Display device

According to one embodiment, a display device includes an insulating layer, a display unit, and an organic EL layer. The display unit is provided on a major surface of the insulating layer and includes a plurality of gate lines, a plurality of signal lines, a plurality of power source lines and a plurality of pixel units arranged in a matrix configuration. The EL layer is provided on the display unit. Each pixel unit includes a drive transistor and a resistor. The drive transistor includes a drive gate electrode, a drive source electrode, and a drive drain electrode. The drive source electrode or the drive drain electrode is connected to one of the power source lines. An end of the resistor is connected to the drive gate electrode. An other end of the resistor is connected to one of the gate line, the signal line, and the power source line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2011-065272, filed on Mar. 24, 2011; the entire contents of which are incorporated herein by reference.

FIELD

BACKGROUND

Organic electroluminescence (EL) display devices, which are self-luminous elements, draw attention these days as flat display devices, and are actively researched. Since the organic EL display device uses a self-luminous element, it has features such as requiring no backlight, having a wide viewing angle of the image, and having high-speed responsiveness to be suitable for video reproduction, as compared to liquid crystal display devices in which the intensity of the transmitted light from a backlight is controlled by a liquid crystal cell including a pixel circuit.

The organic EL display device can use a simple (passive) matrix system and an active matrix system as the driving system of the device similarly to liquid crystal display devices. In the active matrix system, the current flowing through an organic EL element is controlled by an active element, such as a thin-film transistor, provided in the same pixel circuit as that of the organic EL element. The active matrix display device can perform large-size and high-definition display.

In the active matrix organic EL display device, each pixel includes a drive transistor connected in series to the organic EL element and a write transistor that writes a signal voltage in accordance with the image signal on the gate of the drive transistor. The drive transistor operates as a constant current source corresponding to the image signal. Hence, to ensure the uniformity of display, it is required to suppress the characteristic variation of the drive transistor between pixels to a very small level.

DETAILED DESCRIPTION

In general, according to one embodiment, a display device includes an insulating layer, a display unit, and an organic EL layer. The display unit is provided on a major surface of the insulating layer and includes a plurality of gate lines, a plurality of signal lines, a plurality of power source lines and a plurality of pixel units arranged in a matrix configuration. The organic EL layer is provided on the display unit. Each pixel unit includes a drive transistor and a resistor. The drive transistor includes a drive gate electrode, a drive source electrode, and a drive drain electrode. The drive source electrode or the drive drain electrode is connected to one of the power source lines. An end of the resistor is connected to the drive gate electrode, and an other end of the resistor is connected to one of the gate line, the signal line, and the power source line.

Various embodiments will described hereinafter with reference to the accompanying drawings.

The drawings are schematic or conceptual; and the relationships between the thickness and width of portions, the proportions of sizes among portions, etc., are not necessarily the same as the actual values thereof. Further, the dimensions and proportions may be illustrated differently among drawings, even for identical portions.

In the specification of this application and the drawings, components similar to those described in regard to a drawing therein above are marked with the same reference numerals, and a detailed description is omitted as appropriate. An active matrix organic EL display device is used as the display device.

First Embodiment

A display device according to a first embodiment is described.

FIG. 1shows a plan view of the display device501.

The display device501according to the embodiment includes a display region100(a display unit) in which images are displayed and a peripheral region200(a peripheral unit) that is a region other than the display region.

The display region100includes pixel units1. The peripheral region200includes a signal line drive circuit2, a control line drive circuit3, and a controller4. The controller4is connected to the signal line drive circuit2and the control line drive circuit3, and performs the timing control of the operation of the signal line drive circuit2and the control line drive circuit3.

The signal line drive circuit2is connected to the pixel units1by a plurality of signal lines Vsig provided along the column direction inFIG. 1. The control line drive circuit3is connected to the pixel units1by a plurality of control lines CL provided along the row direction inFIG. 1. The signal line drive circuit2supplies a signal voltage corresponding to the image signal to the pixel unit1through the signal line Vsig. The control line drive circuit3supplies a scan line drive signal to the pixel unit1through the control line CL.

The pixel unit1includes an organic EL element that emits light based on the current supplied and the like.

FIG. 2is a view showing a partial cross section of the display region of the display device501according to the first embodiment.

A TFT protection film117is provided on the source electrode116S, the drain electrode116D, and the gate insulating film113. A pixel electrode118is provided on part of the TFT protection film117. The pixel electrode118is connected to the drain electrode116D through a bank provided in the TFT protection film117. An organic EL layer120is provided on the pixel electrode118. A common electrode121is provided on the organic EL layer120. In a region above the thin-film transistor10, the organic EL layer120and the common electrode121are provided above the TFT protection film117via a passivation film119. A sealing film122is provided on the common electrode121.

An insulative substrate such as a glass substrate and a plastic substrate, for example, may be used as the substrate110. As the plastic substrate, for example, PET (polyethylene terephthalate), PEN (polyethylene naphthalate), PES (polyether sulfone), PI (polyimide), and the like may be used. As the array barrier film111, a single layer of silicon oxide (SiOX, X being an arbitrary plus value) and silicon nitride (SiNX, X being an arbitrary plus value) or a stacked layer thereof is used. A conductive material such as a high melting point metal, such as MoW, Ta, and W, for example, may be used for the gate electrode112. In addition, an Al alloy containing Al treated against hillocks as a main component may be used for the gate electrode112. Also a stacked layer of Al and a high melting point metal may be used for the gate electrode112.

An insulative material such as silicon oxide (SiOX), for example, may be used for the gate insulating film113. In addition to silicon oxide, silicon nitride (SiNX), silicon oxynitride (SiON), and the like may be used, and also a stacked film of films thereof may be used.

An In—Ga—Zn—O-based amorphous oxide semiconductor formed by the reactive sputtering method, for example, may be used for the semiconductor layer114. Also an oxide semiconductor having another composition, polycrystalline silicon, microcrystalline silicon, amorphous silicon, an organic semiconductor, and the like may be used as the semiconductor layer114. In the case where an amorphous oxide semiconductor is used as the semiconductor layer114, the thickness may be set to approximately not less than 10 nm and not more than 100 nm, and is preferably approximately 10 nm in view of electrical characteristics.

An insulative material is used for the channel protection layer115. In the case where an amorphous oxide semiconductor is used for the semiconductor layer114, silicon oxide (SiOX) having higher acid resistance than the semiconductor layer114may be used for the channel protection layer115. In addition, silicon nitride (SiNX), silicon oxynitride (SiON), or the like may be used as the channel protection layer115.

Various conductive materials such as a Ti/Al/Ti stacked film and a Mo/Al/Mo stacked film, for example, may be used for the source electrode116S and the drain electrode116D.

Silicon oxide (SiOX), silicon nitride (SiNX), silicon oxynitride (SiON), aluminum oxide (Al2O3), and the like, for example, may be used for the TFT protection film117.

At least one of indium tin oxide (ITO), a stacked structure of ITO/Ag/ITO, AZO, which is ZnO doped with Al, and the like, for example, may be used for the pixel electrode118.

A photosensitive acrylic resin, a photosensitive polyimide, or the like, for example, may be used for the passivation film119.

A material that emits light by voltage application may be used for the organic EL layer120. The organic EL layer120is electrically connected to the pixel electrode118.

A material such as Al and MgAg, for example, may be used for the common electrode121.

An insulative material such as silicon nitride (SiNO, for example, may be used for the sealing film122.

The examples of the configuration of the thin-film transistor10described above may be applied to the configuration of a write transistor and the configuration of a drive transistor described below. However, the connection relationships among the gate electrode, the source electrode, and the drain electrode of a transistor are determined in conformity with each transistor.

The display device501according to the embodiment includes an insulating layer101, a display unit (e.g. the display region100), and the organic EL layer120. The array barrier film111, for example, is used as the insulating layer101. Also a stacked film of the substrate110and the array barrier film111may be used as the insulating layer101. In addition, depending on circumstances, the substrate110may be used as the insulating layer101.

FIG. 3shows the pixel configuration of the display device501according to the first embodiment.

Although not shown inFIG. 1, also a power source line Vdd is provided parallel to the signal line Vsig. Each pixel unit1includes a write transistor11(the thin-film transistor10), a drive transistor12for current control (the thin-film transistor10), a capacitor20, the organic EL layer120, and a resistance for discharge50(a resistor55).

The gate electrode11gof the write transistor11is connected to a gate line CL, the source electrode11sis connected to the signal line Vsig, and the drain electrode11dis connected to the gate electrode12G of the drive transistor12.

The source electrode12S of the drive transistor12(a drive source electrode) is connected to the pixel electrode118connected to the organic EL layer120. The drain electrode12D of the drive transistor12(a drive drain electrode) is connected to the power source line Vdd. One end of the capacitor20and one end of the resistance for discharge50are connected to the gate electrode12G of the drive transistor12(a drive gate electrode). The other end of the capacitor20and the other end of the resistance for discharge50are connected to the drain electrode12D of the drive transistor12.

The same material as that used for the semiconductor layer114of the write transistor11and the drive transistor12, for example, may be used for the resistance for discharge50. That is, the resistor (e.g. the resistance for discharge50) contains the same material as that used for the semiconductor layer (e.g. the semiconductor layer114) included in the drive transistor12.

The inventors have found the following this time. In the active matrix organic EL display device501, since a structure is used in which the gate electrode12G of the drive transistor12is at a floating potential, the static electricity generated in manufacturing processes remains in the gate electrode12G easily. Therefore, the characteristics of each of the drive transistors experience voltage stress deterioration based on the amount of generated static electricity and the retention state. As a consequence, a variation in characteristics has occurred between drive transistors, causing a decrease in the uniformity of display. In this regard, it has been found that the problem can be suppressed by connecting the resistor55for discharge (e.g. the resistance for discharge50) to the gate electrode12G of the drive transistor12.

The resistor55(e.g. the resistance for discharge50) mentioned above similarly carries out the role of releasing static electricity if provided in one of a position between the gate electrode12G of the drive transistor12and the signal line Vsig, a position between the gate electrode12G and the power source line Vdd, and a position between the gate electrode12G and the gate line CL. In other words, one end of the resistor55(e.g. the resistance for discharge50) is connected to the gate electrode12G of the drive transistor12. The other end of the resistor55(e.g. the resistance for discharge50) is connected to one of the signal line Vsig and the gate line CL. Thereby, similar effects can be obtained. In other words, the other end of the resistor55is connected to at least one of the signal line Vsig, the power source line Vdd, and the gate line CL.

In order to obtain good discharge characteristics, it is notable that the resistance of the resistor55is lower than a resistance of the write transistor11in an OFF-state. On the other hand, in order to obtain good holding characteristics, it is notable that the resistance of the resistor55is more than 100 times a resistance of the write transistor11in an ON-state. For example, the resistance of the write transistor11in the OFF-state is about 1 teraohm. The resistance of the write transistor11in the ON-state is about 1 megaohm. Therefore, it is notable that the resistance of the resistor55is not less than 100 megaohms and is lower than 1 teraohm.

Second Embodiment

FIG. 4is a view showing the pixel configuration of an active matrix display device502according to a second embodiment.

In the second embodiment, a diode for discharge51is used as the resistor55. The anode51A of the diode for discharge51is connected to the gate electrode12G of the drive transistor12, and the cathode51C is connected to the power source line Vdd. When the display device502is driven, since the electric potential of the power source line Vdd is higher than the signal potential, the diode for discharge51does not reduce the retention properties. Also such a configuration can prevent the characteristics of the drive transistor12from changing due to the static electricity caused in processes, similarly to the first embodiment.

That is, the diode for discharge51can contain the same material as that used for the semiconductor layer (e.g. the semiconductor layer114) included in the drive transistor12.

In order to obtain good discharge characteristics, it is notable that the resistance in an inverse direction of the diode for discharge51is lower than the resistance of the write transistor in the OFF-state. On the other hand, in order to obtain good holding characteristics, it is notable that the resistance in the inverse direction of the diode for discharge51is more than 100 times the resistance of the write transistor11in the ON-state. For example, it is notable that the resistance in the inverse direction of the diode for discharge51is not less than 100 megaohms and is lower than 1 teraohm.

It is notable that a resistance in a forward direction of the diode for discharge51is less than the resistance of the write transistor11when a voltage (i.e., Vgs of the write transistor11) between the gate electrode11g(the write gate electrode) and the source electrode11(the write source electrode11s) of the write transistor11is zero volt.

Third Embodiment

FIG. 5is a view showing the pixel configuration of an active matrix display device503according to a third embodiment.

In the third embodiment, a transistor for discharge52(a discharge transistor) is used as the diode for discharge51used as a discharger55. The gate electrode52G and the drain electrode52D of the transistor for discharge52are connected to the gate electrode12G of the drive transistor12. The gate electrode52G of the transistor for discharge52is electrically connected to the drain electrode52D. The source electrode52S of the transistor for discharge52is connected to the power source line Vdd.

That is, the transistor for discharge52can contain the same material as that used for the semiconductor layer (e.g. the semiconductor layer114) included in the drive transistor12.

The transistor for discharge52does not require an additional material and an additional process, and can be formed by the same processes as those for the write transistor11and the drive transistor12.

FIG. 6is a diagram showing the TFT characteristics of an oxide semiconductor.

The vertical axis represents the drain current Id (ampere, A) of the drive transistor12, and the horizontal axis represents the gate voltage (volt, V) of the drive transistor12. A curved line301corresponds to the case where Vd is 15 V, and a curved line302corresponds to the case where Vd is 0.1 V.

As shown inFIG. 6, the TFT using an oxide semiconductor exhibits very low OFF properties as compared to TFTs based on other semiconductor materials. Therefore, in the case where the write transistor is formed of an oxide semiconductor, particularly the floating properties of the gate electrode of the drive transistor in processes are significant. Therefore, by using the configurations described in regard to the first to third embodiments and modifications of the configurations, particularly the unifomization of display is effective.

By using an oxide semiconductor for the transistor for discharge52, a display device less susceptible to static electricity can be formed.

In this embodiment, the transistor for discharge52forms the diode. In order to obtain good discharge characteristics, it is notable that the resistance in an inverse direction of the diode formed with the transistor for discharge52is lower than the resistance of the write transistor in the OFF-state. On the other hand, in order to obtain good holding characteristics, it is notable that the resistance in the inverse direction of the diode formed with the transistor for discharge52is more than 100 times the resistance of the write transistor11in the ON-state. For example, it is notable that the resistance in the inverse direction of the diode is not less than 100 megaohms and is lower than 1 teraohm.

It is notable that a resistance in a forward direction of the diode formed with the transistor for discharge52is less than the resistance of the write transistor11when the voltage between the gate electrode11g(the write gate electrode) and the source electrode11(the write source electrode11s) of the write transistor11is zero volt.

Fourth Embodiment

In the embodiment, a plastic substrate is used as the substrate110used as an insulating layer.

FIG. 7illustrates part of the manufacturing processes for the display device504(the display device501-503) according to the embodiment.

In the embodiment, since the substrate110is plastic, when manufacturing the display device504, the substrate110is temporarily attached to an adhesion layer131on a support substrate130. Then, an array and display elements of the thin-film transistors10and the like are formed on the substrate110in this state. After that, the support substrate130and the adhesion layer131are divided from the substrate110.

FIG. 7is a cross-sectional view showing the process of separating the support substrate130.

FIG. 8is a diagram showing the characteristics of the TFT before and after the process of separating the plastic substrate (the substrate110) from the support substrate130.

The vertical axis represents the drain current Id (ampere, A) of the drive transistor12, and the horizontal axis represents the gate voltage Vg (volt, V) of the drive transistor12.FIG. 8shows the characteristics BS before separating the support substrate130and the characteristics AS after separating it. The TFT characteristic curve after separating the support substrate130(the characteristics AS) is shifted to the positive direction as compared to that before separating it (the characteristics BS). In other words, the threshold voltage is shifted to the positive. When the support substrate130is separated, static electricity may be generated. When the support substrate130is separated, the static electricity may change the characteristics of the drive transistor12from the design values undesirably.

However, by connecting a resistor (e.g. at least one of the resistance for discharge50, the diode for discharge51, and the transistor for discharge52) to the gate electrode of the drive transistor12, the static electricity generated in the process of separating the support substrate130can be released. Therefore, a change in the characteristics of the drive transistor12can be suppressed even if the support substrate130is separated from the substrate110.

Also the embodiment can prevent the characteristics of the drive transistor12from changing due to the static electricity caused in processes.

The embodiment can provide a display device with increased uniformity of the display of the display device.

The invention is not limited to these embodiments described above. For example, one skilled in the art may appropriately select specific configurations of components of transistors and display devices from known art and similarly practice the invention. Such practice is included in the scope of the invention to the extent that similar effects thereto can be obtained.

Moreover, all display devices practicable by an appropriate design modification by one skilled in the art based on the display devices described above as embodiments of the invention also are within the scope of the invention to the extent that the spirit of the invention is included.