Design for Mixed Circuits ofOrganic FETs and Plastic MEMS Switches for
Wireless Power Transmission Sheet
Makoto Takamiya, Tsuyoshi Sekitani*, Yoshio Miyamoto, Yoshiaki Noguchi*,Hiroshi Kawaguchi**, Takao Someya*, and Takayasu 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
E-mail: mtaka@iis.u-tokyo.ac.jp
which achieves higher gain than a previously reported OFET
amplifier.

Abstract— Design innovations to overcome the shortcomings ofa wireless power transmission sheet made with plastic MEMSswitches and OFET for printable low-cost electronics are shown.
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 bycontrolling the back-gate voltage, bridge the operation voltage
gap between silicon VLSIs (below 5 V) and OFETs/MEMS (above40 V).

II. WIRELESS POWER TRANSMISSION SHEETThe principle of the wireless power transmission is based on
an array of coils (TX-coil) made on a plastic sheet which is
selectively driven by plastic MEMS switches and is coupledmagnetically with a receiver coil (RX-coil) mounted on apower receiving object. Fig. 1 shows the comparison of thepower transmission efficiency for a large TX-coil and manysmall TX-coils. The measured power transmission efficiencyof the large TX-coil is 0.1%, while that of the many smallTX-coils is 60%. The segmentation and selective activation ofTX-coils prevent the efficiency loss. Position detection of theI. INTRODUCTION
A wireless power delivery system to electronic objects
scattered over tables, walls and ceiling will form an
infrastructure necessary for ubiquitous electronics, wirelesssensor network and ambient intelligence. A first step toward
this goal has been reported as a wireless power transmission
sheet [1]. Although the device demonstrated the feasibility ofsuch a system, there were several limitations and drawbacks inpractical use. This paper describes design innovations [2] toovercome the shortcomings of a wireless power transmission
sheet made with plastic MEMS switches and organic FETs
(OFET) circuits suitable for printable low-cost electronics. By
the novel design solutions, the sheet reduces the number of coilarrays, and takes 5V digital input by using newly developed
level shifter with adaptive biasing and organic analog circuits
1 large TX-coil

30cm2 X 1 coil
Efficiency ~ 0.1%

21 cm
21 cm
RX-coil

MEMS switches
for powertransmission (PT)
Organic FETs (OFETs)
for RX position
detection (PD)8 x 8 array (1-inch pitch)

Many small TX-coils(a)RX-coil
1 inch2
TX-coil

TX-coil array

RXSwitchMEMS
OFETs

TXSpeed~ 1Hz> 100Hz

On-resistance< 10Ω> 1kΩ

1 inch2 X 64 coils(b)

Efficiency ~ 60%

Fig. 2. (a) Device structures of wireless power transmission sheet. (b) Speedand on–resistance of MEMS switches and organic FETs. They arecomplementary switches.

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

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TX RXMEMS switch
f1 =3.5 MHz
for position
detection

TX

13.56 MHz 38 LEDs

RXCP(a)13.56 MHzfor powertransmission

C1
to distinguish2 frequencies

RX-coil

VMON
Monitor for PD

Fig. 5. Mixed circuits of MEMS and OFETs for shared coil.

power transmission.
TX-coils was 5 mm.

m
25.4 m

TX-coil array(Without OFETs)

III. SHARED COIL SHEET

5-mm distance
between RXand TX-coils

Fig. 4 shows the comparison of the sheet configuration ofour previous work [1] and this work. The sheet of TX-coilarray for the power transmission and the coil array for theposition detection were separately fabricated and stacked in theprevious implementation. The separate coil sheets stacked
together, however, increase the fabrication cost and reduce theposition detection efficiency because the power transmission
coils sit over the position detection coils. In order to solve theproblems, new circuits are developed to enable a shared coil
sheet for the power transmission and the RX position detection.
Fig. 5 shows the developed mixed circuits of MEMS switches
and OFETs for the shared coil sheet. The circuits use twodifferent frequencies and carefully eliminate interference of thetwo functions: the power transmission and the position
detection. The circuit makes use of the fact that theconductivity of MEMS switches is more than 102 higher thanthat of OFETs. Thus, the OFET doesn’t degrade the powertransmission efficiency by shorting two frequency signal(b)Fig. 3. Wireless power transmission to 38 LEDs. (a) Measurement setup. (b)Demonstration.

receiving object is needed for the selective activation ofTX-coils. The position detection is carried out by scanning
through many TX-coils to check if an RX-coil exists above aTX-coil by means of impedance change. Fig. 2 (a) shows thedevice structures of wireless power transmission sheet. Three
21-cm-square plastic sheets are stacked. Each sheet has 8 x 8array of TX-coil, MEMS switches, and OFETs with 1-inchpitch. Fig. 2 (b) shows the comparison of the speed and theon–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 cantransfer amperes of current but the switching speed is 1 Hz. Incontrast, OFETs are used for the RX position detection because
the switching speed of OFETs is more than 100 Hz but theon–resistance is more than 1 kΩ. Fig. 3 (a) shows themeasurement setup for the wireless power transmission to 38
LEDs. Fig. 3 (b) shows the demonstration of the wireless

6This work

Coil

Coil

MEMS

MEMS

for PT & PD

for PTfor PT

OFET

OFET

3Maximumfrequency
for OFETs

WithRX-coil

200for PD

3.5 5
10Frequency (f1) (MHz)

15Fig. 6. Measured position detection frequency dependence of monitor voltage
with and without RX-coil. 3.5 MHz was used due to the speed limitation ofOFETs.

Fig. 4. Shared coil sheet. In the previous work [1], two coil sheets for thepower transmission (PT) and the RX position detection (PD) are required. In
this work, the developed circuits enabled a shared coil sheet for PT and PD.

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4

1Coilfor PD

Without
RX-coil

5
VMON (V)Previous work

The distance between RX-coil and2

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In

5V
0V

SF

SF

SF

Out

40 V
Enhancement

0V

pMOS with back gate

40 V

40 V

OutOutGain = 2.6InInInb

InInb

InInb

OutOutb

OutOutb

OutOutb-30V-30VBias

Source follower

Diode-connected
loadSingle amp

sources. C1 is added to decrease the impedance at 13.56 MHz(power transmission frequency) to improve the power
efficiency while monitoring the impedance change for 3.5MHz(position detection frequency). CP is added to increase theposition detection sensitivity by peaking the impedance by LCresonance. As shown in Fig. 6, the measured resonant
frequency of VMON for the position detection system is around 8
MHz to 10 MHz, which is beyond the cutoff frequency ofOFETs (3.5 MHz). As seen from Fig. 6, in order to increase thesensitivity for the position detection, the frequency used forposition detection is higher the better. Thus 3.5MHz is chosen
as the frequency for the position detection.

IV. OFET LEVEL SHIFTERSSFSFSF

20 VOutOut

40 V
20 VInOutOutb

CS load(G = 15)30

Triode load
(G = 4.5)

20

Diode load
(G = 0.6)

10
Inb0Fig. 8. OFET level shifters with replica circuits for adaptive biasing to deal
with PVT variations. The lower figure shows the voltage waveforms of eachnode.

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G: Gain @ In=20 V

40Inb, Out, Outb (V)-SF+

In20 V Vadap-VadapSF+SF

Current-source
load (proposed)

circuits in Fig. 7 are not robust against PVT variations. Fig. 8shows the OFET level shifters with replica circuits for adaptive
biasing to deal with the PVT variations. Fig. 8 also shows thevoltage waveforms of each node in the level shifters. It should
be noted that the circuits in Figs. 7 and 8 are pMOS-only design,
because nMOSFETs are not commonly available in the OFET
designs. In the pMOS-only design, it is difficult to obtain thehigh gain in the amplifiers. The high gain differential
amplifiers in Fig. 8 are the key circuits for the adaptive biasing.
Fig. 9 shows three differential amplifiers with different loads tofind the high gain circuit topologies. Organic pMOS FETs withback gates [3] achieve both enhancement mode and depletion
mode operations by changing the back gate bias. Fig. 10 shows
SPICE-simulated gain comparison of differential amplifiers inFig. 9. Current-source load achieved the highest gain, because
the output impedance of the current-source load is the highest.
Figs. 11 (a) and (b) show the fabricated differential amplifierswith 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 μm.
Fig. 11 (c) shows the measured
characteristics of the differential amplifiers. The gain of 6.4 isachieved, which is higher than 2.3 for an organic analog
amplifier reported in [4]. Figs. 12 (a) and (b) show theThe integration of the silicon LSIs, OFETs, and MEMS
devices is necessary in this power transmission system. Direct
digital drive of OFETs and MEMS by silicon LSIs is, however,difficult, because there is large discrepancy in the operation
voltage between LSIs (below 5 V) and OFETs/MEMS (above40 V). Therefore, OFET level-shifters from 5 V to 40 V aredeveloped. Fig. 7 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
Replica 2.5 VTriode loadFig. 9. Three differential amplifiers with different loads. Organic pMOS
FETs with back gates achieve both enhancement mode and depletion mode
operations by changing the back gate bias.

Fig. 7. Original OFET level shifters from 5 V to 40 V. Only pMOS FETs areused.

5V
2.5 V
0V

Identical

40 VInVadap

Original& Depletion

01020
In (V)

3040Fig. 10. Simulated gain comparison of differential amplifiers with differentloads. Current-source load achieved the highest gain.

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40VInOutSF

8000/50-16000/50+InOut20V
Outb

20 V

(a)

4000/50

6.4 mm(a)22.2 mm(b)

4.9 mmOut (V)402.9 mm

Out, Outb (V)Gain = 6.42010(b)403000Outb

10Out

30203040In (V)(c)

20Fig. 12. Adaptive biasing used in the OFET level shifters. (a) Circuit
schematic. (b) Chip micrograph. (c) Measured characteristics.

100

OFETs/MEMS (above 40 V).

01020In (V)3040

ACKNOWLEDGMENT
This work is partially supported by CREST, JST, andMEXT.

(c)
Fig. 11. Differential amplifiers with current-source load.
schematic. (b) Chip micrograph. (c) Measured characteristics.

(a) Circuit

REFERENCES[1]

fabricated adaptive biasing circuits used in the OFET levelshifters. The chip area is 6.4 mm by 22.2 mm. Fig. 12 (c)shows the measured characteristics of the adaptive biasing.
The developed high gain differential amplifier contributes tothe successful feedback control.

[2]

V. CONCLUSION[3]

Design innovations to overcome the shortcomings of awireless power transmission sheet made with plastic MEMS
switches and OFET for printable low-cost electronics areshown. 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 current-source loads
with enhancement/depletion mixed threshold voltages realized
by controlling the back-gate voltage, bridge the operation
voltage gap between silicon VLSIs (below 5 V) and

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[4]

4

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