AMD3 - 1
InvitedWireless Power Transmission Sheetwith Organic FETs and Plastic MEMS Switches
M. Takamiya, T. Sekitani*, Y. Miyamoto, Y. Noguchi *, H. Kawaguchi**,T. Someya*, and T. Sakurai***
VLSI Design and Education Center, University of Tokyo, Japan*Quantum-Phase Electronics Center, School of Engineering, University of Tokyo, Japan**Department of Computer and Systems Engineering, Kobe University, Japan***Center for Collaborative Research, University of Tokyo, Japan
ABSTRACT
Design innovations to overcome the shortcomings of a wireless power transmission sheet madewith plastic MEMS switches and organic FETs(OFETs) for printable low-cost electronics areshown.
The mixed circuits of MEMS switches
and OFETs with two different frequencies reduce
the number of coil sheets form 2 to 1. OFET
level-shifters, with the current-source loads withenhancement/depletion mixed threshold voltages
realized by controlling the back-gate voltage,
bridge the operation voltage gap between silicon
VLSIs (below 5 V) and OFETs/MEMS (above 40V).

1. INTRODUCTION
A wireless power delivery system to electronic
objects scattered over tables, walls and ceiling willform an infrastructure necessary for ubiquitouselectronics, wireless sensor network and ambient
intelligence. A first step toward this goal has
been reported as a wireless power transmission
sheet [1]. Although the device demonstrated thefeasibility of such a system, there were several
limitations and drawbacks in practical use. This
paper describes design innovations [2] to overcome the shortcomings of a wireless power transmission sheet made with plastic MEMS switches
and organic FETs (OFET) circuits suitable forprintable low-cost electronics. By the novel design
1 large TX-coil

Many small TX-coilsRX-coil
1 inch2
TX-coil

RXsolutions, the sheet reduces the number of coilarrays, and takes 5V digital input by using newly
developed level shifter with adaptive biasing andorganic analog circuits which achieves higher gain
than a previously reported OFET amplifier.

2. WIRELESS POWER TRANSMISSION SHEET
2.1 Principle of Operation
The principle of the wireless power transmission
is based on an array of coils (TX-coil) made on aplastic sheet which is selectively driven by plastic
MEMS switches and is coupled magnetically with areceiver coil (RX-coil) mounted on a power receiving object. Fig. 1 shows the comparison of thepower transmission efficiency for a large TX-coiland many small TX-coils. The measured powertransmission efficiency of the large TX-coil is 0.1%,
while that of the many small TX-coils is 60%. The
segmentation and selective activation of TX-coils
prevent the efficiency loss. Position detection ofthe receiving object is needed for the selective activation of TX-coils. The position detection is carried out by scanning through many TX-coils tocheck if an RX-coil exists above a TX-coil bymeans of impedance change.
MEMS switch

OFET
1mm

GBLViaDS

WL

Pentacene
5mm

Cu21cmTX
Via

30cm2 X 1 coil
Efficiency ~ 0.1%

1 inch2 X 64 coils
Efficiency ~ 60%

Fig. 1. Power transmission efficiency in two
different TX-coils.

TX-Coil
10mm

Fig. 2. Bird's-eye photo of wireless powertransmission sheet and micrographs of a
TX-coil, MEMS switch and OFET.

IDW ’0795

TX-coil array

21 cm
21 cm
RX-coil

TX RX

MEMS switches
for powertransmission (PT)
Organic FETs (OFEfor RX positiondetection (PD)8 x 8 array (1-inch pitch)
Fig. 3. Device structures of wireless powertransmission sheet.

13.56 MHz 38 LEDs(a)

RX-coil

TX-coil array

Table. I. Speed and on–resistance of MEMS
switches and organic FETs.

Switch
MEMS
OFETs

Speed
~ 1Hz
> 100Hz

On-resistance
< 10:
> 1k:

2.2 Device Structures
Fig. 2 shows a bird's-eye photo of the wirelesspower transmission sheet and micrographs of a
TX-coil, MEMS switch and OFET [1]. Fig. 3
shows the device structures of wireless powertransmission sheet. Three 21-cm-square plasticsheets are stacked. Each sheet has 8 x 8 arrayof TX-coil, MEMS switches, and OFETs with1-inch pitch. Table I shows the comparison of thespeed and the on–resistance of MEMS switches
and OFETs. The MEMS switches and OFETs
are complementary switches.
The MEMS
switches are used for the power transmission because they can transfer amperes of current but theswitching speed is 1 Hz. In contrast, OFETs areused for the RX position detection because theswitching speed of OFETs is more than 100 Hz but
the on–resistance is more than 1 k:. Fig. 4 (a)shows the measurement setup for the wirelesspower transmission to 38 LEDs. Fig. 4 (b) shows
the demonstration of the wireless power transmission. The distance between RX-coil and TX-coils
was 5 mm.

m
25.4 m(Without OFETs)

5-mm distance
between RXand TX-coils(b)Fig. 4. Wireless power transmission to 38
LEDs. (a) Measurement setup. (b) Demonstration.
tion cost and reduce the position detection efficiency because the power transmission coils sit
over the position detection coils. In order to solve
the problems, new circuits are developed to enablea shared coil sheet for the power transmission andthe RX position detection.
Fig. 6 shows the developed mixed circuits ofMEMS switches and OFETs for the shared coilsheet. The circuits use two different frequencies
and carefully eliminate interference of the twofunctions: the power transmission and the position
detection. The circuit makes use of the fact thatthe conductivity of MEMS switches is more than
102 higher than that of OFETs. Thus, the OFET
doesn’t degrade the power transmission efficiency
by shorting two frequency signal sources. C1 isadded to decrease the impedance at 13.56 MHz
Previous work

This work

CoilCoilMEMS

MEMS

for PT & PD

for PTfor PT

Coil

3. SHARED COIL SHEET
Fig. 5 shows the comparison of the sheet configuration of our previous work [1] and this work.
The sheet of TX-coil array for the power transmission and the coil array for the position detection
were separately fabricated and stacked in the previous implementation. The separate coil sheets
stacked together, however, increase the fabrica-96

IDW ’07

for PD

OFET

OFETfor PD

Fig. 5. Shared coil sheet. In the previouswork [1], two coil sheets for the power transmission (PT) and the RX position detection(PD) are required. In this work, the developed
circuits enabled a shared coil sheet for PT andPD.

SFSFSFCP40 VMonitor for PD

6WithoutRX-coil

VMON (V)5Maximumfrequency
for OFETs

WithRX-coil

2
1
00

3.5 5
10Frequency (f1) (MHz)Gain = 2.6

In15Fig. 7. Measured position detection frequency
dependence of monitor voltage with and without RX-coil. 3.5 MHz was used due to thespeed limitation of OFETs.
(power transmission frequency) to improve thepower efficiency while monitoring the impedancechange for 3.5MHz (position detection frequency).
CP is added to increase the position detection sensitivity by peaking the impedance by LC resonance.
As shown in Fig. 7, the measured resonant frequency of VMON for the position detection system isaround 8 MHz to 10 MHz, which is beyond thecutoff frequency of OFETs (3.5 MHz). As seenfrom Fig. 7, in order to increase the sensitivity forthe position detection, the frequency used for position detection is higher the better. Thus 3.5MHz
is chosen as the frequency for the position detection.
4. OFET LEVEL SHIFTERS
The integration of the silicon LSIs, OFETs, andMEMS devices is necessary in this power transmission system. Direct digital drive of OFETs andMEMS by silicon LSIs is, however, difficult, because there is large discrepancy in the operation
voltage between LSIs (below 5 V) andBias

Source follower
Single amp

Fig. 8. Original OFET level shifters from 5 V to40 V. Only pMOS FETs are used.
OFETs/MEMS (above 40 V). Therefore, OFET
level-shifters from 5 V to 40 V are developed. Fig.
8 shows the original OFET level shifter circuits.
The DC level of the input for the single amplifier isadjusted by tuning the bias voltage of the source
follower. The circuits in Fig. 8 are not robust
against PVT variations. Fig. 9 shows the OFET
level shifters with replica circuits for adaptive biasing to deal with the PVT variations. Fig. 9 also
shows the voltage waveforms of each node in thelevel shifters. It should be noted that the circuits
in Figs. 8 and 9 are pMOS-only design, becausenMOSFETs are not commonly available in theOFET designs. In the pMOS-only design, it isdifficult to obtain the high gain in the amplifiers.
The high gain differential amplifiers in Fig. 9 are
the key circuits for the adaptive biasing. Figs. 10(a) and (b) show the fabricated high gain differential amplifiers with current-source load with enhancement/depletion mixed threshold voltages
realized by controlling the back-gate voltage [3].
The chip area is 2.9 mm by 4.9 mm. The gate
length of all OFETs is 50 Pm. Fig. 10 (c) shows
the measured characteristics of the differential
amplifiers. The gain of 6.4 is achieved, which isReplica 2.5 VSFSF

Vadap
OriginalInSFSF

SF-VMON

Fig. 6. Mixed circuits of MEMS and OFETs forshared coil.

3Out+to distinguish2 frequencies

4In

Vadap
OutC140 V
0VRX
40 V

13.56 MHzfor powertransmission

Out

20 V Vadap

-TX

In

5V
0V

+

MEMS switch
f1 =3.5 MHz
for position
detectionSF

20 VOut

Out

40 V

20 V
5V
2.5 V
0V

InFig. 9. OFET level shifters with replica circuits
for adaptive biasing to deal with PVT variations. The lower figure shows the voltagewaveforms of each node.

IDW ’07

97

40V

In-

8000/50

20VOutb

403020㩷

Out (V)

4000/50(a)20 V(a) 㩷㩷

16000/50+InOutOutSF

104.9 mm0010㩷203040In (V)(b)
Fig. 11. Adaptive biasing used in the OFET
level shifters.
(a) Circuit schematic.
(b)
Measured characteristics.

2.9 mm(b)

Out, Outb (V)40

Gain = 6.4Outb

Out30206. ACKNOWLEDGMENT
This work is partially supported by CREST, JST,and MEXT.

100203040In (V)(c)
Fig. 10.
Differential amplifiers with current-source load. (a) Circuit schematic. (b)Chip micrograph. (c) Measured characteristics.
higher than 2.3 for an organic analog amplifier reported in [4]. Fig. 11 (a) shows the fabricatedadaptive biasing circuits used in the OFET levelshifters. Fig. 11 (b) shows the measured characteristics of the adaptive biasing. The developed
high gain differential amplifier contributes to thesuccessful feedback control.

0105. CONCLUSION
Design innovations to overcome the shortcomings of a wireless power transmission sheet madewith plastic MEMS switches and OFET for printable low-cost electronics are shown. The mixed
circuits of MEMS switches and OFETs with two
different frequencies (13.56 MHz and 3.5 MHz)
reduce the number of coil sheets form 2 to 1.
OFET level-shifters, from 5 V to 40 V with the cur-98

IDW ’07rent-source loads with enhancement/depletion
mixed threshold voltages realized by controllingthe back-gate voltage, bridge the operation voltage
gap between silicon VLSIs (below 5 V) andOFETs/MEMS (above 40 V).

7. REFERENCES[1] T. Sekitani, M. Takamiya, Y. Noguchi, S. Nakano, Y. Kato, K. Hizu, H. Kawaguchi, T. Sakurai, and T. Someya, "A large-area flexible
wireless power transmission sheet using
printed plastic MEMS switches and organic
field-effect transistors ," IEDM Tech. Dig., pp.
287 - 290, Dec. 2006.
[2] M. Takamiya, T. Sekitani, Y. Miyamoto, Y.
Noguchi, H. Kawaguchi, T. Someya, and T.
Sakurai, “Design solutions for multi-object
wireless power transmission sheet based onplastic switches,” ISSCC Dig. of Tech. Papers,, pp. 362-363, Feb. 2007.
[3] S. Iba, T. Sekitani, Y. Kato, T. Someya, H.
Kawaguchi, M. Takamiya, T. Sakurai, and, S.
Takagi, "Control of thresh-old voltage of organic field-effect transistors with double gate
structure," Applied Physics Letters, 87,023509, July 2005.
[4] N. Gay, W. Fischer, M. Halik, H. Klauk, U.
Zschieschang, and G. Schmid, "Analog signal
processing with organic FETs," ISSCC Dig. ofTech. Papers, pp. 278-279, Feb. 2006.