2011 International Conference on Communication Systems and Network Technologies
Organic Thin Film Transistor Architecture, Parameters and their Applications Poornima Mittal1, Brijesh Kumar2, Y. S. Negi3, B. K. Kaushik4 and R. K. Singh5 Electronics & Communication Engineering, Graphic Era University-Dehradun, Dehradun INDIA 2, 3 Polymer Science and Technology Program, DPT, Indian Institute of Technology-Roorkee, Roorkee, INDIA 4 Department of Electronics and Computer Engineering, Indian Institute of Technology-Roorkee, Roorkee, INDIA 5 Electronics and Communication Engineering, Uttarakhand Technical University-Dehradun, Dehradun, INDIA 1
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semiconducting properties [3]. To make OTFT, need to have organic semiconductor (OSC), gate dielectric insulator, contact electrodes and substrate. Plastic substrate is used for flexible displays, for that the gate insulator should be organic to reduce the thermal stress induced by the difference in the thermal expansion coefficient between TFT organic semiconductor layer and substrate. Many organic semiconductor materials have been analyzed including Pentacene, Poly (3-octylthiophene) (P3OT), Poly (3alkylthiophene) (P3AT) and poly (3-hexylthiophene) (P3HT) are the most extensively used organic materials for semiconducting layer, but pentacene shows the best organic thin film transistor performance [4]. To meet the performance requirements, it is important to fully understand the driving mechanism of OTFTs, which is still under continuous discussion owing to ambiguities of interface energetic and complexities of carrier behavior [5]. Organic thin film transistor fabrication methodology has progressed remarkably in past decade and it appears that OTFT will find use in numerous low-cost, large-area electronic applications such as smart cards, flexible displays, Mobile phones, Price and Inventory tags, Flexible integrated circuits, Sensors and other novel products [6]. OTFTs provide two important advantages over TFTs based on inorganic semiconductors in that they can be fabricated at lower temperature and at considerably lower cost. OTFTs with electrical characteristics comparable to or better than amorphous silicon hydrogenated (a-Si: H) devices have been demonstrated on glass substrates and inexpensive flexible plastic substrates [7]. OTFT has great potential for flexible printed electronics which is considered as a future age band of electronic circuits and devices. Researchers have concentrated aggressively for characterization and enhancement of charge transport properties of organic semiconductors for their meaningful applications. A remarkable progress was observed by researchers that organic transistors could operate with low voltages while maintaining high performance. Researchers have already replaced the semiconductor layer by an organic material and are currently targeting to modify the dielectric layer by suitable organic insulating materials [8].
Abstract- Organic Thin Film Transistors (OTFTs) are promising devices for future development of variety of low-cost and large-area electronics applications such as flexible displays. This paper analyzes the performance of OTFT made of several organic semiconducting and insulating materials and further discusses their applications. Analysis of previous research work demonstrates that the mobility in OTFT decreases when the product of semiconducting film thickness and gate capacitance per unit area increases. The decrease is specified by a power law function with parameters for several organic semiconductors. OTFT characteristics have undergone spectacular improvements during the last few years. This paper explores the effect of variation of channel length from 40 nm to 20 nm on drain current for pentacene bottom contact structure. Variations in these quantities maps to variations in the electrical behaviour of devices. It has been found that drain current increases due to decrease in length of organic thin film conducting channel. It reviews recent progress in parameter properties for device designs and applications related to OTFTs. The performance of OTFTs is evaluated in terms of mobility, on/off current ratio, threshold voltage and sub threshold slope. This paper thoroughly discusses the overall performance and applications of OTFTs in various fields. Keywords- Contact Resistance, Channel Length, Organic Materials, Organic Thin Film Transistor (OTFT) and RFID.
I.
INTRODUCTION
Recently there has been remarkable interest on organic electronics because of their unique advantages such as low cost fabrication, light weight and mechanical flexibility. The contemporary portable communication and computing devices need light weight, high image quality, thin, and low power flat panel displays. The answer to this need is Organic Thin Film Transistor (OTFT). It is likely to have suitable applications requiring large area coverage, structural flexibility and low cost which was not possible with crystalline silicon [1]. However, the innovative human mind soon searched a novel class of TFTs based on organic or polymeric semiconductor as active layer material that shows amazing possibility for integration on to flexible plastic substrates, thus giving the world an idea of futuristic technology of low cost, thin, printable electronics, rugged, flexible and lightweight displays [2]. Organic semiconductors are actually new class of materials comprising small molecules and polymers with 978-0-7695-4437-3/11 $26.00 © 2011 IEEE DOI 10.1109/CSNT.2011.96
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II.
ORGANIC THIN FILM TRANSISTOR STRUCTURE
B. Contact Electrode For electrode metals such as gold, platinum, aluminum, magnesium and chromium prepared by evaporation can be used [12]. Adding nickel on gold improves adhesion of the gold on the oxide. Platinum electrodes are inferior to gold electrodes. Aluminum shows slightly higher electron mobility (2.2cm2/Vs) at room temperature in single crystals.
An OTFT is a transistor composed of a thin film of current carrying semiconductor, an insulator layer and three electrodes. The main difference between the geometry of conventional MOSFET and OTFT is that the latter does not have a fourth terminal that is body, thus making these transistors free of the body effect. Secondly, in the former the conducting channel is formed by an inversion layer while in OTFT, it is because of accumulation layer. OTFTs can be fabricated in different device architectures. Some of these structures are depicted in Fig. 1 and Fig. 2.
C. Materials for Organic Semiconductor Layer The Pentacene, Poly (3-octylthiophene) (P3OT), Poly (3alkylthiophene) (P3AT) and poly (3-hexylthiophene) (P3HT) are the most extensively used p-type organic semiconductors [13]. Among all investigated oligomeric and polymeric materials, pentacene thin films have demonstrated the best electrical performance. For pentacene mobility exceeding 3.2cm2/Vs and on/off current ratio>108 has been quoted [14]. Some n-type organic semiconductors are Pc2Lu (Lutetiumbisphthalocyanines), Pc2Tm (Thulium-bisphthalocyanines), TCNQ (tetracyan-oquinodimethane), C60 and F16CuPc. Gold with work function of 5.0ev has been optimized for S/D electrodes [15], and since most n-type materials have solid state electron affinity levels 4.0ev. Thus charge injection into the semiconductor would be limit by the energy barrier of approximately 1ev, is a issue associted with complexity of ntype devices.
Fig. 1 Bottom gate top contact transistor structure with pentacene as organic semiconductor, gold for contacts and dielectric material SiO2
D. Materials for Dielectric The dielectric material needs to have very high resistivity to prevent the leakage between gate and semiconductor channel and highest possible dielectric constant to have enough capacitance for channel current flow. polymers have good processability and dielectric properties. Some important dielectric materials with their dielectric constant are polymide - 2.6, PMMA (polymethylmethacrylate) - 2.65, Al2O3 - 9, and SOG (spin on glass) - 3.9.[13]. Low switching voltage of OTFTs obtained by high dielectric constant insulators.
Fig. 2 Bottom gate bottom contact transistor structure with pentacene as organic semiconductor, gold for contacts and dielectric materil SiO2.
In top contact architecture contacts are deposited through shadow mask where as in bottom contact microlithography technique is used [9]. Bottom gate top contact structure shows better field effect mobility in comparison with bottom gate bottom contact structure. The reason for this difference is often explained by the large metal-semiconductor contact resistance due to interface contact barrier and irregular deposition or poor morphology of the semiconductor film around the already patterned source and drain contacts [10]. III.
IV.
OTFT OPERATION
Organic materials such as P3HT, P3OT or Pentacene acts as p-type semiconductor having holes as majority carriers. When a negative gate voltage is applied, an electric field is formed across the dielectric, causing an accumulation region of holes at the dielectric-semiconductor interface. Applying a voltage to the source-drain terminals allows a current to flow across this accumulation layer between the contacts. Fig.3 shows the Structure and operation of OTFT.
MATERIALS FOR ORGANIC THIN FILM TRANSISTOR
Organic materials offer strong assurance in terms of properties, processing and cost effectiveness. Following materials are explained here for different layers of OTFT. A. Substrate For substrates quartz, polycarbonate, polyethylene naphthalate (PEN), glass, silicon wafer and polyimide materials can be used [11]. Inorganic substrates have high melting point and good flatness where as polymer substrates have high toughness, flexibility and light weight.
Fig. 3 OTFT operation with Pentacene as active semiconductor layer and gold as contact electrodes in the top contact structure.
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In contrast to conventional Si transistors, organic TFTs normally operate in the accumulation mode, where applying a gate voltage creates mobile charge carriers in the channel, thus switching the device “on”. The semiconductor field-effect mobility is calculated from the I-V data according to current equation of organic transistor. The device current ION /IOFF ratio, and the sub threshold slope (related to how efficiently the gate field modulates the OFF to ON current and how crisply the device turns on) are also important OTFT performance characteristics. To demonstrate operating mode of OTFT, typical current–voltage characteristics are shown in Fig. 5 and 6. The shape of the current-voltage characteristics of OTFTs is similar to those of MOSFET at gate bias voltage Vgs higher than a threshold voltage Vt. The screen shot diagram of bottom contact structures for different channel length (20nm and 40nm) is given in Fig. 4. Both devices are simulated in ATLAS device simulator with identical parameters. Typical dimensions of the device include Channel width of 200 µm, SiO2 insulator thickness of 600 nm, source and drain length of 10 µm and Pentacene active layer of 40 nm. The properties of pentacene organic semiconductor material used in simulation include energy gap of 2.34 eV, electron affinity of 3.22 eV, electron density of state of 2.0 x 1021 per cm3 in valance band and 1.7 x 1021 per cm3 in conduction band and permittivity of 4.0 [16].
(a)
(b) Fig. 6 (a) ID-VDS and (b) ID-VGS curves for bottom contact 20nm channel length.
From the curves it is clear that drain current increases due to decrease in length of conducting channel. The on/off current ratio is higher for short channel devices over long channel devices. V.
PARAMETERS OF ORGANIC TRANSISTOR
A. Mobility Field effect mobility is the average charge carrier drift velocity per unit electric field and measure of how easily charge carriers can move in the device. Large mobility is required for reliable operation of transistor. Due to weak intermolecular interaction in the solid material state, impurities, defects and inefficient carrier injection capability at the metal contacts, OTFTs are characterized with much lower carrier mobility than inorganic MOSFETs. In 2010 Hagen proposed a solution by self assembled monolayer in combination with a high quality insulating layer called multilayer [10]. Mobility increases with increase in channel length and active semiconductor layer thickness. The mobility of organic thin film transistor is gate biased dependant and tends to increase when gate bias increases. The bias dependent mobility, expressed as power law for organic based field effect transistor is given by: µ (VGS) = µ0 (VGS – VT) γ The parameter γ is usually estimated in the range of 0.2 – 0.5 for different OTFTs/PFETs. The mobility increases from very low values about 0.02 cm2V-1s-1 at VG = -14 V to 1.26 cm2V-1s-1 at -146 V [17].
Fig. 4 Screen shot of bottom contact architecture with channel length 40nm and 20nm.
(a)
(b) Fig. 5 (a) ID-VDS and (b) ID-VGS curves for bottom contact 40nm channel length.
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B. On/Off (Ion/Ioff ) Current Ratio The ratio of current in the accumulation mode over the current in the depletion mode is called Ion/Ioff. Current ratio depends upon various factors such as materials, channel length, and thickness of semiconductor. Short channel devices exhibit higher on/off current ratio over larger channel length [18]. This ratio increases with decrease in the thickness of semiconducting layer. It should be more than 106 for memory and display devices. For these applications high on/off current ratio is more important requirement than high mobility. It has been quoted [19] as 108 for bottom contact structure with Pentacene as organic semiconductor, cross linked PVP as insulator and gold S/D contacts. Recently it has been observed around 109 for Pentacene as active organic semiconductor [20].
F. Effect of Active Layer Thickness Electrical parameters of OTFT does not solely depend upon gate capacitance, these can be modulated by film thickness and charge injection from the source electrode. There are trends which can be expressed as a function of the product of thickness of polymeric film and gate capacitance per unit area. It has been observed that with increasing the permittivity of gate insulator and thickness of organic material, the mobility decreases in OTFTs [1]. VI.
LIMITATIONS
Almost all electronic devices used in daily life are based on inorganic semiconductors, silicon or gallium arsenide, since they are extremely stable. It's essentially impossible to destroy silicon. Organic semiconductors are very soft and sophisticated. They degrade and can break easily. Characteristics of organic materials changes with environmental conditions after long duration. So the stability of these devices has to be worked out and modeled properly to better understand the process of degradation. Researchers agree that most of the instability comes from the chemical structure of the compound, and they are trying to find ways to make more stable organic compounds. Furthermore, the OTFT current-voltage characteristics degrade at higher temperatures and the noise at low frequency increases. Scientists throughout globe are intending to develop organic semiconductor with high mobility and fast switching time.
C. Threshold Voltage To extract information about impurity concentrations, interface states and traps it is common practice to use threshold voltage and sub threshold current as device evaluation parameters. In MOSFETs, the sub threshold current exponentially depends on the gate-bias as well as the drainsource bias because below threshold the free carrier density exponentially depends on the local bias. The threshold voltage VT of OTFTs varies with either the gate insulator capacitance or the thickness of the organic film [21]. The devices with shorter channel length and thicker Pentacene/P3HT films tend to have smaller threshold voltages [18]. Lower VT is useful in lowering device power consumption and useful in producing portable devices.
VII.
D. Contact Resistance Ideally the contact resistance should be ohmic and small in order to make enable the whole voltage applied to the device, contributes to the transport current. For top contact devices it strongly depends upon gate bias and sharply increases at low gate-source voltage, while contact resistance appears to be almost independent of the gate bias in bottom contact structures. In top contact structure the field effect mobility is higher and contact resistance is lower due to large injection area whereas contact resistance is higher in bottom contact devices due to poor morphology.
APPLICATIONS
OTFTs find wide applications in RFID tags and OLEDS are discussed as follows. A. Radio Frequency Identification (RFID) Recently, there has been enormous interest in the development of RFID tags for item-level tracking of individual goods of consumer. Such tags are expected to dramatically improve inventory control, automation, and purchasing and checkout operations. Several approaches have been developed to realize item-level RFID. In most conventional approach, low-cost silicon RFID tags were developed. But there usages are restricted in water- and metalcontaminated environments. Moreover, silicon based RFIDs are not bendable which limits their applicability in general flexible items. In contrast, organic electronic devices find wide applications in alternative flexible RFIDs [23]. To realize low cost individual RFID tags, efforts have been made for development of item level RFID tags by use of organic transistors and printed electronics technologies. Organic RFID tags are generally one to three orders of magnitude cheaper than silicon technology per unit area. RFID devices are usually deployed in different frequency bands to execute the required tasks. Operating range of most low-cost RFID technologies is likely to be limited by power delivery from the reader to the passive tag.
E. Effect of Channel Length Drain current strongly depends upon the semiconductor used for channel and it can be modulated by length of the conducting channel. M. Austin et al quoted drain current dependence on the length of channel for P3HT (poly (3hexylthiophene)) in OTFTs with different channel lengths of 1000nm and 70nm. It has been shown that saturation region was present for long channel (1000nm) device but no saturation region appeared in the short channel (70nm) device. Long channel devices are relatively immune to high contact resistance and when scaled to smaller channel lengths, the device performance may degrade [22].
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B. Organic Light Emitting Diode Organic Thin Film Transistor can be utilized to make displays on glass or plastic. Liquid crystal Display (LCD) and E-paper can be fabricated with OTFT and Active Matrix Organic Light Emitting Diode (AMOLED) with dot patterns. But the AMOLED shows significant non-uniformity in the brightness. On the other hand, OTFTs can be used to make superior displays of E-paper or LCD because it requires only high on/off current ratio. An organic LED is a thin film device whose emissive layers are made of organic compound [24] and does not require any back light function. New emissive technology called organic/polymer light emitting diode (OLEDs/PLEDs) displays has been developed. In these displays, generation and combination of electron hole pairs produces a photon in emissive layer. When current is passed through a thin multilayer organic material it should efficiently be converted into light. Many OLEDs together on a screen make up a picture. OLEDs require no backlighting, so they have high luminous efficiencies. They have also shown promise for brighter backgrounds, sharper images, better color quality, larger viewing angles, and lower voltage and faster switching times of the order of nano seconds. OLEDs are used in mobile phones, televisions and displays system. VIII.
[3] [4]
[5] [6] [7] [8]
[9]
[10] [11] [12]
CONCLUSION
[13]
Organic and polymer devices now compete with hydrogenated amorphous silicon in terms of mobility, sub threshold slope, on/off current ratio, and low cost fabrication technologies. The research efforts so far have resulted in organic thin film transistors with active layers such as pentacene deposited by high vacuum system. They show high mobility and on/off ratio which is logically good enough for actual device utilization. Furthermore, the mode of fabrication of the device seems to strongly affect its final performance. The effect of channel length on drain current for pentacene bottom contact structure has been analyzed. It has been found that drain current increases due to decrease in length of conducting channel, But long channel devices are relatively immune to high contact resistance and when scaled to smaller channel lengths, the device performance may degrade The next big leap will be the further development of advanced devices that will exploit properties unique to organic semiconductors and to prepare multifunctional systems that cannot currently be fabricated from inorganic semiconductors. All technologies require improvements in charge mobility to reduce the drive voltage.
[14] [15]
[16] [17] [18]
[19]
[20] [21]
[22]
[23]
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