APPLIED PHYSICS LETTERS 87, 023509 共2005兲

Control of threshold voltage of organic field-effect transistors with doublegate structures
Shingo Iba, Tsuyoshi Sekitani, Yusaku Kato, and Takao Someyaa兲
Quantum-Phase Electronics Center, School of Engineering, The University of Tokyo, 7-3-1 Hongo,Bunkyo-ku, Tokyo 113-8656, Japan

Hiroshi Kawaguchi, Makoto Takamiya, and Takayasu Sakurai
Center for Collaborative Research, the University of Tokyo, 4-6-1 Komaba, Meguro-ku,
Tokyo 153-8505, JapanShinichi Takagi
Graduate School of Frontier Sciences, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,Tokyo 113-8656, Japan

共Received 11 March 2005; accepted 15 June 2005; published online 8 July 2005兲
We fabricated pentacene field-effect transistors with planar-type double-gate structures, where thetop- and bottom-gate electrodes can independently apply voltage biases to channel layers. The
threshold voltage of organic transistors is changed systematically in a wide range from −16 to− 43 V when the voltage bias of the top-gate electrode is changed from 0 to + 60 V. The mobility
in the linear regime is almost constant 共0.2 cm2 / V s兲 at various voltage biases of the top-gate
electrode and the on/off ratio is 106. © 2005 American Institute of Physics.
关DOI: 10.1063/1.1995958兴
Organic field-effect transistors 共FETs兲 have attracted
much attention owing to their excellent properties for realizing flexible, large-area electronic devices. Organic-transistor
integrated circuits 共ICs兲 for driving paper-like displays1,2 andlarge-area sensors3,4 have been manufactured. Althoughmuch progress in organic transistors has been made in thelast decade, one of the major remaining issues on organic
transistor ICs is the control of their threshold voltage 共Vth兲,
which is crucial to the design and manufacture of complicated ICs. Silicon metal-oxide-semiconductor FETs, which
are mainstream electronic devices, can control Vth by changing the doping level of semiconductors and/or by applying
voltage bias to conductive substrates.5,6 However, base films
for organic transistors are usually made of nonconductive
plastic.3 Although there are some pioneering works in which
Vth of organic transistors is changed by doping organic
semiconductors7 and/or by modifying the surface of gate dielectric layers,8,9 such approaches are still far from practicaluse due to the reliability of the manufacturing process and/or
the distribution of the performance of organic transistors.
In this work, we manufactured pentacene FETs withplanar-type double-gate structures on plastic films. In theseFETs, pentacene channel layers are sandwiched between
polyimide and parylene gate dielectric layers, and the topand bottom-gate electrodes can independently apply voltage
biases to channel layers. Vth is systematically decreased from
−16 to − 43 V when top-gate bias is changed from 0 to+ 60 V. The mobility in the linear regime is 0.2 cm2 / V s andshows no significant changes at various Vtop biases.
Pentacene FETs with double-gate structures were fabricated on plastic films. The cross-sectional structure of theFETs is schematically shown in Fig. 1共a兲. First, a bottomgate electrode consisting of 5-nm-thick Cr and 50-nm-thick
Au layers is deposited in a vacuum evaporator on a 75 ␮ma兲Author to whom correspondence should be addressed; electronic mail:someya@ap.t.u-tokyo.ac.jp

thick polyimide film. Polyimide precursors 共Kemitite
CT4112, Kyocera Chemical兲 are then spin-coated and cured
at 180 ° C to form 620-nm-thick gate dielectric layers.3,10 A
50 nm thick pentacene film is deposited in the same vacuum
evaporator. 50-nm-thick Au drain 共D兲 and source 共S兲 electrodes are evaporated through a shadow mask. The channel
length 共L兲 and width 共W兲 of the pentacene FETs are 100 ␮mand 1 mm, respectively. The base film with transistors is thenuniformly coated with a 600 nm thick parylene layer. Finally,a 150 nm thick Au layer is deposited to form top-gate
electrodes.11 Figure 1共b兲 shows a micrograph of the fabricated FETs. In addition to functionality of the gate dielectric
layer, the parylene layer has functionality as a passivation
layer, which prevents organic semiconductors from being directly exposed to ambient air. As shown in Figs. 1共a兲 and1共b兲, both source-drain electrodes and channels are completely covered by a 300⫻ 1000 ␮m2 top-gate electrode.

FIG. 1. 共a兲 The cross-sectional illustration of organic transistors on plastic
films with double-gate structures. 共b兲 A micrograph of the fabricated FETs.
The dashed-line represents the area of pentacene, while the dash-dotted line
represents the bottom-gate electrode.

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Iba et al.

FIG. 2. Typical dc characteristics of the present transistors with double-gatestructures, where the top-gate bias Vtop is floating. 共a兲 The source-draincurrent 共IDS兲 is measured under ambient environment as a function ofsource-drain voltage 共VDS兲. Bottom-gate voltage 共VGS兲 is changed from0 to − 60 V in −10 V steps. 共b兲 The corresponding transfer curve of thesame FET: VGS is swept from +30 to − 60 V with the application of VDS= −60 V, while the top-gate is floating. The inset is the same trace in thelinear scale.

To accurately evaluate the capacitances of the top- andbottom-gate dielectric layers, we also prepared capacitor
structures on the same base films. The capacitances of thebottom-gate dielectric layer of polyimide 共Cbottom兲 and thetop-gate dielectric layer of parylene 共Ctop兲 are 5.4 and
4.5 nF/ cm2, respectively, which are consistent with the relative permittivities of polyimide and parylene provided by themanufacturers, namely, 3.8 and 3.1, respectively.
The current-voltage characteristics of the organic transistors are measured with a precision semiconductor parameter
analyzer 共4156C, Agilent Technologies兲. Figure 2共a兲 shows
the typical dc characteristics obtained when the top-gate
electrode is in the floating state, hereafter referred to as“floating.” Here, we monitor the source-drain current 共IDS兲 ofpentacene transistors as a function of source-drain voltage
共VDS兲. Bottom-gate voltage 共VGS兲 is changed from 0 to− 60 V in −10 V steps. Figure 2共b兲 shows a transfer curve ofthe FET: VGS is swept from +30 to − 60 V with the application of VDS = −60 V, while the top-gate electrode is floating.
The evaluated mobility is 0.2 cm2 / V s in the linear regime
and the on/off ratio is 107.
The FETs are functional when voltage bias is applied tothe top-gate electrode and the, bottom-gate electrode is floating. /The mobility is only 10−4 cm2 / V s, although the channels are produced in the same pentacene layer. In the above
operation mode, note that channel formation occurs at theinterface of the bottom surface of parylene and the top surface of pentacene. This interface should be very rough since
the surface morphology of pentacene shows leaf vein likestructures.12 The leakage current through parylene gate dielectric layers is only 5 pA even at −60 V, demonstrating agood reliability of parylene layers as insulators.

Appl. Phys. Lett. 87, 023509 共2005兲

FIG. 3. The transfer characteristics of the present transistors with doublegate structures are measured with the application of 共a兲 VDS = −0.5 V in thelinear regimes and 共b兲 −60 V in the saturation regimes, while the top-gatebias 共Vtop兲 is varied from −60 to + 60 V.

We measure the transfer characteristics of the present
FETs with double-gate structures in both the linear and saturation regimes at various voltage biases of the top gate 共Vtop兲
from −60 V to + 60 V. Figures 3共a兲 and 3共b兲 show the transfer curves obtained with the application of VDS = −0.5 V 共linear regime兲 and −60 V 共saturation regime兲, respectively.
When Vtop is swept from −60 V to + 60 V, the transfercurves systematically shift from left to right along the horizontal axis, or to the negative gate-source voltage 共VGS兲. In
particular, the shift is large when Vtop is greater than +20 V,
while it is very small when Vtop is less than +20 V. Saturation current is systematically decreased by increasing Vtop. In
contrast, minimum 共off兲 current is slightly increased whenVtop is varied from −60 to + 60 V, which is mainly due to theleakage current through parylene gate dielectric layers.
We plot in Fig. 4 mobility in the linear regime and Vth asfunctions of the top-gate bias Vtop. As shown in Fig. 4, thechange in Vth is very small when Vtop is varied from−60 V to+ 20 V, but Vth markedly decreases from −17 to− 43 V when Vtop is varied from +20 to+ 60 V. Such a large

FIG. 4. The threshold voltage 共Vth兲 and mobility 共␮兲 evaluated in the linearregime with double-gate structures as a function of top-gate bias Vtop. The
dashed line represents the calculated slope 共0.83兲, or the capacitance ofparylene 共4.5 nF/ cm2兲 divided by that of polyimide 共5.4 nF/ cm2兲, which isconsistent with the measured slope, ⌬Vth / ⌬Vtop 共0.85兲.
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Appl. Phys. Lett. 87, 023509 共2005兲

Iba et al.

and systematic change in Vth has not been achieved in organic transistors so far by other methods.7–9 In contrast,
chemical doping and other approaches can control Vth in silicon FETs more conveniently and efficiently,5,6,13 although
double-gate structures are known to change Vth in silicon
FETs.14,15 In the case of silicon transistors with double-gatestructures, the change in Vth can be understood well by asimple analysis of electrostatic potential:13 ⌬Vth / ⌬Vtop= Cx / C1, 1 / Cx = 1 / C2 + / C3, where C1, C2, and C3 are thecapacitances of the bottom-gate dielectric 共polyimide in organic FET兲, channel semiconductor 共pentacene兲, and topgate dielectric 共parylene兲, respectively. The slope
共⌬Vth / ⌬Vtop兲 of the present organic transistor in Fig. 4 is0.85, which is in excellent agreement with Cx / C1 ⬃ 0.83 culated without taking C2 into account since the organic semiconductor is much thinner than the parylene layer. Thus, weconclude that the change in Vth of organic transistors is well
explained by the change in electrostatic potential, similarly
to that of silicon FETs. Note that, in organic transistors, pentacene under a positive bias behaves like an insulator ratherthan a n-type semiconductor, because electrons are not induced at the interfaces of polycrystalline pentacene under apositive bias. As a result, a positive bias directly changes
electrostatic potential without shielding, and carrier concentration at the channel layer decreases drastically.
It is very interesting to note in Fig. 4 that mobility does
not depend on Vtop, but is maintained at 0.2 cm2 / V s. Thissuggests that mobility in the linear regime does not dependon the magnitude of electric field in the vertical direction.
Since the top-gate electrode of the present FETs covers
channel layers as well as source and drain electrodes, one of
the major concerns is that the top-gate electrodes may also
have a large influence on the physical properties of the FETs
near the source and drain contact regions. Thus, we prepareda control FET in which the top-gate electrodes cover only
80 ␮m of the 100 ␮m long channel and no overlap between
the top-gate electrodes and source-drain electrodes exists.16
No significant differences were found between those twoFETs, indicating that top-gate bias mainly affects the channelpart rather than the source-drain electrode part.
In summary, we controlled the threshold voltage 共Vth兲 oforganic FETs by employing novel planar-type double-gatestructures. Vth changes systematically in a wide range from
−16 to − 43 V with the application of voltage bias on thetop-gate electrode, while mobility 共0.2 cm2 / V s兲 in the linear
regime shows no significant changes at various Vtop biases
and the on/off ratio is 106. The control of Vth enables theoptimization of high-speed operation and low-power dissipation of integrated circuits, both of which give rise to a trade-off. Therefore, the present double-gate structures will open
up a possibility of realizing sophisticated integrated circuitswith organic transistors.
We thank Prof. T. Hiramoto for fruitful discussions. We
also thank Kyocera Chemical Cooperation for providing
high-purity polyimide precursors and Daisan Kasei for providing high-purity parylene. This study is partially supportedby the IT Program, MEXT, MPHPT, and NEDO.
Note added to the proof. Recent conference presentations
by Gelinck et al.17 and Li et al.18 report a similar effect.
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FETs with the different top-gate electrodes are manufactured simultaneously on the same base films so that the other structural parameters areexactly the same as the FET used in the measurement on Figs. 2 and 3.
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