Organic Field-Effect Transistors
Introduction
Since Bardeen, Shockley, and Brattain invented the world's first transistor in 1947, field-effect transistors (FETs) have dominated the mainstream microelectronics industry. They are fundamental building blocks for basic analytical circuits, such as amplifiers, as well as the key elements for logic circuits, modern memory devices, and microprocessors used in personal computers and laptops. In all of these applications inorganic semiconductors are used, and processed by using modern, but also complex semiconductor technologies. These processes require sophisticated photolithographic patterning, as well as high-temperatures and high-vacuum, hence the production costs are quite high. Such products of inorganic materials are also not mechanically flexible, and can not be produced on a large-area basis.
Recent developments in the conducting (conjugated) polymer technology, however, offer quite different application opportunities, bringing a new vision to electronics. Since the late 1970s conjugated polymers have been a focus for the worldwide research as they combine the non-insulating electrical properties of a delocalized p-electron system with the material strength of polymers. Initially, the studies were directed towards heavily doped systems searching for metallic conductivity in polymeric materials. In the last 15 years much interest has been diverted to the semiconducting properties of lightly doped or undoped conjugated polymers. Schottky diodes, light-emitting diodes, photodiodes, chemical sensors and organic field-effect transistors (OFETs) have been constructed.
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| Fig. 1 Soluble organic semiconductor and a schematic picture of spin-coating method |
OFETs which are made of conjugated polymers present several potential advantages. First, the deposition techniques for semiconductor films allow large areas to be coated. Many conjugated polymers can be synthesized in a way that they are soluble in organic solvents, so that they can be processed by spin-coating, casting or by various printing methods. (Figure 2) Therefore a high-temperature processing is not necessary for producing films. Second advantage is that polymers are mechanically tough and flexible, therefore compatible with plastic substrates. This offers the possibility to realise foldable, flexible products with extremely light-weight. Since the report of the first solution-processed organic field-effect transistor in 1986, there has been great progress in both the materials' performance and development of new fabrication techniques. OFETs have already been demonstrated in applications like organic solar cells, sensors, memory devices, flexible displays and radio frequency identification tags (RF-IDs). (Figures 2 and 3)
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| Fig. 2 - RF-ID tags |
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| Fig. 3a Organic solar cell | Fig. 3b flexible display |
OFETs are not meant to replace conventional inorganic MOSFETs since their switching speed does not have the level of MOSFETs. This is due to the lower charge-carrier mobility values in organic semiconductors. Typical values of mobility for such materials range from 0.01 to 1.0 cm2V-1s-1, which are at least 1000 times smaller than crystalline silicon, but similar to amorphous silicon. Therefore, OFETs still have great potential for a wide variety of low-performance, inexpensive, large-area applications, as already mentioned above. Another important drawback of organic transistors is that organic semiconductors are sensitive to atmospheric conditions, and react with oxygen and humidity. Therefore stable and reproducible devices can only be produced with a good control of processing conditions and use of an efficient encapsulation.
Research in the field of OFETs has been conducted in an interdisciplinary level on many different disciplines, which have to work parallel in good collaboration. Organic chemists work on the synthesis of high-mobility, high-purity, high-stability semiconductors. Electronic engineers are trying to improve circuitry that can be realized by OFETs. Physicist are working on the physical model of OFETs, whereas material scientists are looking for new materials and investigating the interface related issues in OFETs.
Goals
At the institute of OEM at TUHH the research conducted on OFETs can be classified in 4 sections:
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Characterization of novel dielectric and semiconductor thin-films for OFETs
Pentacene and P3HT are mainly used as semiconductors in the devices. Various thermosetting and thermoplastic materials are used as dielectric in combination with these semiconductors. Compatibility of layers and the electronic properties of transistors are characterized. In cooperation with the company Elantas Beck new dielectric materials are being developed.
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Influence of interfaces on OFET device properties and interface tuning
Dielectric/semiconductor interface plays a crucial role in OFET performance. Therefore a proper interface should be formed during the deposition of these layers. By means of experimental studies on interfaces, like usage of new solvents or surface modifications, it is tried to extract new information on physical properties of interfaces in OFETs. Charge carrier mobility and interface doping are the main investigated mechanisms.
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OFET-sensors
Chemical volatile sensing properties of OFETs are demonstrated. Selective volatile sensing by using organic transistor-sensors is aimed.
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Ferroelectric high-K gate insulator in OFETs for memory applications
Well known superb ferroelectric and dielectric properties of PVDF/TrFE copolymers were modified by blending barium titanate nanopowder and preparing thin nanocomposite films. The films have up to 50 volume % BT powder and exhibit dielectric constant values of up to 51.5 at 1 kHz. These films are introduced as gate insulator in OFETs where ferroelectric memory and low-voltage operation are demonstrated.
Results
Materials & Interfaces
There are two types of OFET configurations being investigated at the institute of OEM: Top-gate and bottom-gate. Main difference between these transistors is the sequence of deposition of semiconductor and dielectric. Since both of the layers are deposited by spin-coating from a solution it is very important to select suitable solvents. Solvent to be used to deposit the upper layer should not dissolve or damage the lower layer. In addition, since OFETs operate at the dielectric/semiconductor interface formation of this interface during solution-processing is very important.
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| Fig. 4 - Top-and bottom-gate OFET configurations |
New insulators with good electrical properties and very high chemical stability were investigated to be used as dielectric thin-films in OFETs. SU-8, NOA74, BCB and Bectron EL37A were some examples which were tested in combination with semiconductor poly(3-heyxlthiophene) (P3HT) in bottom-gate configuration. In the figure below, a hybrid transistor with SiO2 as dielectric was compared with a device with the organic NOA74 layer as the dielectric.
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| Fig. 5 - Output and transfer characteristics of a hybrid OFET with 300 nm thick SiO2 dielectric (a,c) and an OFET with a 1,2 µm thick NOA74 dielectric (b,d). |
Surface functionalization of SiO2 was applied on hybrid transistors, in order to improve field-effect mobility, threshold behaviour and the reproducibility of P3HT-transistors. An HMDS monolayer was used to create a hydrophobic surface with which the P3HT side chains interact and so form a better ordered layer with a well-defined interface.
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| Fig. 6 - Chemical structure of P3HT with its side chains and HMDS modification of silicon oxide surface. Side chains interact with HMDS monolayer and favour ordering of molecules. | |
Insulating layer for low-driving voltages and ferroelectric memory applications
In this study, high-K polymer-ceramic nano-composite films were introduced into the field of ferroelectric OFETs. By carefully blending barium titanate nanopowder into PVDF/TrFE polymer, much higher dielectric constant values were achieved. By integrating this layer into a top-gate OFET, induced gate capacitance was increased and therefore low-voltage operating OFETs were successfully produced.
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| Fig. 7a - Table of dielectric constant values of PVDF/BT nanocomposites | Fig. 7b - and equation of the drain-current where the capacitance of the dielectric is directly proportional to current. |
In addition to low-voltage operation, non-volatile memory function of the transistors was also demonstrated by measuring the hysteresis behaviour and on- and off-state currents of the transistor.
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| Fig. 8 - Demonstration of low-voltage operation and ferroelectric hysteresis, as well as memory retention properties of ferroelectric OFETs produced by PVDF/BT nanocomposites. |
OFET Sensors
Organic field-effect transistor sensors have advantages over plain resistive sensors. Since a transistor can be characterized by different parameters, there is a chance to selectively sense chemicals. For example, a volatile can influence the threshold behaviour of the transistor, whereas another once affects the mobility. Therefore organic volatile sensing properties of OFETs were investigated by using hybrid OFETs. Taking advantage of high sensitivity of semiconductor P3HT and pentacene, OFET sensors were built and measured. Reactions of various transistor properties to different chemicals were measured before and after exposing the transistors to the chemicals.
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| Fig. 9 - Reversible doping effect of acetone on drain current of a pentacene transistor |
References
- A. R. Brown, C. P. Jarrett, D. M. de Leeuw, M. Matters, Field-effect transistors made from solution-processed organic semiconductors, Synthetic metals 88 , (1997) 37-55
- C. Reese, M. Roberts, M. -M. Ling, Z. Bao, Organic thin-film transistors, Materials today, , September 2004, pp 20
- F. A. Yildirim, R. R. Schliewe, R. M. Meixner, H. Göbel, W. Bauhofer, W. Krautschneider, Gate insulators and interface effects in organic thin-film transistors, Elsevier Organic Electronics 9 , (2008), 70.
- F. A. Yildirim, C. Ucurum, R. R. Schliewe, W. Bauhofer, R. M. Meixner, H. Göbel, W. Krautschneider, Spin-cast composite gate insulation for low driving voltages and memory effect inorganic field-effect transistors, Applied Physics Letters 90 , (2007), 083501 Invited paper to the Virtual Journal of Nanoscale Science & Technology.
- R. M. Meixner, F. A. Yildirim, R. Wille, P. Schertling, H. Goebel, H. Harde, K.-H. Steglich, W. Bauhofer and W. Krautschneider, Bottom Gate Organic Field Effect Transistors Made by Laser Structuring Elsevier Organic Electronics 7 , (2006), 586
- R. M. Meixner, H. Göbel, F. A. Yildirim, W. Bauhofer, W. Krautschneider, Wavelength-Selective Organic Field-Effect Phototransistors Based on Dye-Doped Poly-3-hexylthiophene, Physics Letters 89 , (2006), 092110.
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