Research Trends in Thin-film Transistors for Flexible Displays

Research Trends in Thin-film Transistors for Flexible Displays Flexible displays are thin and light and are promising for use in smartphones and large sheet-type displays. They are kinds of the active-matrix type and liquid crystal or organic light-emitting diode (OLED) materials are used as display devices. Each pixel incorporates thinfilm transistors (TFTs) whose characteristics significantly affect those of the display itself. Several types of semiconductor can be used to make TFTs, and a lot of research and development has gone into improving their characteristics. This Article provides an overview of TFTs, outlines the current trends in flexible displays using TFTs, and describes drive methods that can be used for compensating the degradation in image quality caused by variations in the TFT characteristics. 1. Introduction 8K Super Hi-Vision (8K) is a form of ultra-highdefinition television (UHDTV) that creates a strong sense of presence through images having 16 times the number of pixels of Hi-Vision (HDTV) and 22.2-channel surround sound. A variety of research and development activities are now underway at NHK Science & Technology Research Laboratories (STRL) in preparation for the launch of 8K test broadcasting in 2016. For viewers to experience the full sense of being there that 8K can offer, it is preferable that large screens on the order of 100 inches be used, and for this reason, development is proceeding on thin and light sheet type displays for home use. For mobile use, the TV screens have to be even lighter and thinner, and flexible displays are seen as a way to meet these needs as well. The flexible displays being studied at present include those made of liquid crystal, organic light-emitting diode (OLED), and electronic paper. It must be kept in mind, however, that displays targeted for receiving broadcasts must be able to display moving pictures in color, and liquid crystal and/ or OLED is therefore preferable for this purpose. OLED, in particular, negates the need for backlighting, making it ideal for very thin displays, and for this reason there has been significant growth in research and development on OLED. Liquid crystal and OLED displays currently on the market normally use silicon-based thin-film transistors (TFTs) in their pixels. Flexible displays also need TFTs, but the TFTs in this case are made from a variety of semiconductor materials whose choice depends on the substrate used. In this Article, we begin by describing the basic structure and characteristics of TFTs and the types of TFTs used in flexible displays. Next, we outline the current trends in flexible displays using TFTs. Finally, we touch upon the drive methods of OLED displays for alleviating the deterioration in image quality caused by variations in the TFT characteristics. 2. Principle of TFT operation A TFT is a very thin type of field effect transistor (FET). As shown in Figure 1, TFTs can be classified according to the way in which the semiconductor and electrodes are positioned relative to each other. A bottom gate structure is one in which the gate electrode is positioned below the semiconductor layer, and a top gate structure is one in which it is positioned above the semiconductor layer. In addition, a bottom contact structure is one in which the source and drain electrodes are positioned below the semiconductor layer, and a top contact structure is one in which they are positioned above the semiconductor layer. For example, a structure in which the gate electrode is positioned below the semiconductor layer and the source and drain electrodes are positioned above the semiconductor layer is called a bottom-gate/top-contact structure. Since the fabrication process conditions will differ depending on the type of semiconductor material used, the TFT structure will for the most part depend on the type of TFT used. For example, amorphous-silicon (a- Si) and organic TFTs have a bottom gate structure, and polycrystalline silicon TFTs have a top gate structure. In Bottom Top Contact Top Contact Channel Drain Source Substrate insulator Channel Drain Source Substrate insulator Bottom Contact Channel Drain Source Substrate insulator Channel Drain Source Substrate insulator Figure 1: Basic TFT structures 10

Feature such TFTs, the semiconductor exhibits a high insulation value when no voltage is applied to the gate electrode. On the other hand, on applying voltage to the gate electrode, charge begins to accumulate near the interface between the insulator and semiconductor, and the electrical conductivity begins to rise. The drain current also begins to flow because of the voltage applied between the source and drain electrodes. Note that semiconductors are classified into n-type or p-type according to whether the accumulated charge is composed of electrons or holes, respectively. Now, let us describe the principle of operation of a TFT with a bottom-gate/top-contact structure (Figure 2). When the drain voltage (V ds ) is small, the relationship between the drain voltage and drain current (I ds ) is given by Eq. (1): W μ V g V t V ds V L ds I ds C i[ ]... (1) Drain W L Charge Source insulator Here, W is the transistor channel width (width of the source/drain electrode), L is the channel length (distance between the source and drain electrodes), C i is the electrostatic capacitance per unit area of gate insulator, μ is mobility (a measure indicating how easy it is for charges to move), V t is threshold voltage (the voltage at the boundary between the current flow and no current flow states), and V g is gate voltage. According to this equation, I ds increases monotonically with respect to V ds when V ds < (V g -V t ). On the other hand, I ds saturates when V ds is large: i.e., it takes on a fixed value when V ds > (V g -V t ): W I ds μc i V g V L t... (2) An example of TFT characteristics is shown in Figure 3. The drain current I ds increases as the drain voltage V ds increases and it eventually saturates at a fixed current determined by the gate voltage V g. In short, a fixed current can be made to flow regardless of V ds, which means that this voltage region can be used to operate the transistor as a fixed current source. Figure 3 depicts the linear region to the left of the dashed-dotted line and the saturation region to the right. The mobility μ in the saturation region can be determined as follows by performing partial differentiation with respect to V g on the square root of both sides of Eq. (2). μ L WC i I ds V g... (3) Similarly, the mobility μ in the linear region can be determined by performing partial differentiation with respect to V g on both sides of Eq. (1). Drain current IdsA Vg Vds Ids Figure 2: Current generated by applying voltage to TFT 2 10 4 1.5 10 4 1 10 4 0.5 10 4 0 0 VgVt=15V Linear region Saturation region 10 20 30 Drain voltage VdsV VgVt=10V VgVt=5V Figure 3: Example of TFT characteristics (drain current vs. drain voltage) The TFT characteristics can therefore be expressed in terms of parameters like μ, the ON/OFF ratio of the drain current I ds, and the threshold voltage V t. The temporal fluctuation of each of these characteristics is also an important parameter. 3. Role of TFTs in displays Most thin displays, including flexible ones, use active-matrix drive systems *1. In cathode ray tube (CRT) μ L I ds... (4) WC i V ds V g *1 A drive method that incorporates a transistor in each pixel to separately control the light emitted by that pixel. 11

displays, one point within the screen emits light for only one instant within one frame and emits no light for the remainder of that frame, as shown in Figure 4 (a). This is an impulse type of display in which a very large luminance is produced at the instant light is emitted. In contrast, an active-matrix display continuously emits light for a certain amount of time within one frame, as shown in Figure 4 (b). This is a hold type of display in which the light-emitting interval is longer than that of the impulse type and a large instantaneous luminance is unnecessary. A hold-type display system can be made by placing TFTs in each pixel. The basic equivalent circuit of one pixel in an activematrix OLED (AMOLED) display is shown in Figure 5. This equivalent circuit incorporates two TFTs and one storage capacitor. One of these TFTs is used for selecting that pixel (switching TFT), the other for driving the current needed by the OLED device to emit light (driving TFT). Writing in this circuit is performed by turning on the select signal of the switching TFT, inputting the data signal to the pixel, and writing that data to the storage capacitor. Here, the voltage written to the storage capacitor can control the driving TFT even while the select signal is off, which means that a prescribed current can be made to flow in the OLED device and be held in that state until the next write operation. The driving TFT can therefore control the intensity of light emitted by the OLED device. This means, however, that any variation in the characteristics of driving TFTs can give rise to pixel variations on the display. Consequently, circuits have been developed that can compensate for such variations in the driving TFT characteristics, and these are described in a following section. 4. Semiconductor materials for TFTs The semiconductor layers can be formed from amorphous-silicon (a-si), polysilicon (poly-si) or, more recently, oxide semiconductor. The main types of TFTs used in flexible displays are listed in Table 1. 4.1 Amorphous-silicon TFT (a-si TFT) The amorphous-silicon TFT (a-si TFT) uses noncrystalline silicon. It can be fabricated using plasma enhanced chemical vapor deposition (PECVD) *2 or sputtering *3 at a temperature under 350 C. Although its mobility is not that high, it can be formed over a large area, thereby making it the TFT of choice at present for large displays. In addition, although its characteristics change over time, they exhibit good uniformity over a large area. 4.2 Low-temperature polysilicon TFT (LTPS TFT) Polysilicon TFTs (poly-si TFTs) can be broadly divided into TFTs that use crystalline silicon, high-temperature *2 A film-formation method in which a gas including the base material is excited into a plasma state and a thin film is deposited onto a substrate through chemical reactions. *3 A film-formation method in which accelerated ions collide with film-forming material and the ejected material adheres to the substrate. (instantaneous luminance large) Emission intensity Emission intensity (instantaneous luminance small) 1 frame Time 1 frame Time (a) Impulse type (b) Hold type Figure 4: Light emission formats of pixels Select Data + Power supply Switching TFT Strage capacitor OLED Data Driving TFT Select ON Figure 5: Basic equivalent circuit of one pixel in an AMOLED display 12

Table 1: TFT Types a-si TFT LTPS TFT Oxide TFT Organic TFT Charge mobility (cm 2 /Vs) Large area Low-temperature formation Process temperature Features/issues 0.5-1.5 > 100 (sputtering) (thermal processing by excimer laser) < 350 C around 600 C Large temporal fluctuation Large variation in in characteristics characteristics 10-80 (sputtering) < 300 C Relatively stable < 5 (coating) < 100 C Unstable polysilicon TFTs fabricated in a high-temperature environment above 1000 C, and low-temperature polysilicon (LTPS) TFTs that can be formed at a lower substrate temperature (under 600 C). At present, the most common TFT for display use is the LTPS TFT in which an a-si film deposited by PECVD is crystallized using an excimer laser *4. This type of TFT has a mobility greater than 100 cm 2 /Vs, so it can also be used in peripheral circuits elsewhere on the substrate in addition to its use within pixels. This capability is conducive to making compact and low-cost displays. Using LTPS TFTs in large displays, however, faces issues, such as larger manufacturing equipment and non-uniform characteristics over a large area. 4.3 Oxide TFT A transparent oxide semiconductor (TOS) is a material with relatively high charge mobility that can be easily fabricated by sputtering. Research and development of TOS as a TFT for displays has made dramatic progress in recent years, and practical devices have been developed. Typical TOS materials include polycrystalline ZnO semiconductor and amorphous In-Ga-Zn-O (a-igzo) semiconductor; and a-igzo TFTs are now being used to make large displays. In addition, transparent amorphous oxide semiconductors (TAOS) on flexible plastic substrates have become a major topic of study 1). Being non-crystalline in nature, a-igzo TFTs show less variation than LTPS TFTs, and that makes them more promising for use in large-screen displays. Another recent development is oxide semiconductors that can be formed by a coating process; the aim here is raising production efficiency and lowering costs 2). 4.4 Organic TFT Organic semiconductors can be formed at room temperature by using a coating process. As such, they exhibit good flexibility compared with silicon and oxide semiconductor materials and can withstand the shock of being dropped. These properties make organic TFTs promising for flexible displays on plastic substrates. In particular, a solution containing the semiconductor material can be formed over a large area at low cost by screen printing *5 or ink-jet printing. Organic semiconductor materials can be broadly divided into two types according to the size of their molecules: smallmolecule organic semiconductors and polymer organic semiconductors. Pentacene is a typical small-molecule organic semiconductor. It can be formed into a thin film with relatively good characteristics by using the vacuum deposition method *6. On the other hand, polymer semiconductor materials, typified by polythiophene, can easily form a film by coating, but suffer from low mobility. For this reason, interest has arisen in developing small-molecule semiconductor materials that have a substitution group to make the material soluble 3). Organic TFT devices having a crystal particle diameter larger than ordinary organic semiconductors and featuring a mobility greater than 10 cm 2 /Vs have been reported 4), but forming such TFTs over a large enough area has been difficult. At NHK STRL, we have succeeded in developing an organic semiconductor formation method that works over a relatively large area while achieving a mobility of 1.3 cm 2 /Vs 5). The mobility and atmospheric stability of organic TFTs are issues that need to be addressed, and researchers are pursuing a variety of approaches to resolve them. 5. Substrate As described in the previous section, there is a close relationship between the semiconductor material used in the TFTs and the temperature of the fabrication process. Moreover, the heat resistance of the base substrate is also a concern. The main flexible substrates are listed in Table 2. Plastic substrates are low-cost, very flexible, and durable, but the table indicates that their heat resistance is low. Hence, although plastic substrates are seen as the most promising for use in flexible displays, what is needed is a more heat-resistant film that can be subjected to fabrication temperatures in excess of 200 C. Polyimide *7 is more heat resistant, but it is more expensive than ordinary plastic films and can t be used on the light emitting side of the device because its color is dark brown. Its suitability as a substrate has however increased with the advent of a transparent form. Thin sheet glass has high heat resistance. Furthermore, it is impermeable to oxygen and water vapor that can seriously shorten the lifetime of the OLED material, and it exhibits little elasticity during the fabrication process. These features make it conducive to forming highperformance devices. However, this sort of substrate is susceptible to fractures and requires special handling. *4 Ultraviolet pulse laser generated using rare and halogen gases. *5 A method for forming a pattern on a substrate by applying ink to a screen having a fine mesh in the shape to be formed and extruding the ink by using a squeegee type of tool. *6 A technique that evaporates material in a vacuum by applying heat or other means so that the vaporized material adheres to the surface of the substrate. *7 A polymer material with a robust molecular structure and high heat resistance. 13

Sheet glass with a thickness less than 100 μm is now under development, and this material is now being used as a substrate for flexible displays. Metallic foils such as stainless steel foil also have high heat resistance. Their application to flexible displays, however, faces problems such as high surface roughness and large stray capacitance. As a result, there have been few reports of displays using metal foil. Substrates composed of cellulose - a component of paper - are another possibility, and cellulose backplanes *8 combined with flexible organic TFTs are being studied with an eye to flexibility and environmental safety 6). 6. Fabrication methods for flexible TFT arrays More problems have to be dealt with when fabricating a flexible TFT array than when fabricating a TFT on a solid substrate such as glass. Special concern must be paid to handling, process temperature, and plastic substrate shrinkage. A support substrate such as glass or silicon can be used *8 A substrate on which TFTs are formed. Forming display devices such as OLED ones on this substrate completes the display. during the process of fabricating devices on a flexible substrate. Fabrication methods that use a support substrate can be broadly divided into direct formation and transfer methods, as shown in Figure 6. In the direct formation method, the target film is laminated to the support substrate via a temporary adhesive layer. Devices are then formed on the film, and the film is peeled from the support substrate. This method features a relatively easy fabrication process, but the TFT fabrication temperature is limited by the film s heat-resistance capacity, and shrinkage of the film substrate is substantial. The transfer method, in contrast, begins by forming devices on a temporary adhesive layer formed on a glass support substrate. The film substrate is then laminated to the top of this assembly, and the fabricated devices with the film are peeled from the support substrate. This method uses no film in the device formation stage, which means that the temperature at the time of fabrication does not affect the properties of the film. On the other hand, this method requires an appropriate balance between the adhesive strength of the temporary adhesive layer and that between the film and devices. Thus, to fabricate a good flexible TFT array, the chosen fabrication method must have the right combination of Table 2: Main types of flexible substrate Plastic High heat-resistant film Thin sheet glass Metallic foil Maximum process temperature 180 C > 300 C 600 C > 600 C Flatness Conductivity No No No Yes Optical transparency Gas (oxygen, water vapor) barrier property Flexibility Film Temporary adhesive layer Film lamination Device formation Support substrate Film Device formation Film lamination Peelng from support substrate Peelng from support substrate (a) Direct formation method (b) Transfer method Figure 6: Two main methods of fabricating flexible TFT array using plastic film 14

Feature semiconductor material (as described in section 4) and substrate material (as described in section 5). 7. Application to flexible displays Many flexible displays have been developed in which organic TFTs are formed on a plastic substrate. In particular, there has been a lot of research on flexible AMOLED displays that require no backlight and are even thinner and more flexible. Prototypes include, for example, of a 5.8-inch (diagonal) display 7) and a 4.1- inch rollable display 8) with a radius of curvature of 4 mm. However, organic TFTs do not have sufficient mobility for use in high-definition, high-brightness AMOLED displays that have large screens. As a result, studies on such TFTs are currently focused on smaller displays such as those made of electronic paper 9). The use of a-si TFTs in flexible AMOLED displays with a stainless-steel foil or plastic-film substrate has also been studied 10). However, like organic TFTs, the mobility of a-si TFTs is not high, and the prototypes have so far been restricted to small displays. Oxide TFTs, though, can be fabricated in a relatively easy manner using the sputtering method, and their mobility is at least one order of magnitude higher than that of a-si TFTs and organic TFTs. They consequently have potential for larger flexible displays. Flexible AMOLED displays ranging from 8 inches to 13 inches have been prepared using IGZO TFTs. Most of these displays use plastic substrates and are still small to medium sized; however, a prototype display with 2,160 scanning lines has been reported 11). The stability of oxide TFTs can be improved by using thermal processing at 300 C and higher. For this reason, high-performance TFT arrays are often made on heatresistant film like polyimide 12) 13) or by transferring a TFT array fabricated at high temperature on a glass substrate onto plastic film 11). High-temperature fabrication processes, however, tend to raise costs, and for this reason, research has been ongoing on forming IGZO TFTs at low temperatures 14). At NHK STRL, we have been studying low-temperature formation of IGZO TFTs. We have succeeded in fabricating TFT arrays at temperatures below 200 C by using pulsed DC sputtering *9 for depositing the gate insulator and semiconductor layer 15), and we plan to test the stability of these TFT arrays. Although we have not yet reached the prototyping stage, we have developed a self-aligned *10 fabrication method for oxide TFTs that decreases parasitic capacitance and produces short channels by irradiating from the back side of the substrate with an excimer laser. Moreover, we have devised a method for decreasing the parasitic capacitance beyond that of conventional TFTs 16). *9 A sputtering method that uses a pulse power supply to accelerate ions that go on to collide with the material used to form the film. High-frequency power supplies or DC power supplies are often used for this purpose. *10 A film-formation method that uses a previously formed device pattern as a mask to form and pattern more devices. This method can simplify the pattern alignment process and reduce the number of masks. Poly-Si TFTs, meanwhile, have been fabricated on thin sheet glass or metallic foil substrates that can withstand high temperatures 17). In addition, progress has been made on increasing the heat resistance of polyimide films, and a small display with high-mobility poly-si TFTs has been reported 18). The use of poly-si TFTs will enable driver circuits to be integrated and arranged on the periphery of the display surface, and this development should lead to more compact flexible displays. On the other hand, since heat-resistant plastic film is somewhat expensive, there is a need to lower its cost. As described above, various TFTs have been studied for use in flexible displays. Among these, the focus is now on using oxide TFTs to make high-performance displays that can show high-quality images. Organic TFTs, meanwhile, are very promising for extremely flexible displays such as rollable displays, but they have issues such as low mobility and low atmospheric stability. 8. High-picture-quality drive technology for AMO- LED displays As described earlier, a pixel in an AMOLED display is equipped with a switching TFT for selecting that pixel and a driving TFT for controlling the emission of light from an OLED device. In general, TFTs show variations in their mobility and threshold voltage, and these variations fluctuate over time in operation. Since driving TFTs determine the intensity of light emitted by the OLED devices, any variation that arises in the electrical characteristics of the TFTs can lead to non-uniform screen characteristics. A common approach to solving this problem is to make a pixel circuit that can compensate for this variation in the TFT characteristics. There have been many reports on displays that incorporate five or six TFTs in one pixel. The idea here is to compensate for unevenness in brightness due to variation in the driving TFTs through a procedure consisting of initialization, threshold correction, signal writing (mobility correction), and light emission. As an example, a threshold compensation circuit using five TFTs in one pixel is shown in Figure 7 (a) 19). In this circuit, GL1, GL2, and GL3 denote control signals for threshold correction, V 0 denotes the reference voltage used in threshold correction, V anode denotes the powersupply voltage for driving light emission from the OLED device, and V cath denotes the cathode voltage. As the name implies, the initialization period involves a process by which the voltage between the source and drain is made higher than the threshold voltage by having current flow through the driving TFT. Next, in the threshold correction and signal writing period, the source potential of the driving TFT is varied by putting it in a floating state and the voltage between the gate and source of the driving TFT is set to the threshold voltage V t. In this state, writing the data voltage of the signal to be displayed by that pixel will cause the voltage of the threshold-corrected data signal to be recorded in the capacitor C. Finally, in the light emission period, current is made to flow through the driving TFT by applying the threshold-corrected voltage between the gate and source (the voltage recorded in C) and the OLED 15

DATA GL1 GL2 GL3 V 0 V anode Initialization Threshold correction and signal writing Light emission Driving TFT DATA GL1 C GL2 OLED GL3 V cath (a) A pixel-variation compensation circuit using five TFTs DATA WS C A V cc (pulse) OLED Driving TFT B V cath WS V cc DATA Point A potential Point B potential Light emission V ofs V ofs Threshold correction preparation (b) A pixel-variation compensation circuit using two TFTs Figure 7: Examples of pixel-variation compensation circuits V t Threshold correction V sig Light emission Signal writing and mobility correction device is made to emit light. Next, the pixel circuit shown in Figure 7 (b) has the same basic configuration as the one shown in Figure 5 consisting of two TFTs and one storage capacitor, but it also has high-mobility poly-si TFTs and a variationcompensation circuit that can perform the operations described above at high speed 20). In this figure, WS denotes the pixel-selection signal, V cc denotes the voltage for driving the OLED device to emit light (varying this voltage enables it to be used for threshold correction as well), V cath denotes the cathode voltage, V ofs denotes the reference voltage used in the threshold correction, V sig denotes the data voltage of the signal to be displayed, and V t denotes the threshold voltage. In the threshold correction preparation period, point B takes on the same potential as V cc when it is set to a low value, the potential at point A simultaneously drops, and light stops being emitted. Then, in the state in which V ofs is applied to DATA, WS is turned ON and V ofs is written to the potential at point A. At this time, the voltage V gs between the gate and source of the driving TFT can be expressed as V gs = V ofs - V cc > V t. Next, in the threshold correction period, the potential at point B rises to a voltage (V ofs - V t) at which the voltage between points A and B (V gs) becomes the threshold voltage (V t), thereby correcting the threshold value. Then, in the signal writing and mobility correction period, the potential at point A takes on the data voltage to be displayed at that pixel, and at the same time, the mobility is corrected in accordance with the performance of the driving TFT. Finally, in the light emission period, the OLED device is driven in a state in which the threshold and mobility have been corrected. 9. Conclusion Good progress is being made in the research and development of various TFTs, and practical flexible displays are nearly at hand. TFT semiconductors for use in flexible displays mainly consist of the silicon, oxide, and organic types, and flexible TFT arrays using substrates appropriate for each type of semiconductor fabrication process have been manufactured. Among these different TFTs, oxide TFTs are promising for large-screen displays, since they feature relatively high charge mobility and make it easy to fabricate large-area TFT arrays. Many institutions have researched oxide TFTs, but issues remain such as the need to improve the devices stability and electrical characteristics. Furthermore, to develop TFTs for use in the ultra-flexible sheet-type displays of the future, suitable manufacturing processes will have to be established and new semiconductor materials centered on organic semiconductors will have to be developed. 16

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