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An Introduction to Organic Field-Effect Transistor (OFET)

An OFET is a field-effect transistor where organic semiconductor is the semiconducting material inside the channel. Organic semiconductors could be deposited by various methods like vacuum evaporation, spin-coating or dip-coating e.t.c to get an OFET. An OFET can be fabricated on different substrated like:glass, plastics or conventional crystalline doped silicon wafer. OFETs when fabricated on the plastic substrates, it gives an OFET flexiblity for the realization of RFID tags and wearable electronic gadgets. When OFETs are fabricated on the glass substrates, we get a transistor for the realization of a transparent electronic circuits (as depicted in the video from Corning Inc.). Economically, OFETs eliminate expensive fabrication technique which are relatively low-temperature processable as compared to conventional silicon based FETs. Due to low processing temperature, the OFETs could be easily fabricated on very large area because substrates themself are very cheaper compared to silicon. OFET could be very useful electronic device in the field of large area electronics due to its features like: transparancy, flexibility, cheaper and also stretchable. The same deposition process could be also applied towards the realization of cheaper thin-film OFETs or thin-film solar cells or thin-film light emitting diodes (LEDs).

Historically, it was back 1986 when the first organic field-effect transistor (OFET) was developed with polythiophene organic semiconductor at Materials and Electronic Devices Laboratory, Mitsubishi Electric Corp. [1]. Later, in the year 2000, the Nobel-prize in chemistry was assigned to A. J Heeger, A. G. MacDiarmid and H. Shrikawa for the discovery of conducting polymers [2].  Then, in 2003, a research group from Institute of Material Sciences, Darmstadt University of Technology reported the first organic light-emitting field-effect transistor (OLET) based on tetracene organic semiconductor [3] . Organic thin-film transistors (OTFT), organic light emitting diodes (OLEDs), organic display unit (ODU) and organic solar cells are such notable applications of organic semiconductor in growing electronics industry [4]  [5] [6] [7].  Commercially, in early 2010 Samsung Corp. has demonstrated an OLED based transparent laptop with screen size as thin as 1.8-inch [8] . Similarly, Sony Corp. has also launched its first active matrix OLED (AMOLED) 14-inch TV in 2010 [9].

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Source: Corning Inc.

 

          

Figure (1): Flexible OLED based display unit from ITRI (Tiwan Research Institute) (Left) and Printed RFID tag from PolyIC GmbH (Right).

 

The OFET shown below in figure (2) is termed as bottom-gate bottom contact architect. In this architect, organic semiconductor (P3HT) is deposited directly above the contacts giving bottom-contact geometry. Similarly, gate is attached below the substrate giving bottom contact architect. Similar to FETs, channel length and channel width describes the device geometry. Channel length (L) is the distance between two contacts namely: the source (S) and the drain (D). For electrical measurement, source is grounded, drain is applied with suitable bias potential (Vds), and gate (G) is applied with suitable control potential (Vgs).

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OFET

Figure (2): Schematic diagram of a p-type bottom-gate bottom-contact OFET.

 

SEM Picture

Figure (3): SEM picture of a typical OFET depicting finger-like gold (Au) contacts with channel length (L) of 2 µm and chanel width (W) of 200 µm. The picture has been obtained in the SEM at Jacobs University Bremen.

 

AFM Picture

Figure (4): AFM pictures (left-topography and right-phase contrast) of a L = 240 nm OFET after deposition of organic semiconductor (P3HT). The pictures has been obtained in the AFM at Jacobs University Bremen.

 

In figure (2), a typical OFET with L = 2 µm and W = 200 µm (fingure-like contacts) is depicted. The SEM (scanning electron microscope) picture is taken before deposition of P3HT. The distance between two consecutive contacts is termed as the channel length (L). Similarly, in figure (4), atomic force microscope (AFM) measuremen of a OFET with L = 240 nm is depicted. SEM fails to give information regarding the semicondutor (which is 45 nm thick) physical surface property because conductivity of gold is very high compared to the semiconductor. Hence, the AFM measurement has been carried out after the deposition of P3HT. In topography picture (left), the channel is clearly visible between the contacts. However, the topographic measurement still doesn't give sufficient imformation whether channel is covered with the semiconductor or not. Interestingly, in phase contrast AFM picture (figure (4)-right), the measurement clearly reveals that the channel is properly covered with P3HT. The phase image is without distinct change in phase inside and outside the channel.

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Refrences

[1] A. Tsumura, H. Koezuka, and T. Ando, "Macromolecular Electronic Device: Field‐Effect Transistor with a Polythiophene Thin-film," Appl. Phys. Lett., pp. 1210-1212, 1986.

[2] C. K. Chiang, C. R. Fincher, Jr., Y. W. Park, and A. J. Heeger, "Electrical Conductivity in Doped Polyacetylene," Phys. Rev. Lett., vol. 39, pp. 1098-1101, 1977.

[3] A. Hepp, H. Heil, W. Weise, M. Ahles, R. Schmechel, and H. von Seggern, "Light-Emitting Field-Effect Transistor Based on a Tetracene Thin-film," Phys. Rev. Lett., vol. 91, pp. 1574061-1574064, 2003.

[4] (2011) RFID. [Online]. http://www.polyic.com/rfid.php.

[5] (2010) HardwareSphere. [Online]. http://www.hardwaresphere.com/2010/04/23/lg-to-fight-samsungs-led-tv-with-their-oled-screen-tv/

[6] C White. (2007) Flexible, full-color OLED display. [Online]. http://pinktentacle.com/2007/05/flexible-full-color-organic-el-display/

[7] (2010) Axom. [Online]. http://www.azom.com/article.aspx?ArticleID=4548

[8] J. L. Fatley. (2010) Engadget. [Online]. http://www.engadget.com/2010/01/07/samsungs-14-inch-transparent-oled-laptop-video/

[9] E. Strasser. (2010) Sony shows 33 OLED Prototypes at CEATEC 2008. [Online]. http://www.oled-display.net/oled-tv-television

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Thin film Technology (TFT)

Organic Electronics

 

 

⇒ Ultra-low Cost RFID Circuit

Flexible Electronic Circuit

Organic Semiconductors

Introducton to OFETs

OFET Electrical Characterization (a, b)

Gate Field Dependent Moblity

 

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Source: PoyIC GmbH

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Last Updated on July 14th, 2012 at 19:00 pm