Invited Paper

47-1 / A.K.M. M. Islam

Sensor and Circuit Solutions for Organic Flexible
A.K.M. Mahfuzul Islam1,2, Hiroshi Fuketa1,3, Koichi Ishida1,4, Tomoyuki Yokota1,2,
Tsuyoshi Sekitani1,2,5, Makoto Takamiya1,2, Takao Someya1,2, and Takayasu Sakurai1,2
1

University of Tokyo, Tokyo, Japan
2
JST/ERATO, Tokyo, Japan3
Now with National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan4
Now with Dresden University of Technology, Dresden, Germany5
Now with Osaka University, Osaka, Japan

Abstract

8 x 2 amplifier array

2V organic
PMOS circuits

8 x 8 EMG electrode array

Stacked 2 sheets

EMG electrodes(0.7mm x 0.7mm)

Piezoelectric energy harvester(PVDF sheet)

Organic transistors

1m-thickultra-flexible
PEN film
40mm

Electrode pitch = 5mm(a) An insole pedometer sheet with (b) An electromyogram
piezoelectric energy harvester [3].
measurement sheet withintegrated amplifiers [4].
Patient

Introduction

Underarm temperatureis measured.

Organic transistors can be fabricated on a plastic or a glass sheet,and because of the material properties such transistors are not as
rigid as the silicon transistors. Thus, flexible, large- area, low-cost
and transparent electronic devices become possible. However,
because the materials used for organic transistors are inherently
different from silicon, newer circuit solutions need to beestablished to realize smart sensors with various functions
integrated [1, 2].
In Sec. 2, we present several examples of sensor solutions
developed in our group. Then, we discuss some digital and analog
circuit solutions to realize the sensors in Secs. 3 and 4. Finally,
Sec. 5 concludes the paper.

Sensor Solutions

As the key feature of organic electronics is its flexibility, they are
most suitable for sensor devices that require flexibility. Fig. 1
shows four such sensor devices to show the capability andversatility of organic electronics.

53 mm

Diaper40 mm

Temperature detector(15mm x 5mm)
30 cm

78 mm

With the emergence of IoT (Internet of Things) era, more andmore physical objects are being connected to the internet
providing information of our surrounding to enable a more secureand comfortable life. The gap between physical world and cyberworld is being narrowed continuously. Newer devices are being
investigated that can provide tighter integration between the twoworlds. In order to gather information from the physical world,
various forms of devices are required including devices capable ofsensing large area. Especially for health-care and bio-medical
devices, flexible and large area devices that can be attached tohuman skin for example will provide greater options. Organic
transistors can be an attractive option to fulfil the above needs.

2.

Surface electromyogram measurement sheet (SEMS)

Organic
circuits

12.5m-thickflexible PCB(polyimide)(under speaker)

Piezoelectric
All flexible
components
speaker(10cm x 5cm)

Solar cells

18 cm

1.

Prosthetic hand

45 mmThis paper describes several sensor and circuit solutions fororganic flexible electronic devices. Organic field effect transistors(OFET) enable low-cost, high-flexibility and large-area which
can be utilized to implement smart sensors. These sensors can bein contact with the physical objects of any shape. Furthermore, as
the devices are very light and flexible, they can even be placed incontact with the human skins. However, design of organic flexible
circuits poses great challenges because of several intrinsic
properties of organic materials. Thus, newer circuit techniques
need to be adopted for robust operation. We give an overview ofvarious sensor and circuit techniques developed in our researchgroup along with other literature reports.

Organic circuits
Sensor electrodes(11cm x 13cm)

Fever alarm armband(d) Wireless wet sensor sheet with(c) Fever alarm sheet with energy wireless power and data
harvesting from solar cell sheet [5]. transmission capability [6].

Figure 1. Various sensor applications developed in our group.
Fig. 1(a) shows an example of energy harvesting device realized
on organic electronics [3]. This is an insole pedometer sheet thatis self-powered with energy harvesting capability. The device usesa polyvinylidene difluoride (PVDF) sheet as piezoelectric energy
harvester. Then all-pMOS organic circuits are used to utilize theharvested energy and count the stepping. Pseudo-CMOS circuit
techniques are used to reshaping the pulses generated from theenergy harvester and to implement the counters.
Fig. 1(b) shows an electromyogram measurement sheet which canbe used for prosthetic hand control [4]. The key feature is toamplify the small signals from the electrodes. Details of theamplifier techniques will be discussed in Sec. 4.
Fig. 1(c) shows a flexible fever alarm sheet with energy
harvesting capability from a flexible solar cell sheet for medical
application [5]. The device features an active voltage limitercircuit to protect the device from excess voltage levels. The
device also features a piezoelectric speaker sheet for alarming. As
the whole system consists of flexible sheets, the device can beused as an armband so that the need for constant monitoring offever manually is eliminated.

ISSN 0097-966X/16/4702-0629-$1.00 © 2016 SID

SID 2016 DIGEST • 629

47-1 / A.K.M. M. Islam

20

IN

OUT

Digittal Circuit Solutions
S
Prob
blems

Lower mobility and staability for nMOS
S transistor,

2)

Limited number of inteerconnect layers, and3)gher variation.
Low reeliability and hig

nnMOS transistorrs have much lower mobility than the pMO
OS
ccounterpart. Thiss poses a great challengecin dessigning robust anndeenergy-efficientcomplementaary
MOS(CMOS)
log
gic
immplementation. Although it iss possible to reealize full CMO
OS
ccircuits using org
ganic materials, researchers hav
ve also looked forfaalternative solutions for higher gain and noise immmunity.

33.2

IN
OUT

Gain=
=148

15
10

5

5

0V

VSS(-20V) GND(0V)

0

5

10

15

20

VIN[V](a) VOUT-V- IN of conventional CM
MOS inverter

0

5

10

15

20

156Hz

VOUT
8pF

VOUT

7.1V

4.7mm

10M

0

VIN[V](b) VOUT-VIN of Pseudo-CM
MOS inverter

VDD=2
20V

T
The main reasonn for the tremeendous success of silicon CMO
OSteechnology is thee ease of integrrating large num
mber of devices torealize digital ciircuits that enab
ble robust, low
w power and hig
ghsspeed computing
g. However, because of the inheerent properties ofthhe organic matterials [7], orgaanic devices faace the followinngcchallenges.
1)

Gain=3.2

10

0

33.
33.1

20

VDD (20V)(

15

VOUT[V]

20V

VOUT[V]F
Finally, Fig. 1(d)) shows a flexib
ble wet sensor shheet with wireleessppower and data transmissiontcappability [6]. Thee device featuress a
ccoil for power annd data transmission, and organiic Schottky dioddesffor rectifying an
nd ESD protecttion. Using orgaanic diodes, ESSD
toolerance of more than 2 kV is achievedamakingg the device robu
ustw
when in contact withw human skinn.

Invited Paper

11mm(c) Meaasured circuit, photogrraph, and waveform off Pseudo-CMOS ring oscillatoroFFigure 3. Convenntional and pseeudo-CMOS invverter
schematicss and their DC ccharacteristics.
which is sufficient to drive transistorr gates. Flip-fllop is a key
buildinng block for diigital circuits. Successful opeeration of afrequenncy divider usinng pseudo-CMO
OS based flip-floop has been

Pseudo-CMOS Circuit
C

B
Because of the lowlmobility off nMOS transisttors, CMOS log
gic
ggates have low gain.
g
In order to improve the gaiin, pseudo-CMO
OSloogic topology has
h been demonsstrated that improves the gain bybmmultiple times [8–11]. Fig. 2 shows a geneeral schematic ofppseudo-CMOS logic circuit usinng either pMOS
S- or nMOS-onnly
trransistors [8]. Fig.
F 3 shows a pseudo-CMOSpinverterischemattic
aalong with a conventional CMOS
C
inverterr schematic. DCDccharacteristics of the two inverrter types are alsoashown in thheffigure. For the conventional CMOS
C
topology, the gain of thheinnverter is only 3.2.
3 Whereas, thhe pseudo-CMOS
S version showss aggain of as high as
a 148 [11]. Fig. 3(c) shows the layout and outp
putw
wave of a threee-stage ring osccillator consistin
ng of the pseud
doC
CMOS inverters of Fig. 3(b). Th
he output frequeency is about 15
56
H
Hz which could be enough for realizingr
simple logics to providdeinntelligence intoo devices. The pseudo-CMOS
S logic topologgyrequires a negattive supply volttage which may
y pose additionnal
ddifficulty. An on
n-sheet negative voltage generattor is proposed in[3]. Fig. 4 show
ws the schematicc and measurem
ment results of thhennegative voltage generator. In Fig. 4(b), a negativ
ve voltage of −1
1.6
V is generated fromfr
a 10 Hz clo
ock with 2 V vo
oltage swing. Fiig.
44(c) shows the output voltage against output load current. Th
hevvoltage generatoor has a load currentccapabilitty of several μAμ

Figure 2. Ex
xample of pseud
do-CMOS logicc gates [8].

630 • SID 2016 DIGEST

Figuree 4. A pseudo-C
CMOS based neegative voltage generator
for biasingg pseudo-CMOS
S logic cells [3].
demonsstrated in [11]. Because of the low conductivitty of pMOS
transisttor switches, bboosted gate vooltage techniquee has been
appliedd for robust operration. Fig. 5(a) shows the schemmatic of theflip-floop where level -shifted clock buffer is usedd for gateboostinng. The level shhift has been achhieved by applyiing −1 V as
the low
wer power rail inn the pseudo-CM
MOS inverter toppology. Fig.
5(b) annd Fig. 5(c) show
w the divided waves without annd with gateboostinng respectively. Without gate--boosting, retenntion occurs
becausee of the low coonductivity of thhe switches. Im
mproving the
conducctivity of switcches by gate-bboosting, data rretention ispreventted.

4.

Analog C
Circuit Soluttions

4.1

Passive C
Componentss

Passivee components suuch as resistor, capacitor and iinductor are
key coomponents for analog circuits.. Compared to the siliconcounterrpart, organic devices havee limitations iin material
resourcces. Area-efficieent resistors are difficult to reaalize, but anInvited Paper

47-1 / A.K.M. M. Islam

Q1

VDD

VDD

Power Tr.

VDD

0.4

ID

IN
VSS

VSS

ID (mA)

0.3
V2

V1

0.1

VSS

0

(a)

AVL (Proposed)
1Hz

1Hz

VSS=0V

+2V

IN

+1V

0

5

10

VDD (V)

15

Figure 7. Active voltage limiter acting as a diode [5].

3V

R1

VSS=-1V(47nF)V DD
Q1

Diode clamp(Conv.)

0.2

Level-shifted clock buffer

IN

ID

+1V

Q1

IN

C2
M1

M1
R2

OUT

VDD

(47nF)

(b)

(c)

Load

C1

Figure 5. A flip-flop topology using pMOS pass-gates withgate-boosting and level-shifted clock buffers [11].

Diode load
(Conv.)(a)30

Measured
VDD=2V

Gain (dB)20

AC coupled load
(This work)

10

Diode load (Conv.)

0-10

Figure 6. IDS–VDS characteristics of an DNTT organic pMOS
transistor with channel length and width of 50 μm and 1600 μm
respectively. (a) Large VDS, (b) Small VDS.
workaround is to use the transistors in the triode region as
resistors [4]. Fig. 6 shows IDS–VDS characteristics of pMOS
transistor under large and small VDS regions. Fig. 6(b) shows that
pMOS transistor can be used as resistors under small VDS region.
Although this workaround has some limitations such as the
reduced operating range, high resistance values can be realized
because of the intrinsically lower mobility.
Inductors are essential components especially for wirelesscommunication. Because of low material properties, on-sheet
inductors are not available. As an alternative, flexible printed
circuit board (PCB) can be used where the PCB sheet and theorganic sheet are stacked together to realize flexible devices. A
successful operation of wireless power and data transmission
using stacked inductor sheet is demonstrated in [4]. On-sheet
capacitors can be realized by MIM capacitors. MIM capacitor
with capacitance of 700 nF/cm2 is reported in [10].

4.2Floating Voltage Regulation

Energy harvesting devices such as a solar cell may generate
excessive power which may damage the large organic devices.
Thus, voltage regulation is required for devices with energy
harvesting capability. Conventionally, a diode clamp is used tolimit the voltage level. However, because of the material
properties, voltage references and Zener diodes are not available.
As an alternate solution, a transistor-based active voltage limitercircuit is proposed for voltage regulation [5]. Fig. 7 shows theschematic of the proposed voltage limiter. Here, the first fourAC-coupled load
(This work)

1

10

100

1k

10k

Frequency (Hz)(b)

Figure 8. AC coupled load for amplifier gain boosting [4].
pMOS transistors from the left realizes a constant voltage
generator. Followed by two CMOS inverters and a power pMOS
transistor, the circuit acts as a diode but with higher current
sensitivity. The right hand figure shows the I–V characteristics ofthe active voltage limiter along with the conventional diodeconnected pMOS transistor. The active voltage limiter showshigher sensitivity than the conventional diode clamp.

4.3

Amplifier with AC Coupled Load

Amplifier is a basic circuit block especially for smart sensor
devices as the sensed signals are often small analog signals.
Conventional diode connected load does not give the required
gain for OFET based amplifiers. In order to improve amplifier’s
gain and performance, an AC-coupled load technique is proposed[4]. Fig. 8(a) shows an amplifier schematic, and Fig. 8(b) shows
the gain of the amplifier with two types of load. One is the diodeconnected pMOS load and the other is the proposed AC-coupled
load. As explained in [4], amplifier with AC-coupled load shows
larger gain which is demonstrated in the figure. However, ACcoupled load has a disadvantage that the gain decreases at low
frequency region. The lower cut-off frequency is determined by
R2C2, thus their values need to be set to meet application
requirements.

4.4

pMOS-only Rectifier

For wireless power and data transmission transponders, rectifying
circuits are required. Fig. 9 shows a pMOS-only rectifier circuit[11]. Cross-coupled topology is used here instead of conventional
diode-connected topology to improve the driving current. Thus,

SID 2016 DIGEST • 631

47-1 / A.K.M. M. Islam
bby adopting circu
uit topology acccordingly, perforrmances of analoog
ccircuits can be inncreased.

Figuree 9. pMOS-only rectifier topolo
ogy [11].

44.5

Otheers

B
Besides the abovve discussed anaalog design solu
utions, researcheers
hhave developed various analog
g circuit blocks. For example, accurrent steering basedbdigital-to--analog converteer is developed in[12]. A differen
ntial amplifier isi proposed in [13]. With mooreuunderstanding off the organic tran
nsistors and withh the improvemeent
inn their performaances, more inn
novative analog circuit blocks areaeexpected to be deeveloped.

55.

ConcclusionW
We have revieweed various senso
or solutions for flexible
flelectroniics
uusing organic fieeld effect transiistors. Various circuit techniquuesw
were discussed tot realize the sensors. Because of the differencces
inn organic materiial properties thaan their silicon counterpart,c
digittalaand analog circuit techniques need to adopt accoordingly. We havvereviewed the diigital circuit soolutions that usse pseudo-CMOOS
wn various analo
og techniques thhatteechniques. We have also show
ccan be used to im
mprove gain and other circuit characteristics. Wiiththhe help of suchh circuit techniq
ques, robust opeeration of flexib
bleeelectronics can be achieved whhich can make way for a wid
devvariety of sensor applications in thet IoT era.

66.

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