Download Analog and Digital Circuits Using Organic Thin-Film Transistors on Polyester Substrates

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IEEE ELECTRON DEVICE LETTERS, VOL. 21, NO. 11, NOVEMBER 2000
Analog and Digital Circuits Using Organic Thin-Film
Transistors on Polyester Substrates
M. G. Kane, J. Campi, M. S. Hammond, F. P. Cuomo, B. Greening, C. D. Sheraw, J. A. Nichols, D. J. Gundlach,
J. R. Huang, C. C. Kuo, L. Jia, H. Klauk, and T. N. Jackson
Abstract—We have fabricated and characterized analog and
digital circuits using organic thin-film transistors on polyester film
substrates. These are the first reported dynamic results for organic
circuits fabricated on polyester substrates. The high-performance
pentacene transistors yield circuits with the highest reported clock
frequencies for organic circuits.
Index Terms—Organic electronics, organic semiconductors,
plastic substrates, thin-film circuits, thin-film transistors.
I. INTRODUCTION
O
RGANIC thin-film transistors (OTFTs) are gaining attention as a technology that enables electronic circuits and
displays to be fabricated using low-cost processing on plastic
substrates. The entire fabrication process can be performed at
temperatures of about 100 C or less, allowing great freedom
in the choice of substrate materials. Plastic substrates, in particular, allow circuits to be fabricated directly onto smart cards
and inventory tags, and open up the possibility of rapid, highvolume web processing. Rugged, flexible displays can also be
built using an OTFT active-matrix on plastic, with liquid crystal
or organic light-emitting diodes (OLEDs) as the electro-optic
element.
OTFT circuits have been reported on silicon substrates [1],
[2], on glass [3], [4], and on polyimide, a high-temperature engineering polymer [5]. Functional circuits containing as many as
864 transistors have been built [6]. Single “smart pixels” have
also been fabricated by combining an OTFT with an OLED
[7], [8]. The first organic circuits on low-cost polyester film
were recently reported, consisting of single two-transistor inverters and smart pixels [9]. Here we report analog and digital organic circuits with complexities up to 48 transistors fabricated using pentacene OTFTs on polyester film. We present the
first reported dynamic results for organic circuits fabricated on
polyester film. The high performance of our organic transistors
yields circuits with the shortest gate-delay reported for organic
circuits on plastic substrates, and the highest clock frequencies
reported to date for any organic circuits.
Manuscript received May 1, 2000. This work was supported by the Defense
Advanced Research Projects Agency under Contract N61331-98-C-0021. The
review of this letter was arranged by Editor T.-J. King.
M. G. Kane, J. Campi, M. S. Hammond, F. P. Cuomo, and B. Greening are
with the Solid State Display Laboratory, Sarnoff Corporation, Princeton, NJ
08543 USA (e-mail: [email protected]).
C. D. Sheraw, J. A. Nichols, D. J. Gundlach, J. R. Huang, C. C. Kuo, L. Jia,
H. Klauk, and T. N. Jackson are with the Electronic Materials and Processing
Research Laboratory, Pennsylvania State University, University Park, PA 16802
USA.
Publisher Item Identifier S 0741-3106(00)09276-4.
II. ORGANIC TFT STRUCTURE AND FABRICATION PROCESS
The substrate material was a 75 m-thick transparent, flexible
polyethylene naphthalate (PEN) film mounted on a removable
glass support for ease of processing. After an initial 150 C,
2 h heat treatment to improve thermal dimensional stability,
the maximum processing temperature was 110 C (photoresist
bake). Nickel gate metallization, a 190 nm SiO gate dielectric,
and palladium source-drain metallization were deposited and
patterned. The surface of the gate dielectric was vapor-treated
with octyldecyltrichlorosilane, followed by thermal evaporation
of the active pentacene layer at a substrate temperature of about
60 C. Further details of the pentacene OTFT process can be
found in [10].
To pattern and passivate the pentacene layer without exposure to common solvents and resist developers, a water-based
solution of polyvinyl alcohol (PVA) with ammonium dichromate photosensitizer was spun onto the substrate, UV-exposed
through a chrome mask, and developed in water. Pentacene outside the active TFT regions, unprotected by the patterned PVA,
was removed in an oxygen plasma. Because our OTFTs are typically depletion-mode, unless the pentacene islands are isolated
there is excessive off-state leakage through the field regions;
the only alternative is to use a geometry in which the source
surrounds the drain, but this requires a third level of metallization in order to contact the drain. The PVA process isolates the
OTFTs effectively without significantly modifying device characteristics. The passivation layer also permits different types of
organic devices to be integrated on one substrate, since it can
protect lower organic layers from the deposition and patterning
of upper layers. In addition, passivation will be important for
liquid-crystal display applications, because we have found that
pentacene OTFTs are degraded when they come into contact
with typical liquid-crystal materials.
Each substrate includes a 1 cm array of 200 transistors with
25 m and
20 m. Typical drain-current characteristics of a passivated device are shown in Fig. 1. A high degree of device uniformity is possible. On one array, the average
threshold voltage was +3.2 V and the average field-effect mobility was 0.45 cm /V-s, with a standard deviation of 0.2 V for
threshold voltage and 0.03 cm /V-s for mobility.
III. ORGANIC CIRCUITS
Several types of analog and digital circuits were fabricated
and characterized. All the circuits used transistors with
10 m. Yields were acceptable, with 65% yield, for example,
on 48-transistor divide-by-two frequency dividers. Because the
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KANE et al.: ORGANIC THIN-FILM TRANSISTORS ON POLYESTER SUBSTRATES
535
Fig. 1. Drain-current characteristics of a typical passivated pentacene OTFT
on a flexible polyethylene naphthalate (PEN) substrate. The device is a
depletion-mode p-channel transistor, with a threshold voltage of +6 V and a
field-effect mobility of 0.6 cm /Vs.
Fig. 3. Transfer characteristics of an organic differential amplfiier. The gate of
the tail current source was brought out to an external pad for adjusting the bias
is plotted
current. The voltage of one of the differentially driven inputs V
on the horizontal axis, and the voltage of the two outputs V
and V
are plotted on the vertical axes. The differential voltage gain in the region of
maximum gain is 8.5. The inset shows the schematic of the amplifier. Supply
V and V
V
V.
voltages are V
0
= 020
Fig. 2. Logic diagram and operation of the organic divide-by-two frequency
divider. The top trace shows one of the two nonoverlapping complementary
clock input signals, with a frequency of 1.1 kHz. The bottom trace shows the
two complementary output signals. (Vertical scales are 2 V/div.).
transistors were depletion-mode, in addition to an input stage
each logic gate included a two-transistor level-shifting output
stage using a source-follower biased by a current source. We
measured a maximum oscillation frequency of 1.7 kHz on fivestage ring oscillators operated at 20 V, corresponding to a 59 s
gate delay. This is the shortest gate-delay reported for organic
circuits on plastic substrates.
Fig. 2 shows the logic diagram and operation of the divide-by-two frequency divider. This circuit provides a more
realistic assessment of speed than ring oscillators, since proper
functionality requires large internal voltage swings that approach saturated binary logic levels. At a supply voltage of
25 V the circuit operated at 1.1 kHz, the highest operating
frequency reported to date for clocked organic circuits.
Fig. 3 shows results from an organic differential amplifier
consisting of a source-coupled pair with depletion loads, biased by a current source. Transfer characteristics are shown for
=
= +10
both outputs with differential input drive. Both sweep directions are shown in order to note that the amplifier typically exhibits some hysteresis, possibly due to slow trapping states at
the pentacene-SiO interface. By averaging the characteristics
for the two sweep directions, the input offset voltage is found
to be 0.6 V. In the high-gain region the differential-mode gain
is 8.5. On other amplifiers the
input offset ranged from 1 to 1 V, and the differential gain
from 5 to 10.
The low voltage gain of our OTFT amplifiers results from
to drain conductance
a small ratio of transconductance
in the saturation region (Fig. 1). The differential gain could be
increased by adding cascode transistors to raise the output impedances of the gain stage and the current source, at the expense
of supply voltage. Low gain also affects the logic gates. Our inverters typically have an input-stage voltage gain of about 3
in the region of highest gain. The unity-gain level-shifters have
a gain of about 0.75, leading to an overall inverter gain of only
about 2.25. The low gain reduces noise margins, especially
in gates with multiple inputs. In logic gates cascoding is impractical because it significantly increases transistor count and
power dissipation, and a better solution may be to use complementary n- and p-channel transistors [11]. Alternatively, new
materials and processes may be required.
IV. CONCLUSION
Our results demonstrate the usefulness of OTFT technology
for fabricating analog and digital circuits on low-cost polyester
substrates. Ring oscillators yield gate delays under 60 s, the
shortest gate-delay reported for organic circuits on plastic substrates. Clocked circuits function at frequencies above 1 kHz,
the highest operating frequency reported to date for clocked organic circuits. Future work is needed to increase the ratio of
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IEEE ELECTRON DEVICE LETTERS, VOL. 21, NO. 11, NOVEMBER 2000
transconductance
to drain conductance
in order to improve the voltage gain of analog circuits, and the noise margin
of digital logic.
REFERENCES
[1] A. R. Brown, A. Pomp, C. M. Hart, and D. M. de Leeuw, “Logic gates
made from polymer transistors and their use in ring oscillators,” Science,
vol. 270, pp. 972–974, 1995.
[2] A. Dodabalapur, J. Laquindanum, H. E. Katz, and Z. Bao, “Complementary circuits with organic transistors,” Appl. Phys. Lett., vol. 69, pp.
4227–4229, 1996.
[3] M. Bonse, D. B. Thomasson, H. Klauk, D. J. Gundlach, and T. N.
Jackson, “Integrated a-Si:H/pentacene inorganic/organic complementary circuits,” in IEDM Tech. Dig., 1998, pp. 249–252.
[4] H. Klauk, D. J. Gundlach, and T. N. Jackson, “Fast organic thin-film
transistor circuits,” IEEE Electron Device Lett., vol. 20, pp. 289–291,
1999.
[5] C. J. Drury et al., “Low-cost all-polymer integrated circuits,” Appl. Phys.
Lett., vol. 73, pp. 108–110, 1998.
[6] B. Crone et al., “Large-scale complementary integrated circuits based
on organic transistors,” Nature, vol. 403, pp. 521–523, 2000.
[7] H. Sirringhaus, N. Tessler, and R. H. Friend, “Integrated optoelectronic devices based on conjugated polymers,” Science, vol. 280, pp.
1741–1744, 1998.
[8] A. Dodabalapur et al., “Organic smart pixels,” Appl. Phys. Lett., vol. 73,
pp. 142–144, 1998.
[9] J. A. Rogers, Z. Bao, A. Dodabalapur, and A. Makhija, “Organic smart
pixels and complementary inverter circuits formed on plastic substrates
by casting and rubber stamping,” IEEE Electron Device Lett., vol. 21,
pp. 100–103, 2000.
[10] H. Klauk, D. J. Gundlach, J. A. Nichols, and T. N. Jackson, “Pentacene
organic thin-film transistors for circuit and display applications,” IEEE
Trans. Electron Devices, vol. 46, pp. 1258–1263, 1999.
[11] Y.-Y. Lin et al., “Organic complementary ring oscillators,” Appl. Phys.
Lett., vol. 74, pp. 2714–2716, 1999.