
Triaxiallarger than the off current of the p-type TFT
cable
FET probeat Vgs = 0, and the on current of the p-type
Cgd
TFT at Vgs = - ( Vdd - Vss ) is larger thanVin
Vout
the off current of the n-type TFT at Vgs = 0.
Cgd
This necessary condition can be satisfied byCcable
Vdd = 10 V and Vss = GND in spite of theinsufficient performance of the polymer
organic TFT. Therefore, these Vdd and Vss
Amorphousoxide TFT
are used below, and it is expected that theMicroVss
GND
complementary inverter correctly works.
chamber
Inverter characteristic
The complementary inverter using theamorphous oxide TFT and polymer organic
TFT is shown in Fig. 3. The amorphousoxide TFT is used as the n-type TFT while
the polymer organic TFT is used as thep-type TFT, and they are located in themicro chamber and connected by the triaxial
cables outside the micro chamber. The load
capacitance of the complementary inverter(Cload) consists of the gate-drain
capacitances (Cgd) of the amorphous oxide

Polymerorganic TFT
Microchamber
FET probe
Amorphousoxide TFTFig. 3Complementary
inverterusing
theamorphous oxide TFT and polymer organicTFT.

TFT and polymer organic TFT and the wiring capacitance of the triaxial cables and is no lessthan the order of 1 nF. Vdd = 10 V, Vss = GND, the input voltage (Vin) is -10 ∼ 10 V in the DC
operation, and the sweep rate of the triangle wave of Vin is 20 ∼ 2k Vs-1 in the transient
operation. The output voltage is measured using the FET probe with excellently low input
capacitance and leakage current. Although Vin should be Vss ∼ Vdd in general, since thetransistor characteristic of the amorphous oxide TFT is unwillingly shifted to depletion side andthe transistor characteristic of the polymer organic TFT is unwillingly shifted to enhancement
side, Vin is reluctantly set to be lower than Vss. However, the following conclusion will bereasonable in general.
The input-output characteristic of the complementary inverter using the amorphous oxide
TFT and polymer organic TFT in the DC operation is shown in Fig. 4. It is found that thecomplementary inverter correctly works in DC operation with a proper gain. The inversion
voltage is about 2 V, and the gain is about 80.
The input-output characteristic of the complementary inverter using the amorphous oxide
TFT and polymer organic TFT in the transient operation is shown in Fig. 5. The input-output
characteristic does not delays very much for Vin > 2 V and translation from the higher Vout tothe lower Vout. This is due to the sufficient performance of the amorphous oxide TFT. On theother hand, the input-output characteristic gradually delays at a high sweep rate in transient
operation for Vin < 2 V and translation from the lower Vout to the higher Vout. This is due tothe insufficient performance of the polymer organic TFT and large Cload. Moreover, the direct
capacitance coupling also affects the input-output characteristic due to the large Cgd. As a result,
it is concluded that the design parameters such as transistor performances, Cgd, Cload andoperation frequency should be optimized in order to produce the actual circuit.

15Vin (V)
1.9 2

1.4 1.5

2.41510020Vs-150

Vdd=10V
Vss=GND

5Vout (V)

Vout (V)

10Gain105400

Vdd=10V
Vss=GND
Cl=1nF200

1k00-5-1002k-50-50Vin (V)510Fig. 4
Input-output
characteristicof
thecomplementary inverter using theamorphous oxide TFT and polymer
organic TFT in the DC operation.

-5-10-50Vin (V)510Fig. 5
Input-output
characteristicof
thecomplementary inverter using theamorphous oxide TFT and polymer
organic TFT in the transient operation.

Conclusion
A complementary inverter was composed by combining an amorphous oxide TFT andpolymer organic TFT, and its characteristic was evaluated. It was found that the complementary
inverter correctly works in DC operation with a proper gain. However, the input-output
characteristic gradually delays at a high sweep rate in transient operation due to the insufficientperformance of the polymer organic TFT and large Cload. It was concluded that the design
parameters such as transistor performances, Cgd, Cload and operation frequency should beoptimized in order to produce the actual circuit.

Acknowledgments
The authors wish to thank Dr. Junichi Karasawa, Dr. Soichi Moriya, Dr. Tsutomu Miyamoto,Dr. Takeo Kawase, Dr. Hideyuki Kawai and Dr. Satoshi Inoue of Seiko Epson Corporation.
This research is partially supported by a grant for Industrial Technology Research from theNew Energy and Industrial Technology Development Organization (NEDO), collaborative
research with Seiko Epson, a research project of the Joint Research Center for Science andTechnology at Ryukoku University, a grant from the High-Tech Research Center Program(HRC) for private universities from the Ministry of Education, Culture, Sports, Science andTechnology (MEXT), a grant for research facility equipment for private universities fromMEXT, and a grant for special research facilities from the Faculty of Science and Technologyof Ryukoku University.

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