Organic and polymer transistors for electronics

Keywords: organic thin-film transistors ; contact length ; contact resistance ; cutoff frequency ; Thin-Film Transistors ; Field-Effect Transistors ; Artificial Skin ; Performance ; Fabrication ; Dielectrics ; Matrix Reference EPFL-ARTICLE-175161doi:10.1002/smll.201101677View record in Web of Science Record created on 2012-02-23, modified on 2017-05-12

Contact Resistance and Megahertz Operation of Aggressively Scaled Organic Transistors

The charge injection process in top-contact organic field-effect transistors was energetically observed with displacement of the Fermi level as a result of scanning the gate voltage. Doping of charge-transfer molecules into the metal/organic interface resulted in low interface resistance, which unveiled the bulk transport of the injected charges from the contact metal into the channel. The authors found that the bulk transport clearly obeys the Meyer-Neldel rule, according to which the exponential density of states near the band edge limits the charge injection.

Charge injection process in organic field-effect transistors

In this study we report on the optimization of the contact resistance by surface treatment in short-channel bottom-contact OTFTs based on pentacene as semiconductor and SiO 2 as gate dielectric. The devices have been fabricated by means of nanoimprint lithography with channel lengths in the range of 0.3 μm < L < 3.0 μm. In order to reduce the contact resistance the Au source- and drain-contacts were subjected to a special UV/ozone treatment, which induced the formation of a thin AuO x layer. It turned out, that the treatment is very effective (i) in decreasing the hole-injection barrier between Au and pentacene and (ii) in improving the morphology of pentacene on top of the Au contacts and thus reducing the access resistance of carriers to the channel. Contact resistance values as low as 80 Ω cm were achieved for gate voltages well above the threshold. In devices with untreated contacts, the charge carrier mobility shows a power-law dependence on the channel length, which is closely related to the contact resistance and to the grain-size of the pentacene crystallites. Devices with UV/ozone treated contacts of very low resistance, however, exhibit a charge carrier mobility in the range of 0.3 cm 2 V -1 s -1 < μ <0.4 cm 2 V -1 s -1 independent of the channel length.

Orders-of-Magnitude Reduction of the Contact Resistance in Short-Channel Hot Embossed Organic Thin Film Transistors by Oxidative Treatment of Au-Electrodes†

A model is presented that describes the gate-voltage-dependent contact resistance and channel-length-dependent charge carrier mobility of small-molecule-based organic thin-film transistors in top and bottom drain/source contact configuration.

On the Origin of Contact Resistances of Organic Thin Film Transistors

Grain size effects on contact resistance of top-contact pentacene TFTs

A full-swing pentacene organic thin-film transistor (OTFT) inverter, which is composed of an enhancement-mode driver and a depletion-mode load, is proposed and confirmed at the discrete device level. The depletion-mode OTFT can be easily converted to the enhancement-mode by adding a thin PMMA layer between insulator and pentacene to the depletion-mode fabrication sequence. It is demonstrated that the proposed inverter shows a full-swing characteristic, large inverting gain and high noise margin with only two OTFTs.

Full-swing pentacene organic inverter with enhancement-mode driver and depletion-mode load

When scaling down to the channel length (L/sub c/) of 1.8 /spl mu/m using a membrane shadow mask, top-contact pentacene thin-film transistors (TFTs) show that grain size dependency on the anomalous leakage current becomes conspicuous as L/sub c/ is comparable to the grain size. For scaled-down OTFTs with large and small grain, the obvious difference of off-current in the depletion regime can be attributed to various reasons such as pentacene conductivity, parasitic resistance, locally ill-defined source/drain edge, and Au interdiffusion. To improve mobility as well as I/sub on//I/sub off/ ratio for scaled-down OTFTs, two-step-deposition (TSD) technique that enables us to control the channel conductivity in the depletion and accumulation regime as well as to improve the film continuity was proposed. To the best of our knowledge, the I/sub on//I/sub off/ ratio of 10/sup 7/ and the mobility of 0.20 cm/sup 2//V/spl middot/s for OTFTs with L/sub c/ of 1.8 /spl mu/m deposited by using the TSD technique was one of the best results in the literature.

Performance improvement of scaled-down top-contact OTFTs by two-step-deposition of pentacene

In this paper, we propose a pentacene organic thin-film transistor (OTFT) logic inverter circuit with a depletion mode load OTFT and an enhancement mode driving OTFT. It is confirmed that, with only two OTFTs, the minimum output voltage goes down to 0 V and it improves the noise margin characteristics. We believe that this scheme is a first but significant step for simple and reliable pentacene OTFT logic circuits.

http://ieeexplore.ieee.org/iel5/9434/29942/01367850.pdf

Full-swing pentacene organic thin-film transistor inverter with enhancement-mode driver and depletion-mode load

Top-contact pentacene TFTs were successfully scaled down from 20 μm to 1.8 μm for the channel length (LC) by using silicon nitride membrane shadow mask. When LC was reduced to 1.8 μm, pentacene TFTs with the grain size of a few micrometer showed poor on/off current ratio (Ion/Ioff) of 103 and high mobility of 0.25 cm2/Vsec, whereas pentacene TFTs with the grain size of sub-micrometer resulted in low mobility of 0.08 cm2/Vsec, but good Ion/Ioff of 5.7×106. To obtain both high mobility and good Ion/Ioff for the scaled-down OTFTs at the same time, two-step-deposition (TSD) technique was proposed and resulted in Ion/Ioff of 107 as well as mobility of 0.19 cm2/Vsec even for the TFT with LC of 1.8 μm. To our best knowledge, this result is one of the best electrical characteristics among the reported electrical performance of OTFTs scaled down to LC of 1.8 μm.

Keum Dong Jung

Papers

Grain size effects on contact resistance of top-contact pentacene TFTs

Full-swing pentacene organic inverter with enhancement-mode driver and depletion-mode load

Performance improvement of scaled-down top-contact OTFTs by two-step-deposition of pentacene

Full-swing pentacene organic thin-film transistor inverter with enhancement-mode driver and depletion-mode load

P‐18: Performance Improvement of Scaled‐down Top‐contact OTFTs by Two‐Step‐Deposition of Pentacene