IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 5, MAY 2010995

Large-Area Flexible Ultrasonic Imaging System
With an Organic Transistor Active Matrix
Yusaku Kato, Student Member, IEEE, Tsuyoshi Sekitani, Yoshiaki Noguchi, Tomoyuki Yokota,
Makoto Takamiya, Member, IEEE, Takayasu Sakurai, Fellow, IEEE, and Takao Someya, Member, IEEE

Abstract—We have successfully fabricated a large-area flexible
ultrasonic imaging system by integrating a polymeric ultrasonictransducer array sheet with an active matrix of organic fieldeffect transistors. The ultrasonic sheet comprises 8 × 8 ultrasonicsensing cells with an effective size of 25 × 25 cm2 . The organic
transistors exhibit mobility of 0.1 and 0.5 cm2 /V · s at the low operation voltage of 1 mV and in the saturation regime, respectively.
When an ac signal is applied between source and drain electrodes
for the transistors with a grounded gate, the on/off ratio is larger
than 104 at the carrier frequency of 40 kHz. In the linear sensing
array comprising eight ultrasonic cells, crosstalk is suppressedsufficiently low, and the on/off ratio exceeds 104 . Images in freespace are obtained for multiple-target objects over this sheet.
Index Terms—Large-area sensor, organic transistor, polyvinylidene fluoride (PVDF), ultrasonic wave.

I. I NTRODUCTIONORGANIC FETs [1]–[5] are suitable for large-area flexible
electronics since organic transistors can be fabricated ona plastic substrate at an ambient and/or low temperature byusing printing processes. In the past decade, utilizing theseadvantages, various types of novel applications have been
proposed and demonstrated. In particular, applications suchas flexible displays [6]–[8] and RF identification (RFID) tags
[9]–[11] have attracted considerable attention. In addition tothese applications, large-area sensors and actuators are expected to open up a new class of large-area electronics.
Recently, many large-area devices such as large-area gas sensors [12], [13], pressure and temperature sensors [14], [15],sheet-type scanners [16], large-area actuators [17], wirelesspower transmission sheets [18], and wireless communication
sheets [19] have been reported.

Manuscript received April 9, 2009; revised November 18, 2009. Current
version published April 21, 2010. This work was supported in part by the Grantin-Aid for Young Scientists (S) (WAKATE S), MEXT, Special Coordination
Funds for Promoting Science and Technology, NEDO, and JST. The review ofthis paper was arranged by Editor J. Kanicki.
Y. Kato was with the Quantum-Phase Electronics Center, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan. He is now with Sony
Corporation, Tokyo 108-0075, Japan.
T. Sekitani, Y. Noguchi, and T. Yokota are with the Quantum-Phase Electronics Center, School of Engineering, The University of Tokyo, Tokyo 113-8656,
Japan.
M. Takamiya and T. Sakurai are with the Institute of Industrial Science,
The University of Tokyo, Tokyo 153-8505, Japan.
T. Someya is with the Quantum-Phase Electronics Center, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan, and also with theInstitute for Nano Quantum Information Electronics, The University of Tokyo,
Tokyo 153-8505, Japan (e-mail: someya@ap.t.u-tokyo.ac.jp).
Digital Object Identifier 10.1109/TED.2010.2044278

Fig. 1. Image of an ultrasonic imaging sheet comprising an 8 × 8 array ofultrasonic sensing cells. The sensing cells are formed by integrating a printedorganic transistor sheet and an ultrasonic transducer array sheet. This system ismechanically flexible and can be wrapped around a robot’s body.

We have recently reported, at an IEEE International Electronic Devices Meeting [20], a successful fabrication of a largearea flexible ultrasonic 3-D imaging system by integrating an
ultrasonic-transducer array sheet, which is made of a polymer
piezoelectric material, with an active matrix of printed organic
FETs. In this paper, we report a systematic characterizationof ultrasonic sensor sheets and provide considerable technical details of the device fabrication process. The proposedsystem comprises 8 × 8 ultrasonic sensing cells or others
and has a printed area of 25 × 25 cm2 . The printed transistors (µsaturation ∼ 0.5 cm2 /V · s) with a grounded gate canswitch 40-kHz signals with an on/off ratio of > 104 . Threedimensional ultrasonic images can be obtained for multipletarget objects over this sheet. This system can detect target
objects behind a cloth and a paper. It is mechanically flexible
and can be wrapped around a cylindrical bar, as shown in Fig. 1;
this configuration is suitable for obtaining a viewing angle of360◦ for a medium-length (about several meters) proximity
robotic skinlike sensor.
II. M ATERIALS AND M ANUFACTURING P ROCESSThe ultrasonic sensing cell comprises one organic transistor
and one ultrasonic transducer. The cross-sectional illustration of
an ultrasonic sensing cell is shown in Fig. 2. A transistor activematrix sheet and an ultrasonic-transducer array sheet (Fig. 3)
are separately manufactured and then electrically connected toeach other. The circuit diagram is shown in Fig. 4. One side of
the transducer is connected to the drain of the transistor.

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996IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 5, MAY 2010

A. Printed Organic Transistor Active Matrix

Fig. 2. Cross-sectional view of an ultrasonic sensing cell. An organic transistor active matrix is laminated with an array of ultrasonic transducers using
a silver paste. The ultrasonic transducers are bent by the spacer to improvesensitivity. The transducer and spacer sheets are patterned to produce the pixel
structure using a numerically controlled cutting system.

The organic transistors with top-contact geometry are fabricated on a 75-µm-thick polyimide film. Compared with theorganic transistors with bottom-contact geometry, the organic
transistors with top-contact geometry show relatively small
contact resistance [21]; this is suitable for small-signal operations of devices such as ultrasonic devices. First, a gateelectrode is deposited on the substrate film by inkjet printing or
vacuum evaporation. As a gate dielectric layer, a 500-nm-thicklayer of polyimide (CT4112, Kyocera Chemical Corporation)is formed by inkjet printing or spin coating and cured in aclean oven at 180 ◦ C for 1 h. Then, a 50-nm-thick pentacene
channel layer is deposited by vacuum evaporation using aprinted shadow mask [22]. Finally, source and drain electrodes
are formed by inkjet printing or vacuum evaporation. The manufactured transistor matrix is encapsulated within a 5-µm-thicklayer of parylene (Daisan Kasei, Ltd., diX-SR) by using chemical vapor deposition. Regardless of manufacturing processes,
printing, or vacuum evaporation, transistor performances arealmost the same; however, yields of vacuum-evaporated transistors are higher than those of printed transistors. Therefore, inthe following, we used organic transistors manufactured using
vacuum evaporation, unless otherwise noted.
B. Ultrasonic-Transducer Array Sheet

Fig. 3. Photographs of the ultrasonic imaging sheet. The sensing cells arespaced by intervals of 1 in. To achieve a low channel resistance, 16 transistors
are connected in parallel. The dimensions of the ultrasonic transducer are
5 mm × 10 mm (W × L); the transducer is patterned using a numericallycontrolled cutting system.

A 28-µm-thick piezoelectric polyvinylidene fluoride (PVDF)
film whose surfaces are covered by NiCu is used as a sheet-typeultrasonic transducer. Although the piezoelectricity of PVDF issmaller than that of lead zirconate titanate (PZT), PVDF hasmany advantages with respect to its application to an ultrasonicimaging sheet, such as mechanical flexibility, light weight, lowcost, compatibility to large-area manufacturing in sheet-typetransducers, low acoustic impedance (compatibility to air-range
transducer), and a small resonance Q value that is requiredfor obtaining a narrow pulsewidth. Since position sensing is
explored in free space, the frequency of the ultrasonic wave
is adjusted to be 40 kHz to avoid significant attenuation.1 The
transducers are used in the transverse mode (d31 mode) to set
the resonance frequency to 40 kHz. The resonance frequencyof the ultrasonic transducer in the d31 mode can be controlled
by changing the bending radius of the transducer [23], [24].
In this device, one side of the transducer is attached to thesubstrate, whereas the other side is kept as a free end and liftedby a spacer sheet to improve the sensitivity at 40 kHz. Thisbending structure is easy to fabricate in sheet-type and largearea devices. The manufactured transducers can be used as both
receivers and transmitters without any change in their structure.
III. D EVICE C HARACTERISTICS
A. High-Speed Printed Organic FETs
Stand-alone organic transistors are characterized. All themeasurements are carried out in ambient air. First, we measure

Fig. 4. Circuit diagram. Each sensing cell has one ultrasonic transducer and
one transistor.

1 We should consider attenuation of ultrasonic wave in free space for aircoupled application. Attenuation of ultrasonic wave is proportional to thesquare of its frequency.

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KATO et al.: ULTRASONIC IMAGING SYSTEM WITH AN ORGANIC TRANSISTOR ACTIVE MATRIX

997

Fig. 6. High-speed printed organic FETs. (a) Measurement circuit ofgrounded-gate organic transistors. (b) One-megahertz operation of the organic
FETs with a grounded gate. (c) Power spectrum of the output waveform shownin (a). A high on/off ratio of 103 is obtained at 1 MHz.
Fig. 5. Small-signal operation of organic FETs. (a) Plot of the source–draincurrent IDS versus the source–drain voltage VDS . (b) Operation of the organic
FETs with a small VDS value. A plot of IDS versus VDS in a linear regime.
(c) IDS versus gate–source voltage VGS of the organic transistors in a linear
regime (VDS = −1 mV).

the dc characteristics prior to measuring the ac characteristics.
The typical dc characteristics of the manufactured transistors
are shown in Fig. 5. For an ultrasonic imaging device application, the characteristics in a linear regime are very importantfor sensing and switching a small signal (∼1 mV) generated
from the ultrasonic receivers. As can be seen in Fig. 5(b)and (c), the manufactured transistors exhibit good linear behavior in the small-signal regime, indicating that good ohmic
contacts can be obtained in the manufactured organic transistors. This characteristic is very important since the ultrasonictransducers are operated at a very low voltage of ∼1 mVp−p .
The mobility in the saturation regime is 0.5 cm2 /V · s, and theon/off ratio is 107 when the off current is defined as a minimum
current at the gate bias of +10 V in dc measurements.
In the ultrasonic device, the organic transistor active matrix switches the ac signal that is generated at the ultrasonicreceivers or applied to the ultrasonic transmitters. In thesesituations, a dc bias is applied to the gate, whereas an ac
signal is applied to the drain. Note that organic transistors haveconsiderably large capacitance coupling between electrodes
because of their process limit and structure, and this causes anunintentional signal flow from the source to the drain electrode.
This flow is independent of the gate voltage at a high frequency.
Fig. 6 shows the output waveform: The ac voltage of 1 Vp−psine wave is applied to the source and the drain by using afunction generator, whereas the constant voltage (−40 V for
the ON state or +20 V for the OFF state) is applied to the gate
[Fig. 6(a)]. The output voltage Vout at the source is measured
using an oscilloscope. Note that the output voltage at the OFF
state is large and limits the on/off ratio in the low-frequency
regime, even below the cutoff frequency. When the output
voltage at the ON and OFF states [Fig. 6(b)] is transformed by a
Fourier transformer using a power spectrum, we obtain a goodon/off ratio of > 103 at 1 MHz and > 104 at 40 kHz [Fig. 6(c)].
The output voltage in the OFF state is mainly due to capacitive
coupling between source and drain electrodes.
B. Stand-Alone Ultrasonic Transducer
The stand-alone characteristics of ultrasonic transducers arecharacterized. A commercial PVDF ultrasonic transducer witha resonance frequency of 40 kHz is used for the transmission of
the ultrasonic wave with the application of an input voltage of
200 Vp−p using the pulser/receiver. The frequency of the input
voltage is 40 kHz. The output waveform from ultrasonic receivers is amplified by 40 dB by the pulser/receiver. The typicalreceived waveform of the manufactured ultrasonic transducers
is shown in Fig. 7(a). The manufactured ultrasonic receivers
show an output voltage of ∼0.8 Vp−p after an amplification of
40 dB by the incidence of an ultrasonic wave. The manufactured
transducer shows a small Q value of 4 and a narrow pulsewidthof 100 µs, which is suitable for a high-resolution proximity
sensor.

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 5, MAY 2010

Fig. 8. Ultrasonic sensing cells comprising organic FETs and ultrasonictransducers. (a) Switching characteristics of the ultrasonic cell in the receivermode with changing VGS of the organic transistor. (b) Power spectrum of
the ultrasonic sensing cell for (solid line) VGS = −100 V and (dashed line)
VGS = 0 V. The on/off ratio is > 104 . (c) Switching characteristics of theultrasonic cell in the transmitter mode. (d) Power spectrum of the ultrasonictransmitter cell for (solid line) VGS = −130 V and (dashed line) VGS = 20 V.
The on/off ratio is > 105 .
Fig. 7. Stand-alone ultrasonic transducer. (a) Typical waveform Vout received
at the transducers. The signal is amplified by a 40-dB gain ac amplifier. The
frequency of the transmitting signal is 40 kHz. (b) Received waveform reflectedfrom a stiff object behind a cloth. (c) Angular dependence of Vout . The
ultrasonic transducers can detect signals at an angle of 180◦ .

An ultrasonic wave can permeate through a perforated object,which has holes whose dimensions are approximately equal
to the wavelength of the ultrasonic wave. We have conducted
experiments for detecting an object behind a cloth by using an
ultrasonic wave. We place the manufactured ultrasonic transducers in front of a cloth and a reflector behind the cloth. The
ultrasonic wave transmitted from the transmitter is reflected on
the cloth, whereas a part of the ultrasonic wave goes thoroughthe cloth and is reflected on the target object behind the cloth.
Then, the reflected ultrasonic wave is detected by the ultrasonicreceiver. The result is shown in Fig. 7(b). The reflected wave
from both the cloth and the target object is shown in the output
waveform. The frequency of the proposed ultrasonic device is
40 kHz, which corresponds to the wavelength of 8.5 mm inair. Furthermore, a part of the ultrasonic wave can transmit
through a cloth. In our experiment, 60% of the ultrasonicwave permeates through a cloth. The ultrasonic transducers candetect a stiff object that is hidden inside a cloth and whichcannot be detected by other imaging devices such as a camera.
Fig. 7(c) shows the directivity of the manufactured ultrasonicreceiver. This figure shows a broad directivity, i.e., > −5 dB for
an incident angle of ±60◦ . This broad directivity is suitable foruse in a synthetic aperture technique. In the width direction,
the receiver shows a symmetrical directivity and peak at ±60◦
because of the superposition of the incident ultrasonic wave. In
the length direction, it shows an almost symmetrical directivity,

but the peak at 60◦ is observed because of the bending structureof the transducers. An almost identical directivity is observedin the case of all the transmitters.
C. Integrated Imaging System
The characteristics of the integrated ultrasonic sensing cellcomprising one ultrasonic transducer and one organic FET areinvestigated. Fig. 8(a) shows the gate voltage dependence ofthe output voltage of an ultrasonic sensing cell. One of theelectrodes of the ultrasonic receiver is connected to the drain of
the organic transistor, whereas the other electrode is connectedto the ground. The output signal generated at the ultrasonicreceiver by the incidence of the ultrasonic wave is controlled bya bias voltage applied to the gate of the organic transistor. The
input voltage of 200 Vp−p is applied to the transmitter by using
a pulser/receiver. The output waveform from the source of theorganic transistor is amplified by the pulser/receiver at 50 dB.
The output voltage at a gate voltage of −60 V is approximately
89% of that of the stand-alone transducer. The power spectrumof the output wave is shown in Fig. 8(b) for gate voltages of−100 and 0 V. The signal from the ultrasonic transducer is wellswitched by the organic FET at 40 kHz, and the on/off ratio at
40 kHz is > 104 .
The operation of ultrasonic cells as a transmitter is also
investigated. The test circuit is similar to that of the receivermode. The input signal to the ultrasonic transducer is switchedby the gate bias voltage. The gate voltage dependence and its
power spectrum are shown in Fig. 8(c) and (d), respectively.
The switching of the signal to the ultrasonic transducer canbe controlled by the organic FET. The output voltage is approximately 22% smaller than that of stand-alone ultrasonic

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KATO et al.: ULTRASONIC IMAGING SYSTEM WITH AN ORGANIC TRANSISTOR ACTIVE MATRIX

Fig. 9. Crosstalk in the ultrasonic cells linear array. (a) Experimental setup
for a crosstalk measurement. The eight sensing cells are connected to eachother through the common BL. All the eight cells are used as receiver cells.
(b) Output waveforms Vout of the ultrasonic sensing cell in the 1 × 8 ultrasonicsensing array for VGS = −40 V (on) and 20 V (off). (c) Power spectrum of theoutput waveforms shown in (b) for (solid line) VGS = −40 V and (dashed line)
VGS = 20 V. The ultrasonic sensing cell in the 1 × 8 ultrasonic sensing arrayalso exhibits a sufficient on/off ratio of > 104 .

transducers when the gate voltage is −130 V. In the transmittermode, an on/off ratio of > 105 is obtained at 40 kHz. The
different characteristic between the receiver and the transmitter
is mainly due to both variation in transistor performances andtransducer coupling.
Moreover, the crosstalk of the integrated ultrasonic sensing
cells is characterized. The eight ultrasonic transducers are connected to the eight transistors that have a common bit line (BL).
All the eight cells are used as receiver cells. In this configuration, the integrated ultrasonic sensing cells have a sufficient
on/off ratio of 104 at 40 kHz when the gate voltage is −40 V
for the ON state or 20 V for the OFF state, as shown in Fig. 9.
D. Imaging
An imaging experiment using the ultrasonic sensing arrayin free space is carried out. The spatial images are obtainedby using a synthetic aperture method [25], [26]. The reflected
ultrasonic wave from target objects is detected by an array of ultrasonic receivers, and the image of the object is reconstructed.
A high-resolution 3-D spatial image can be reconstructed using
a 2-D array of ultrasonic receivers. First, the target object whose
dimensions are 50 mm × 50 mm is detected by a 1 × 8 linear

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Fig. 10. Ultrasonic imaging. (a) Experimental setup of ultrasonic imaging.
A 3-D image of the object can be reconstructed (synthetic aperture method)using all the received signals. (b) 2-D image obtained by a 1 × 8 ultrasonicsensing array comprising ultrasonic transducers and organic transistors. The
x-axis represents the direction of the linear array of ultrasonic sensing cells,
and the z-axis represents the direction perpendicular to the linear array. (c) 3-D
image obtained by 17 × 11 stand-alone ultrasonic transducers.

array of ultrasonic sensing cells. The measurement setup andthe result are shown in Fig. 10(a). A 2-D image, representing
the direction of the linear array of the ultrasonic sensing cellson the x-axis and the direction perpendicular to the linear
array on the z-axis, is obtained by the ultrasonic sensor array.
In this experiment, all the 1 × 8 ultrasonic sensing cells areused as receivers, and one PVDF ultrasonic transmitter is usedin addition to the linear array. The readout of the reflected
waveform is scanned one by one by the organic transistors.
Since >99% of the ultrasonic wave is reflected at the surfaceof the target object in this case, only the bottom surface ofthe target object can be detected. A clear image is obtained, as
shown in Fig. 10(b). According to the principles of the synthetic
aperture technique, the resolution along the z-axis is better thanthat along the x- and y-axes. However, further improvement
will be feasible only if the area or number of receivers and/ortransmitters is increased. As shown in Fig. 10(c), a 3-D image
is obtained by using the 17 × 11 two-dimensional stand-alone
ultrasonic transducers without organic transistors.
IV. D ISCUSSION
Here, we would like to address the advantages of ultrasonicimaging devices in comparison with other imaging devices. In
recent times, 3-D sensing of the position of the people andAuthorized licensed use limited to: UNIVERSITY OF TOKYO. Downloaded on May 25,2010 at 06:53:52 UTC from IEEE Xplore. Restrictions apply.

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 5, MAY 2010

objects in free space has been carried out by using several
methods that are based on visible light, radio waves, millimeterwaves, and/or ultrasonic waves. All of these methods have both
merits and demerits, and an optimal method that considers target materials, measurement environment, required resolution,
distance, and cost is used. Ultrasonic imaging [27]–[30] canoffer attractive features such as real-time nondestructive 3-D
imaging in free space at an ultralow cost. Ultrasonic waves
are now widely used for nondistractive testing [31], fabricationof intervehicular distance sensors [32], and medical ultrasonicimaging [33]. Unlike these applications, the proposed device
can detect 3-D spatial information in free space. The resolutionof the spatial image obtained by the synthetic aperture method
depends on the number, density, and area of the array ofultrasonic receivers. As shown in Fig. 10(b) and (c), a large
aperture and a large number of receivers are needed to obtainhigh-resolution spatial images, particularly along the plane thatis parallel to the receiver array. A large-area array of ultrasonicreceivers is required to obtain a high-resolution image; however, it is difficult to manufacture a large-area receiver arrayusing PZT or other ceramic piezoelectric material integrated
with inorganic electronics at a low cost. In this paper, we
realized a large-area ultrasonic imaging sheet compatible with
3-D imaging using a polymer piezoelectric material and anorganic transistor active matrix.
In the ultrasonic imaging sheet with an active matrix oforganic transistors that switch ac input and/or output signals,
it is important to significantly improve the ac characteristics oforganic transistors. For this purpose, improvement of mobility
and reduction of channel length are crucial. In addition topentacene, some organic semiconductors are known to exhibit a high mobility value of > 1 cm2 /V · s in ambient air
such as [1]Benzothieno[3,2-b]benzothiophene derivatives [34]and 6,13-bis(triisopropyl-silylethynyl) pentacene [35]. Furthermore, with an increase in mobility, the frequency responseof organic transistors has been improved [36]. It is important
not only to improve mobility but also to decrease the channel
length for high-frequency operation. Indeed, by reducing thechannel length to 0.48 µm, a transition frequency fT of 2 MHz
has been achieved in the case of polymer FETs [37]. Moreover, in the case of logic circuits based on organic transistors,
64-bit code generators for RFID systems [11], complementary
D flip-flops [38], and ultralow-power organic complementaryring oscillators that operate at a supply voltage of 1.5 V [39]have been reported. In all these ICs, organic transistors have asource-grounded configuration.
Meanwhile, organic transistors in the ultrasonic device havea gate-grounded configuration; an ac signal from/to ultrasonictransducers is applied to the drain, whereas a constant voltage
is applied to the gate. Gate-grounded ac characteristics oforganic transistors are as important as the source-grounded ac
characteristics in many applications such as active matrices
in sensors; however, little attention has been paid to thesegate-grounded ac characteristics. In ultrasonic devices, as in
the case of organic active matrices that are used for display
applications, not only a high cutoff frequency but also a highon/off ratio is required when the number of ultrasonic cells
becomes large. Note that organic transistors have a rather large

overlap area and large capacitance coupling between electrodes
because they are generally fabricated using a simple mechanics
for low-cost registration, as compared with advanced silicon
technology. Such capacitance coupling causes an unintentionalac signal that limits the on/off ratio flows from the drain to thesource, even in the frequency region below the cutoff frequency.
Therefore, reducing capacitance coupling between electrodes isimportant for obtaining a high on/off ratio with grounded-gateorganic transistors.
To decrease the capacitance, we have cut down the width of
the source/drain electrodes. Low channel resistance is needed
for high sensitivity of the ultrasonic cells; however, the sourceand drain electrodes that have large channel width cause
large parasitic capacitance between electrodes. This capacitance drastically degrades the OFF state of the transistors. In
this paper, we have successfully obtained a high on/off ratioof > 104 with gate-grounded organic transistors, even at an
ultrasonic frequency of 40 kHz, by reducing the width of thesource and drain electrodes. Furthermore, to reduce the width ofelectrodes with large-area scalability, inkjet printing technologies with subfemtoliter accuracy have recently been developed[40], [41]. These technologies will drastically improve the ac
characteristics of organic transistors.
Another possible method is electrical shielding using an electrically grounded top-gate electrode placed over source/drainelectrodes. The capacitive coupling between the source and thedrain can be suppressed by electric shielding on source/drainelectrodes. By placing the grounded electrode immediately overthe source/drain electrode such as the top gate connected to theground, the electric flux line from the drain to the source is
cut by the shielding electrode. Then, the capacitance between
the source and the drain decreases. Moreover, in the large-area
circuit, the resistance and the capacitance are large becauseof the interconnections and the circuit layout. In considerablylarge-area sensors or actuators, suppressing the capacitance
between interconnections, such as word lines and BLs, is also
very important for obtaining smaller crosstalk between the transistors at a high frequency. If grounded-gate organic transistors
can operate at a high frequency of > 1 MHz with a sufficiently
high on/off ratio after these improvements, the resolution of
the ultrasonic imaging sheet becomes higher; this increase in
resolution expands the sheet’s scope of application in the fieldof medicine.
In this paper, the spatial image is obtained by using thesynthetic aperture method; however, we can use the ultrasonictransducer matrix as a proximity sensor matrix because eachcell of the ultrasonic transducer matrix in the proposed device
can be used as both a receiver and a transmitter without any
change. Moreover, we do not have to use all transducers every
time. We can choose the area and number of transducers that
we wish to use according to the measurement area, resolution,and scan rate that we expect.
V. S UMMARY
In this paper, we have successfully fabricated a large-area
flexible ultrasonic 3-D imaging system by integrating an array
sheet of ultrasonic transducers, which is made of a polymer

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KATO et al.: ULTRASONIC IMAGING SYSTEM WITH AN ORGANIC TRANSISTOR ACTIVE MATRIX

piezoelectric material, with an active matrix of printed organic FETs. The printed transistors (µ ∼ 0.5 cm2 /V · s) witha grounded gate can switch 40-kHz signals with an on/off
ratio of > 104 . The system comprises 8 × 8 ultrasonic sensing
cells or others and has a printed area of 25 × 25 cm2 . Threedimensional ultrasonic images can be obtained for multipletarget objects over this sheet. This system can detect target
objects behind a cloth and a paper. It is mechanically flexible
and can be wrapped around a cylindrical bar; this configuration
is suitable for obtaining a viewing angle of 360◦ for a mediumlength (about several meters) proximity robotic skinlike sensor.
ACKNOWLEDGMENT
The authors would like to thank K. Ishibe for fruitful discussion and technical support.
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pp. 4976–4980.

Yusaku Kato (S’05) was born in Tokyo, Japan,
in 1981. He received the B.S. and Ph.D. degrees
in applied physics from The University of Tokyo,
Tokyo, in 2004 and 2009, respectively.
He is currently with Sony Corporation, Tokyo.

Tsuyoshi Sekitani received the B.S. degree from
Osaka University, Suita, Japan, in 1999 and the Ph.D.
degree in applied physics from The University ofTokyo, Tokyo, Japan, in 2003.
From 1999 to 2003, he was with the Institute for
Solid State Physics, The University of Tokyo. From2003 to 2009, he was a Research Associate with
the Quantum-Phase Electronics Center, School ofEngineering, The University of Tokyo. Since 2009,he has been a Research Associate with the Department of Electrical and Electronic Engineering,
The University of Tokyo.

Yoshiaki Noguchi was born in Kanagawa, Japan, in
1983. He received the M.S. degree in applied physics
in 2007 from The University of Tokyo, Tokyo, Japan,
where he is currently working toward the Ph.D.
degree in applied physics.
His research interests include organic transistors,
large-area electronics, and printed electrical devices.
Mr. Noguchi is a Student Member of the JapaneseSociety of Applied Physics and a member of theJapan Society for the Promotion of Science Research
Fellowships.

Tomoyuki Yokota was born in Tokyo, Japan, in
1985. He received the B.S. degree in applied physics
in 2008 from The University of Tokyo, Tokyo, where
he is currently working toward the M.S. degree inapplied physics.
His research interests include printed electricaldevices, large-area electronics, organic memories,
and organic transistors.
Mr. Yokota is a Student Member of the JapaneseSociety of Applied Physics.

Makoto Takamiya (S’98–M’00) received the B.S.,
M.S., and Ph.D. degrees from The University ofTokyo, Tokyo, Japan, in 1995, 1997, and 2000, respectively, all in electronic engineering.
In 2000, he joined NEC Corporation, Tokyo,
where he was engaged in the circuit design of highspeed digital LSIs and developed the field of on-chipmeasurement macros to solve the power integrity
issues. In 2005, he joined The University of Tokyo,
where he is currently an Associate Professor with
the VLSI Design and Education Center. His researchinterests include power and signal integrity issues in LSIs, low-power RF
integrated circuits, low-power digital circuits, and large-area electronics withorganic transistors.

Takayasu Sakurai (S’77–M’78–SM’01–F’03) received the Ph.D. degree in electrical engineering from The University of Tokyo, Tokyo, Japan,
in 1981.
In 1981, he joined Toshiba Corporation, where
he designed CMOS DRAM, SRAM, RISC processors, DSPs, and SoC solutions. He has extensively
worked on interconnect delay and capacitance modeling known as Sakurai model and alpha-power-law
MOS model. From 1988 to 1990, he was a Visiting Researcher with the University of California,
Berkeley, where he conducted research in the field of VLSI CAD. Since 1996,
he has been a Professor with The University of Tokyo, working on low-powerhigh-speed VLSI, memory design, interconnects, and wireless systems. He
is also a consultant to U.S. startup companies. He has published more than
400 technical papers, including 70 invited papers and several books, and has
filed more than 100 patents.
Prof. Sakurai served as a Conference Chair for the IEEE/JSAP Symposium on VLSI Circuits and IEEE International Conference on IC Design
and Technology, a Vice Chair for the ACM/IEEE Asia and South Pacific
Design Automation Conference, and a Program Committee Member for theIEEE International Solid-State Circuits Conference, IEEE Custom Integrated
Circuits Conference, ACM/IEEE Design Automation Conference, ACM/IEEEInternational Conference on Computer-Aided Design, ACM Workshop on
Field-Programmable Gate Arrays, ACM/IEEE International Symposiumon Low-Power Electronics and Design, ACM/IEEE International Workshop on
Timing Issues, and other international conferences. He was a Plenary Speaker
for the 2003 IEEE International Solid-State Circuits Conference. He is anelected AdCom Member for the IEEE Solid-State Circuits Society and an IEEE
Circuits and Systems Society Distinguished Lecturer.

Takao Someya (M’03) received the Ph.D. degree inelectrical engineering from The University of Tokyo,
Tokyo, Japan, in 1997.
In 1997, he joined the Institute of Industrial Science, The University of Tokyo, as a Research Associate. He was appointed to be a Lecturer with the
Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, in 1998
and an Associate Professor of RCAST in 2002.
From 2001 to 2003, he was a Visiting Scholarwith the Nanocenter (NSEC), Columbia University,
New York, NY, and Bell Laboratories, Lucent Technologies. From 2003 to
2009, he was an Associate Professor with the Department of Applied Physics
and Quantum-Phase Electronics Center, School of Engineering, The Universityof Tokyo. Since 2009, he has been a Professor of electrical and electronicengineering with The University of Tokyo. His current research interests
include organic transistors, flexible electronics, plastic integrated circuits, largearea sensors, and plastic actuators.
Prof. Someya has been a Board of Directors of the Materials ResearchSociety since 2009; a Distinguished Lecturer of the IEEE Electron Devices
Society since 2004; an International Academic Member at NSEC, Columbia
University, since 2003; and a Global Scholar of Princeton University, Princeton,
NJ, since 2009. He was the recipient of numerous awards, including theIEEE/ISSCC Sugano Award in 2005, the Marubun Award in 2006, the Japan
Society for the Promotion of Science Award in 2008, and the Japan IBM
Science Award in 2009.

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