Angle speeds plastic transistor
Technology Research News
Plastic computer chips have recently received
a lot of attention because they promise to imbue everyday objects with
inexpensive electronic intelligence and enable flexible displays and electronic
Though they are flexible and potentially very inexpensive, organic
electronic devices perform relatively poorly. This is because organic
materials have low charge carrier mobility, which is a measure of how
readily electricity -- or negatively-charged electrons and positively-charged
holes -- moves through the material.
Researchers from Lucent Technologies' Bell Laboratories, Rutgers
University and the University of Illinois have found that the orientation
of crystalline organic semiconductors plays a big role in organic transistor
performance. The researchers have developed a simple lamination manufacturing
process for making transistors from the fragile organic material, and
the resulting transistors have set a record for carrier mobility in organic
The researchers' method could lead to mass production techniques
for organic transistors and light-emitting diodes.
The researchers' field-effect transistor is formed from organic
rubrene crystal and titanium and gold electrodes and has a carrier mobility
of 15.4 square centimeters per volt second, compared to typical organic
semiconductor carrier mobilities of less than one square centimeter per
volt second, according to John Rogers, a professor of materials science
and engineering at the University of Illinois.
The silicon transistors commonly used in today's computer chips
have carrier mobilities of 1,500 square centimeters per volt second, and
other inorganic crystalline semiconductors can have carrier mobilities
an order of magnitude higher than silicon.
The orientation of the molecules within the organic crystal and
the spacing between the molecules contribute to the prototype's relatively
high carrier mobility, said Rogers. Crystal molecules in the prototype
transistor are spaced 1.44 nanometers apart in one direction and 0.72
nanometers in the perpendicular direction. Carrier mobility dropped to
4.4 square centimeters per volt second when the wide spacing of the crystal
was aligned with the electrodes. A nanometer is one millionth of a millimeter.
The rubrene molecule has groups of atoms attached to its sides,
and electrons flow along these side groups and along the backbone of the
molecule. In the high-mobility orientation, the molecules' side groups
are aligned, facilitating electron flow from molecule to molecule. "The
orientation of the molecules relative to the electrodes of the transistors
has a profound impact on the way [the] devices behave," said Rogers.
To test the relationship between orientation and performance in
the organic crystal, the researchers developed a method of making field
effect transistors that allowed them to repeatedly place, remove, rotate
and replace the relatively fragile crystal on the transistor's electrodes.
"We build all components of the transistor -- source/drain electrodes,
gate dielectric, and gate electrode -- out of soft, conformable materials
built on a soft, elastomeric substrate," said Rogers. "We then, at room
temperature and without applied pressure, gently place the organic crystal,
which is grown in a separate process... on the surface of this transistor
To make the transistors, the researchers placed a titanium-gold
gate electrode on a silicone rubber surface, covered it with a thin film
of silicone rubber and placed titanium-gold source and drain electrodes
on top. They then simply placed the organic crystal over the electrodes
and gently pressed one edge of the crystal. This caused the crystal to
adhere to the silicone and metal due to the van der Waals force, which
is the electrostatic attraction between atoms and molecules. "The soft
contact forms very high-performance transistors in a way that avoids all
of the hazards that conventional semiconductor processing poses to the
organics," said Rogers.
The lamination method could be used in practical applications
in three to five years, said Rogers.
Rogers' research colleagues were Bell Laboratories researchers
Vikram Sundar, now at IBM, Jana Zaumseil, now at the University of Cambridge,
Robert Willett, and Takao Someya, now at the University of Tokyo; Vitaly
Podzorov and Michael Gershenson of Rutgers University; and Etienne Menard
of the University of Illinois.
They published the research in the March 12, 2004 issue of Science.
The research was funded by the National Science Foundation (NSF) and the
U.S. Department of Energy.
Timeline: 3-5 years
TRN Categories: Integrated Circuits; Materials Science
Story Type: News
Related Elements: Technical paper, "Elastomeric Transistor
Stamps: Reversible Probing of Charge Transport in Organic Crystals," Science,
March 12, 2004
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