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Flexible organic static induction transistors using pentacene thin films

Yasuyuki Watanabea

Venture Business Laboratory, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

Kazuhiro KudoFaculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

Received 1 April 2005; accepted 4 October 2005; published online 23 November 2005

Flexible organic static induction transistors OSITs based on pentacene thin films are fabricated onplastic substrates and their static characteristics are measured. The basic transistor characteristics of

the flexible pentacene OSITs were comparable to those of nonflexible pentacene OSITs fabricated

on glass substrates. In addition, variation in the static characteristics was negligible up to a bending

radius of 20 mm. These characteristics suggest the potential for the OSITs as drivers in flexible sheet

displays, such as organic light-emitting transistors in which the OSITs are combined with organic

light-emitting transistors. 2005 American Institute of Physics. DOI: 10.1063/1.2137900

One exciting application for organic transistors, arising

from their flexibility, is their use as drivers for paperlike

electronic displays.1,2

In general, to realize flexible displays,

thin-film transistors TFTs with a lateral configuration such

as amorphous silicon TFTs Refs. 3,4 or organic field effecttransistors OFETs

5have been used with pixel circuits to

drive stable current through to organic light-emitting diodes

OLEDs. However, OFETs have several disadvantages in-cluding low current density, high operational voltage, and

low speed of operation due to their high resistivity and low

carrier mobility.59

To improve the device performance, static induction

transistors SITs, which utilize organic semiconductors,have been employed.

1015The SIT is a promising device be-

cause it enables high-speed and high-power operation.16,17

The excellent characteristics of the SIT are known to be due

to the vertical structure, with a very short distance between

the source, drain, and gate electrodes. Organic SITs OSITsbased on copper phthalocyanine CuPc thin films and A1Schottky gate electrodes have been fabricated and their static

characteristics investigated. The OSITs were confirmed to

operate as typical SITs without saturation.10,11

In comparison

with lateral-type field effect transistors based on CuPc, the

OSITs show higher-frequency and higher-current character-

istics under a relatively low-voltage condition.1214

Thus the

OSITs, with vertical geometry, exhibit superior performance

over the lateral devices above. Based on these results, we

have proposed OSITs based on CuPc thin films for display

devices and have investigated their basis electrical character-

istics. In addition, organic light emitting transistors OLETs,

which combine OSITs with OLEDs, have been fabricatedand their static and dynamic characteristics investigated.18,19

The observed results demonstrate that OLETs, which have an

enhanced high aperture structure due to their vertical struc-

ture, could be used for flexible sheet displays. However, fab-

rication of OSITs and OLET together on a flexible substrate

has yet to be achieved and the electrical properties of such a

device remain uninvestigated.

In this study, OSITs based on pentacene thin films were

fabricated on flexible substrates, such as polyethylene naph-

thalate PEN. By choosing a high-mobility organic semicon-

ductor, such as pentacene, we expected the cutoff frequency

to be suitable for application. Here, we report the fabrication

method and basic characteristics. Figure 1 shows a schematic

diagram and photograph of a fabricated pentacene SIT. The

effective area of the source and drain electrodes of the OSITis approximately 2.25 mm2. The devices are fabricated by

conventional vacuum evaporation with the substrate tem-

perature maintained at room temperature during the vacuum

deposition. The fabrication process is as follows. First, a 100

nm pentacene thin film is deposited on the indium tin oxide

ITO formed on the flexible PEN substrate. Second, a slit-type Al gate electrode with a thickness of 30 nm is formed

on the pentacene film. Third, the Al gate electrode is covered

with a second 100 nm pentacene film. Finally, the drain Au

electrode is fabricated on the pentacene film. The pentacene

films are evaporated under a vacuum of 2104 Pa and the

source temperature of the pentacene and the evaporation rate

are 200 C and 0.1 nm/s, respectively. When fabricating theOSIT, it is important to control the width and parallel align-

ment of the slit-type Al gate electrodes as the characteristics

of the SIT are strongly affected by the very thin Al film. In

our previous reports, the ideal Al gate electrode was consid-

ered to be a mesh type,10,11

which blocks the current flow

from the source to the drain electrodes through the formation

of double Schottky barriers. On the other hand, if there is a

wide gap between the gate electrodes, the current flow is not

controlled effectively.11,14

In fact, we confirmed that the gate

aElectronic mail: watanabe@restaff.chiba-u.jp

FIG. 1. Schematic diagram and photograph of pentacene SIT fabricated on

PEN substrate.

APPLIED PHYSICS LETTERS 87, 223505 2005

0003-6951/2005/8722 /223505/3/$22.50 2005 American Institute of Physic87, 223505-1Downloaded 19 May 2011 to 115.249.41.221. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissio

http://dx.doi.org/10.1063/1.2137900http://dx.doi.org/10.1063/1.2137900http://dx.doi.org/10.1063/1.2137900http://dx.doi.org/10.1063/1.2137900

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could not control the current flow for a gap region of 20 m.

In the present experiments, a slit-type Al gate was used and

the gap between the gate electrodes was set to be approxi-

mately 1 m by the shadow evaporation method as shown in

Fig. 2a. Both the line L and space S of the shadowevaporation mask are 20 m as shown in Fig. 2b. Thedimensions and geometry of the evaporated Al electrode are

controlled by adjusting the size X of the evaporation source,

the distance between the evaporation source and the shadow

mask Y, and the spacer thickness d. The gap region formedwas confirmed to be narrower than that of the estimated gap

value of 1 m and seemed to be consistent with a semitrans-

parent film having continuous and discontinuous parts as re-

ported previously.10,11

However, the edge features of the

evaporated Al electrodes cannot be distinguished clearly in

the scanning electron microscopy SEM images, as shownin Fig. 2c. If the gap region formed was 1 m wide, asestimated here, the depletion region must be 500 nm wide to

control the carrier flow from the source and the drain elec-

trodes by the gate voltage VG applied to the Al Schottky

gate electrode. However, other authors have reported the

width of the depletion layers of pentacene to be 5 nm Ref.

20 or 21.6 nm,21 up to two orders of magnitude smaller thanthe value for the gate region in the present research. From

these reports, in fact, the gap region at the Al semitransparent

film seemed to be approximately 50 nm. As mentioned

above, the edge features of the gate electrodes are not clear at

the present stage.

We monitored the electrical properties using a semicon-

ductor parameter analyzer 4156C, Agilent. All electricalmeasurements were performed in air at room temperature.

The measurements were carried out in the dark in order to

obtain the OSIT characteristics without the photovoltaic ef-

fect of the Schottky barrier contact. Before measuring the

static characteristics of the OSIT, the current-voltage I-V

characteristics between the source and drain electrodes weremeasured under the floating gate condition. In the resulting

I-V curve, some asymmetry was exhibited. When a drain

electrode voltage of 3 V was applied, the current was

0.5 A. However, when a drain electrode voltage of 3 V

was applied, the absolute value of the current was lower, at

0.1 A. This indicates that the ITO injects holes more effec-

tively than Au. This can be explained as follows. The highest

occupied molecular orbital HOMO, which represents thehole transport level, of pentacene is about 5.0 eV as deter-

mined from ultraviolet photoemission spectroscopy

measurements.22

The measured work function of ITO varies

from 4.3 to 4.8 eV depending on the surface condition.23,24

Based on these results, the pentacene/ITO contact is consid-

ered to be capable of injecting holes efficiently due to a small

difference between the work function of ITO and the HOMO

of pentacene. The relatively low-energy barrier at the

pentacene/ITO interface leads to the formation of a hole in-

jection barrier. On the other hand, the Au on the surface of

the pentacene has a work function of 4.3 eV, more than 1 eV

lower than that of pure Au.25

The difference in the work

functions between pentacene on ITO and Au on pentacene

results in asymmetric I-V characteristics. The differences in

I-V characteristics between the gate and source electrodes,and the gate and drain electrodes are measured in order to

confirm the formation of a Schottky barrier near the Al gate

electrode. The resulting I-V curves exhibit a rectification

property, with a forward bias corresponding to a negative

voltage applied to the gate Al electrode. The experimental

results are in good agreement with the results reported by the

other author;7

that is, the pentacene films show p-type semi-

conducting properties, forming a Schottky barrier contact

with the Al gate electrode. In addition, the avalanche break-

down point was not seen until an applied gate voltage of

+5 V. We also measured static characteristics, such as the

source-drain current IDS as a function of the source-drain

voltage VDS. To operate the OSITs, it is important that thevoltage range applied to each electrode does not adversely

affect the Schottky barrier around the gate electrode. In an

experiment, VDS was changed continuously from 0 V to

3 V while the gate voltage VGS was changed from 0.9 V

to +0.9 V in +0.2 V steps. The static characteristics of the

pentacene SIT, having the previously described structure of

Au/pentacene/Al/pentacene/ITO/PEN substrate, are shown

in Fig. 3a. The slope of the IDS-VDS curves increases withincreasing VDS without current saturation and the IDS values

are similar to those of typical inorganic SITs.16,17

There is an

exponential increase in IDS with VGS at a constant VDS. In

measurement of the leak gate current IG, it was found that IG

increases with increasing VDS. However, the IG observedhere is about two magnitudes lower than IDS. Thus, at a

FIG. 2. Schematic ofa shadow evaporation method, b evaporation mask,

and c SEM image of the formed Al gate electrode.

FIG. 3. Static SIT characteristics of pentacene SIT fabricated on a PEN

substrate and b glass substrate.

223505-2 Y. Watanabe and K. Kudo Appl. Phys. Lett. 87, 223505 2005

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constant VDS, IDS increases with decreasing VGS decreasing

from 0 V to 0.9 V, and decreases with VGS increasing from

0 V to +0.9 V. In our previous reports,12

these phenomena

were explained as transport of the injected hole carriers from

the source flow toward the drain region through the saddle

point of the potential barrier. The current flow of the OSIT is

controlled by the potential barrier height and depends on the

Al gate voltage. For comparison, a pentacene SIT was fabri-

cated on an ITO/glass substrate. The characteristics of the

flexible pentacene SIT were comparable to those of a non-

flexible pentacene SIT as shown in Fig. 3b. Figure 4 showsthe IDS-VG curve at a VDS of 3 V for the pentacene SIT on a

PEN substrate. The mutual conductance gm=dIDS/dVG was

also obtained from the IDS-VG curve. It was found that the

absolute value of IDS decreases with increasing VG and gmwas calculated as 1.8106 S from the slope of the IDSVGcurve in the range of 10.6 V. In the present study, both

the on/off ratio and the low gm of the pentacene SIT seemed

to be low for driving OLETs by combining the OSITs with

OLEDs. However, in our previous study, it was confirmed

that the luminescence of OLETs could be controlled by gate

voltages as low as 1 V and dynamic characteristics wereobtained at 60 Hz Refs. 18,19 using OSITs with compa-rable characteristics to the present pentacene SIT. In addi-

tion, we confirmed in the present research that the cutoff

frequency of the pentacene SIT fabricated on glass was ap-

proximately 7 kHz. This value is higher than that observed

for a CuPc SIT fabricated on a glass substrate.12

In addition,

there is negligible variation in the SIT characteristics even

when bending the flexible SIT to a radius of 20 mm.

In this letter, we have presented the characteristics of an

OSIT fabricated on a flexible substrate. The static character-

istics suggest that the OSIT has potential in driving OLEDs,

although currently the on/off ratio and the mutual conduc-

tance are too low for practical application. The dependence

of the OSIT characteristics on the device structure, gate ge-

ometry, and energy band condition at the interface of the

pentacene/ITO require further investigation. Research is cur-

rently progressing toward this end.

The authors are grateful to Dr. Masakazu Nakamura, Dr.

Masaaki Iizuka, and Hiroyuki Iechi for many helpful discus-

sions. This work was partially supported by the 21st CenturyCenter of Excellence Program.

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FIG. 4. IDS VG curves of pentacene SIT fabricated on PEN substrate. VDwas 3 V.

223505-3 Y. Watanabe and K. Kudo Appl. Phys. Lett. 87, 223505 2005

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