Using direct-write electron-beam lithography, low-voltage organic thin-film transistors (TFTs) with channel lengths and the parasitic gate-to-source and gate-to-drain overlaps as small as 100 nm have been fabricated on flexible polymeric substrates. Despite the small channel lengths and gate-to-contact overlaps, these TFTs display good static current-voltage characteristics, including on/off current ratios of nine orders of magnitude, subthreshold swings of about 100 mV/decade, turn-on voltages of 0 V, negligibly small threshold-voltage roll-off, and contact resistances below 1 kOhm-cm.
Using direct-write electron-beam lithography, low-voltage organic thin-film transistors (TFTs) with channel lengths and parasitic gate-to-source and gate-to-drain overlaps as small as 100 nm have been fabricated on flexible polymeric substrates. Despite the small channel lengths and gate-to-contact overlaps, these TFTs display good static current-voltage characteristics, including on/off current ratios of nine orders of magnitude, subthreshold swings of about 100 mV/decade, turn-on voltages of 0 V, negligibly small threshold-voltage roll-off, and contact resistances below 1 kOhm-cm. TFTs with such small critical dimensions are of interest for high-frequency applications.
Using Kelvin Probe Force Microscopy (KPFM), we performed surface-potential measurements on operating organic thin-film transistors (TFTs). Several parameters inaccessible through current-voltage measurements were determined, namely the source and drain resistances separately, the threshold voltage and the electric field along the channel. We show that the source resistance is always higher than the drain resistance, and non-linear intrinsic behavior is demonstrated in some cases. The threshold voltage extracted by KPFM is different from that extracted from current-voltage measurements. By analyzing the tip response using calibration samples, the electric field at the source/channel interface can be estimated within a small error margin.
Achieving gigahertz transit frequencies in low-voltage organic thin-film transistors (TFTs) will require a contact resistance below about 1 Ohm-cm [1,2]. A general approach to reduce the contact resistance in organic devices is to modify the surface of the metal contacts with a chemisorbed interface layer, ostensibly by reducing the nominal injection barrier. Combined with a thin gate dielectric, this approach can enable contact resistances below 30 Ohm-cm and transit frequencies above 10 MHz at low voltages in coplanar organic TFTs [3,4]. However, further reduction of the contact resistance depends strongly on non-idealities of the interface other than the nominal barrier height according to the Schottky-Mott limit. We show a detailed study on the efficacy of interface layers based on various thiols to improve the contact resistance in coplanar dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) TFTs. We compare the contact resistance of multiple sets of TFTs to results from ultraviolet photoelectron spectroscopy measurements and find strong evidence that Fermi-level pinning prevents a significant reduction of the contact resistance below about 100 Ohm-cm in DNTT TFTs. Therefore, we conclude that this approach may not be a generally sufficient method by itself to eliminate the contact resistance in organic TFTs.
The choice of a staggered or coplanar geometry for organic thin-film transistors (TFT) has significant effects on the static and dynamic electronic properties of the transistors. Using two-port network analysis, we find that the parasitic capacitances and thus the unity current-gain (transit) frequencies are significantly more dependent on the gate-to-source overlap in the staggered TFTs than in coplanar TFTs, and that the transit frequency is higher overall when a coplanar geometry is implemented. We show that these differences are primarily attributed to the lower contact resistance in the coplanar TFTs (10 Ohm-cm) as well as smaller parasitic capacitances associated with the gate-to-contact overlaps.
The characteristics of low-voltage n-channel organic thin-film transistors (TFTs) based on the small-molecule semiconductors N,N’-bis(2,2,3,3,4,4,4-fluorobutyl)-(1,7 & 1,6)-dicyano-perylene-tetracarboxylic diimide (ActivInk N1100) and 2,9-bis(heptafluoropropyl)-4,7,11,14-tetrabromo-1,3,8,10-tetraazaperopyrene (TAPP) are compared. Staggered and coplanar TFTs were fabricated, and all measurements were performed in ambient air. In the coplanar TFTs, the source and drain contacts were treated with one of three different thiols. Overall, the two semiconductors provide similar performance, with electron mobilities up to 0.15 cm2/Vs, on/off ratios up to five orders of magnitude, subthreshold slopes as small as 130 mV/decade and contact resistances (measured using the transmission line method) as small as 21 kOhm-cm.
A process for the fabrication of integrated circuits based on bottom-gate, top-contact organic thin-film transistors (TFTs) with channel lengths as short as 1 µm on flexible plastic substrates has been developed. In this process, all TFT layers (gate electrodes, organic semiconductors, source/drain contacts) are patterned with the help of high-resolution silicon stencil masks, thus eliminating the need for subtractive patterning and avoiding the exposure of the organic semiconductors to potentially harmful organic solvents or resists. The TFTs employ a low-temperature-processed gate dielectric that is sufficiently thin to allow the TFTs and circuits to operate with voltages of about 3 V. Using the vacuum-deposited small-molecule organic semiconductor 2,9-didecyl-dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (C10 DNTT), TFTs with an effective field-effect mobility of 1.2 cm2/Vs, an on/off current ratio of 107, a width-normalized transconductance of 1.2 S/m (with a standard deviation of 6%), and a signal propagation delay (measured in 11-stage ring oscillators) of 420 nsec per stage at a supply voltage of 3 V have been obtained. To our knowledge, this is the first time that megahertz operation has been achieved in flexible organic transistors at supply voltages of less than 10 V. In addition to flexible ring oscillators, we have also demonstrated a 6-bit digital-to-analog converter (DAC) in a binary-weighted current-steering architecture, based on TFTs with a channel length of 4 µm and fabricated on a glass substrate. This DAC has a supply voltage of 3.3 V, a circuit area of 2.6 × 4.6 mm2, and a maximum sampling rate of 100 kS/s.
We have successfully manufactured rubber-like large-area stretchable integrated circuits comprising printed elastic
conductors, organic transistor-based circuits, and silicon transistor-based circuits. Employing the first direct
integration of organic and silicon (Si) integrated circuits, we have realized to develop a stretchable electromagnetic
interference (EMI) measurement sheet that can detect EMI distribution on the surface of electronic devices by
wrapping the devices in the sheet. The stretchable devices can spread over arbitrary surfaces including free-formed
curvatures and movable parts, thereby significantly increasing the applications of electrical circuits.
Five core-dichlorinated naphthalene diimides (NDIs) bearing several fluoroalkyl-substituents at the imide nitrogens were
synthesized, characterized and employed in organic n-channel
thin-film transistors with a vacuum-deposited
semiconductor layer on 110 nm thick SiO2 (100 nm)/AlOx (8 nm)/SAM (1.7 nm) and 5.7 nm thick AlOx (3.6 nm)/SAM
(2.1 nm) gate dielectrics. The electron mobility of the thin-film transistors under ambient conditions is as large as
1.3 cm2/Vs on the thicker gate dielectric. On the thinner gate dielectric the mobility is lower (0.4 cm2/Vs) but enables
switching at gate-source voltages of only 3V. Such outstanding performance together with the feasible synthetic access
to these compounds make these semiconductors highly promising for
low-cost, large-area, and flexible electronics.
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