The present invention relates to a method for driving a liquid crystal display device. Further, the present invention relates to a liquid crystal display device or an electronic device including the liquid crystal display device.
Liquid crystal display devices ranging from a large display device such as a television receiver to a small display device such as a mobile phone have been spreading. From now on, products with higher added values will be needed and are being developed. In recent years, in view of increase in concern about global environment and improvement in convenience of mobile equipment, development of liquid crystal display devices with low power consumption has attracted attention.
In Non-Patent Document 1, is disclosed a structure of a liquid crystal display device where refresh rates differ between the mode of moving image display and the mode of still image display for reducing power consumed by the liquid crystal display device.
In Non-Patent Document 2, is disclosed a structure of a transflective liquid crystal display device where a color image is displayed using transmitted light and a monochrome image is displayed using reflected light for reducing power consumed by the liquid crystal display device.
As in Non-Patent Document 1, consumed power can be reduced by lowering refresh rate of when a still image is displayed. However, the structure disclosed in Non-Patent Document 1 has a problem of an insufficient reduction in power consumption because the power of the liquid crystal display is mainly consumed by lighting a backlight. Further, the structure disclosed in Non-Patent Document 2 has a problem of insufficient contrast of a displayed image particularly under high-intensity external light, due to light scattering in a reflective pixel portion or the like.
It is an object of one embodiment of the present invention is to suppress a reduction in contrast due to light scattering in a reflective pixel portion or the like to reduce consumed power.
One embodiment of the present invention is a method for driving a transflective liquid crystal display device including a plurality of pixels each including a plurality of light-transmitting pixel portions and a reflective pixel portion, which includes the steps of: in a first period, supplying a first image signal to the plurality of light-transmitting pixel portions and a signal for black display to the reflective pixel portion; and in a second period, supplying a second image signal to the plurality of light-transmitting pixel portions and the reflective pixel portion.
One embodiment of the present invention is a method for driving a transflective liquid crystal display device including: a plurality of pixels each including first to third light-transmitting pixel portions and a reflective pixel portion; and a first scan line and a second scan line which are configured to drive the liquid crystal display device, which includes the steps of: in a first period, supplying a first image signal to the first to third light-transmitting pixel portions and a signal for black display to the reflective pixel portion; and in a second period, supplying a second image signal to the first to third light-transmitting pixel portions and the reflective pixel portion. The first light-transmitting pixel portion and the reflective pixel portion are driven by the first scan line, and the second light-transmitting pixel portion and the third light-transmitting pixel portion are driven by the second scan line.
One embodiment of the present invention is a method for driving a transflective liquid crystal display device including: a plurality of pixels each including first to third light-transmitting pixel portions and a reflective pixel portion; and a first scan line and a second scan line which are configured to drive the liquid crystal display device, which includes the steps of: in a first period, supplying a first image signal to the first to third light-transmitting pixel portions and a signal for black display to the reflective pixel portion; and in a second period, supplying a second image signal to the first to third light-transmitting pixel portions and the reflective pixel portion and holding an image of the second image. The first light-transmitting pixel portion and the reflective pixel portion are driven by the first scan line, and the second light-transmitting pixel portion and the third light-transmitting pixel portion are driven by the second scan line.
In the method for driving a liquid crystal display device which is one embodiment of the present invention, the first scan line and the second scan line may drive in this order.
In the method for driving a liquid crystal display device which is one embodiment of the present invention, an operation frequency of a driver circuit which drives the first scan line and the second scan line in the second period may be lower than an operation frequency of the driver circuit which drives the first scan line and the second scan line in the first period.
In the method for driving a liquid crystal display device which is one embodiment of the present invention, the first to third light-transmitting pixel portions may be light-transmitting pixel portions emitting respective colors of red, green, and blue, and the first image signal supplied in the first period may be an image signal corresponding to any of colors of red, green, and blue.
In the method for driving a liquid crystal display device which is one embodiment of the present invention, the second image signal may be a grayscale image signal.
In the method for driving a liquid crystal display device which is one embodiment of the present invention, the holding an image of the second image signal in the second period may be performed by stopping supply of a driver-circuit control signal for driving the first scan line and the second scan line.
According to one embodiment of the present invention, a reduction in contrast due to light scattering in the reflective pixel portion or the like can be suppressed without increasing the number of driver circuits, wirings, and the like, so that consumed power can be reduced.
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, the present invention can be carried out in many different modes, and it is easily understood by those skilled in the art that modes and details of the present invention can be modified in various ways without departing from the purpose and the scope of the present invention. Accordingly, the present invention is not construed as being limited to the described content of the embodiments and included herein. Note that identical portions or portions having the same function in all drawings illustrating the structure of the invention that are described below are denoted by the same reference numerals.
Note that the size, the thickness of a layer, distortion of the waveform of a signal, and a region of each structure illustrated in the drawings and the like in the embodiments are exaggerated for simplicity in some cases. Therefore, embodiments of the present invention are not limited to such scales.
Note that in this specification, terms such as “first”, “second”, “third”, and “N-th” (N is a natural number) are used in order to avoid confusion among components and do not limit the number of the components.
In this embodiment, a method for driving a liquid crystal display device will be described with reference to a circuit diagram of a pixel in the liquid crystal display device, a timing chart for describing operation thereof, and the like.
In the first light-transmitting pixel portion 103, a first terminal of the pixel transistor 107R is connected to the first signal line 102A, and a gate of the pixel transistor 107R is connected to the first scan line 101A. A first electrode (pixel electrode) of the liquid crystal element 108R is connected to a second terminal of the pixel transistor 107R, and a second electrode (counter electrode) of the liquid crystal element 108R is connected to a common potential line 110 (common line). A first electrode of the capacitor 109R is connected to the second terminal of the pixel transistor 107R, and a second electrode of the capacitor 109R is connected to a capacitor line 111.
In the second light-transmitting pixel portion 104, a first terminal of the pixel transistor 107B is connected to the first signal line 102A, and a gate of the pixel transistor 107B is connected to the second scan line 101B. A first electrode (pixel electrode) of the liquid crystal element 108B is connected to a second terminal of the pixel transistor 107B, and a second electrode (counter electrode) of the liquid crystal element 108B is connected to the common potential line 110 (common line). A first electrode of the capacitor 109B is connected to the second terminal of the pixel transistor 107B, and a second electrode of the capacitor 109B is connected to the capacitor line 111.
In the third light-transmitting pixel portion 105, a first terminal of the pixel transistor 107G is connected to the second signal line 102B, and a gate of the pixel transistor 107G is connected to the second scan line 101B. A first electrode (pixel electrode) of the liquid crystal element 108G is connected to a second terminal of the pixel transistor 1070, and a second electrode (counter electrode) of the liquid crystal element 108G is connected to the common potential line 110 (common line). A first electrode of the capacitor 109G is connected to the second terminal of the pixel transistor 107G, and a second electrode of the capacitor 109G is connected to the capacitor line 111.
In the reflective pixel portion 106, a first terminal of the pixel transistor 107ref is connected to the second signal line 102B, and a gate of the pixel transistor 107ref is connected to the first scan line 101A. A first electrode (pixel electrode) of the liquid crystal element 108ref is connected to a second terminal of the pixel transistor 107ref, and a second electrode (counter electrode) of the liquid crystal element 108ref is connected to the common potential line 110. A first electrode of the capacitor 109ref is connected to the second terminal of the pixel transistor 107ref, and a second electrode of the capacitor 109ref is connected to the capacitor line 111.
Note that the pixel transistor 1078, the pixel transistor 1070, the pixel transistor 107B, and the pixel transistor 107ref preferably each include an oxide semiconductor in a semiconductor layer. The oxide semiconductor here is an intrinsic (i-type) oxide semiconductor which is highly purified by removal of hydrogen that is an n-type impurity so that impurities other than main components of the oxide semiconductor are contained as little as possible. In addition, the highly purified oxide semiconductor includes extremely few carriers (close to zero), and the carrier concentration thereof is lower than 1×1014/cm3, preferably lower than 1×1012/cm3, much preferably 1×1011/cm3. A considerable reduction in carriers in the oxide semiconductor enables the off current of the transistor to decrease. Specifically, a transistor including the above oxide semiconductor layer can realize the off current which is less than or equal to 10 aA/μm (1×10−17 A/μm), preferably less than or equal to 1 aA/μm (1×10−18 A/μm), much preferably less than or equal to 10 zA/μm (1×10−20 A/μm), per micrometer in channel width at room temperature. In other words, in circuit design, the oxide semiconductor layer can be regarded as an insulator when the transistor is off. In the pixel 100 including pixel portions provided with transistors each of which includes an oxide semiconductor and has significantly low off current, an image can be maintained even when the writing frequencies of an image signal (also referred to as a video voltage, a video signal, or a video data) are low, and thus the refresh rate can be reduced. Therefore, a period during which a driver circuit is stopped driving the first scan line, the second scan line, and the signal line can be provided; accordingly, consumed power can be reduced.
Note that a transistor is an element having at least three terminals of gate, drain, and source. The transistor includes a channel region between a drain region and a source region, and current can flow through the drain region, the channel region, and the source region. Here, since the source and the drain of the transistor may change depending on the structure, the operating condition, and the like of the transistor, it is difficult to define which is a source or a drain. Therefore, in this document (the specification, the claims, the drawings, and the like), a region functioning as a source and a drain is not called the source or the drain in some cases. In such a case, for example, one of the source and the drain may be referred to as a first terminal and the other thereof may be referred to as a second terminal. Alternatively, one of the source and the drain may be referred to as a first electrode and the other thereof may be referred to as a second electrode. Further alternatively, one of the source and the drain may be referred to as a source region and the other thereof may be called a drain region.
Note that when it is explicitly described that “A and B are connected”, the case where A and B are electrically connected, the case where A and B are functionally connected, and the case where A and B are directly connected are included therein.
Note that a pixel corresponds to a display unit where the first to third light-transmitting pixel portions and the reflective pixel portion which are elements capable of controlling brightness are combined. For example, the first to third light-transmitting pixel portions (also referred to as subpixels) function as display units capable of controlling brightness of color elements R (red), G (green), and B (blue) which are combined for displaying color images when moving images are displayed. The reflective pixel portion functions as a display unit capable of controlling brightness of grayscale (or monochrome) images when a still image is displayed.
Since in this embodiment, an example in which color display is performed using three color elements of RGB in the light-transmitting pixel portions is given, the specific structures of the first to third light-transmitting pixel portions are described. However, the structure described in this embodiment does not particularly limit the number of light-transmitting pixel portions, and a plurality of light-transmitting pixel portions can be employed instead of the first to third light-transmitting pixel portions. For example, four element colors where Y (yellow) is added to RGB may be used for a plurality of light-transmitting pixel portions, and alternatively, combination of colors other than RGB may be used. Further, a signal line and a scan line connected to the pixel may be provided as appropriate in accordance with a plurality of light-transmitting pixel portions and connected to the plurality of light-transmitting pixel portions.
Note that “voltage” refers to a potential difference between a given potential and a reference potential (e.g., a ground potential) in many cases. Accordingly, voltage, potential, and a potential difference can be referred to as potential, voltage, and a voltage difference, respectively.
The common potential supplied to the common potential line 110 may be any potential as long as it serves as a reference with respect to a potential of an image signal supplied to the first electrode of the liquid crystal element. For example, the common potential may be a ground potential.
The image signal may be appropriately inverted in accordance with dot inversion driving, source line inversion driving, gate line inversion driving, frame inversion driving, or the like to be input to each pixel.
The potential of the capacitor line 111 may be the same as the common potential. Further, the capacitor line 111 may be supplied with another signal.
Of each of the liquid crystal elements 108R, 108G, 108B, and 108ref, the second electrode is preferably provided to overlap with the first electrode thereof. The first electrode and the second electrode of each liquid crystal element may have a variety of opening patterns. A liquid crystal material sandwiched between the first electrode and the second electrode in each liquid crystal element may be any of a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal (PDLC), a ferroelectric liquid crystal, or an anti-ferroelectric liquid crystal. These liquid crystal materials exhibit a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions. Alternatively, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used.
The first electrode of each of the liquid crystal elements 108R, 1086, and 108B is formed using a light-transmitting material. As examples of the light-transmitting material, indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), gallium-doped zinc oxide (GZO), and the like can be given. On the other hand, the first electrode of the liquid crystal element 108ref is a metal electrode with high reflectivity. Specifically, aluminum, silver, or the like is used. When the surface of the pixel electrode of the liquid crystal element 108ref has unevenness, incident external light can be reflected diffusely. Note that the first electrode, the second electrode, and the liquid crystal material may be collectively referred to as a liquid crystal element.
In
Although it is preferable that the first scan line driver circuit 152A, the second scan line driver circuit 152B, and the signal line driver circuit 153 are provided over the same substrate as the pixel region 151, it is not necessarily to provide them over the same substrate. When the first scan line driver circuit 152A, the second scan line driver circuit 152B, and the signal line driver circuit 153 are provided over the same substrate as the pixel region 151, the number of terminals for external connection to can be reduced; thus downsizing the liquid crystal display device can be achieved.
In the pixel region 151, the pixels 100 are provided (arranged) in matrix. Here, description that “the pixels are provided (arranged) in matrix” includes the case where the pixels are arranged in a straight line and the case where the pixels are arranged in a jagged line, in a longitudinal direction or a lateral direction.
Signals supplied from the terminal portion 154 include a signal for controlling the first scan line driver circuit 152A, the second scan line driver circuit 1528, and the signal line driver circuit 153 (high power supply potential Vdd, low power supply potential Vss, a start pulse SP, and a clock signal CK: hereinafter, referred to as a driver-circuit control signal) and the like in addition to the signal supplied to the common potential line 110 and the capacitor line 111. The first scan line driver circuit 152A, the second scan line driver circuit 152B, and the signal line driver circuit 153 to each of which a driver-circuit control signal is supplied may include a shift register in which flip-flop circuits or the like are cascaded. Image signals for color display are supplied through the first signal line 102A to the liquid crystal element 108R in the first light-transmitting pixel portion 103 and the liquid crystal element 108B in the second light-transmitting pixel portion 104 and through the second signal line 102B to the liquid crystal element 108G in the third light-transmitting pixel portion 105. An image signal for black grayscale display is supplied through the second signal line 102B to the liquid crystal element 108ref in the reflective pixel portion 106.
Next, operation of the liquid crystal display device is described with reference to
As shown in
The cycle of one frame period (or frame frequency) is preferably less than or equal to 1/60 sec (more than or equal to 60 Hz) in the moving-image display period 301. The high frame frequency can prevent a viewer from perceiving flickering. In the still-image display period 302, the cycle of one frame period is extremely long, for example, longer than or equal to one minute (less than or equal to 0.017 Hz), so that eye strain can be alleviated as compared to the case where the same image is switched plural times.
When the pixel transistors 107R, 107G, 107B, and 107ref each include an oxide semiconductor as a semiconductor layer, carriers in the oxide semiconductor can be drastically reduced as described above, which results in a decrease in off current. Thus, in the pixel, an electrical signal such as an image signal can be held for a longer time, and a writing interval can be set longer. As a result, the cycle of one frame can be set long, and a reduction in the number of operations of writing an image signal the same as that written in the previous frame period, i.e., a reduction in refresh rates can be achieved in the still-image display period 302. Therefore, the effect of reducing consumed power can be improved.
The moving-image display period 301 shown in
In the moving-image display period 301, an image signal is supplied to the first signal line 102A and the second signal line 102B from the signal line driver circuit 153 in order to perform color display (in
In the still-image display period 302 shown in
Note that the image signal of black-and-white grayscale indicates an image signal for displaying a grayscale or monochrome image on the reflective pixel portion. Therefore, in the case where the image signal of black-and-white grayscale is written to the pixel portions provided with color filters, the pixel portions function as monochrome pixel portions, and colors of the monochrome pixel portions are mixed, so that the grayscale or monochrome images are displayed. The image signal of black-and-white grayscale supplied to the reflective pixel portion 106 in the still-image display period 302 is referred to as a second image signal.
As for the stop of the supply of the driver-circuit control signals in the still-image display period 302, in the case where the holding period of the image signal which has been written is short, a configuration in which supply of the high power supply potential Vdd and the low power supply potential Vss is not stopped may be originally employed. Thus, an increase in power consumption due to repetition of stop and start of supply of the high power supply potential Vdd and the low power supply potential Vss can be reduced, which is favorable.
In the case where a plurality of pixel portions are employed for the first to third light-transmitting pixel portions, the first image signal and the image signal for displaying black grayscale may be supplied to the plurality of light-transmitting pixel portions and the reflective pixel portion, respectively in the moving-image display period 301, and the second image signal may be supplied to both the plurality of light-transmitting pixel portions and the reflective pixel portion in the still-image display period 302.
Next, the moving-image display period 301 and the still-image display period 302 of
First,
The first scan signal shown in
In the moving-image display period 301, the scan lines are sequentially selected by inputting the first scan signal and the second scan signal alternately. Specifically, the scan line in a first row is selected by input of the first scan signal, and the scan line in a second row is selected by input of the second scan signal, i.e., the scan line in a (2n−1)-th row is selected by input of the first scan signal, and then, the scan line in a 2n-th row is selected by input of the second scan signal. As shown in
Further, image signals corresponding to pixels are supplied from the first signal line 102A and the second signal line 102B to the pixels in accordance with the first scan signal and the second scan signal which are the signals controlling on/off of the pixel transistors and supplied to the scan lines. Specifically, as shown in
In addition, in the moving-image display period 301, the backlight for transmitting light through the first to third light-transmitting pixel portions 103 to 105 each provided with the color filter operates. Further, in the moving-image display period 301, the first scan line driver circuit 152A, the second scan line driver circuit 152B, and the signal line driver circuit 153 are supplied with the driver-circuit control signals for outputting the following signals at the given timing: the first scan signal, the second scan signal, the image signal supplied to the first signal line 102A, and the image signal supplied to the second signal line 102B.
In other words, the moving-image display period 301 is a period for selecting the first light-transmitting pixel portion and the reflective pixel portion by inputting the first scan signal, selecting the second light-transmitting pixel portion and the third light-transmitting pixel portion by inputting the second scan signal, supplying the first image signal for performing color display on the first to third light-transmitting pixel portions, and supplying the image signal for displaying black on the reflective pixel portion. Specifically, the image signals written to the pixel portions in the moving-image display period 301 are shown in
By repeating the above operation, the image signal for R (red) display supplied to the first light-transmitting pixel portion 103, the image signal for B (blue) display supplied to the second light-transmitting pixel portion 104, and the image signal for G (green) display supplied to the third light-transmitting pixel portion 105 are changed while the image signal of the black grayscale is supplied to the reflective pixel portion 106. As a result, a viewer can perceive color display of a moving image. In addition, the image signal for black grayscale display on the reflective pixel portion 106 is supplied in the moving-image display period 301 as shown in
Note that although a structure in which the first image signal is supplied on the condition that the first light-transmitting pixel portion 103, the second light-transmitting pixel portion 104, and the third light-transmitting pixel portion 105 correspond to RGB respectively is described with
Next,
During the still-image signal writing period in the still-image display period 302, the scan lines are selected by the first scan signal and the second scan signal in order to write an image signal for displaying a black-and-white grayscale image depending on whether reflected light is transmitted or not. The scan lines are sequentially selected in such a manner that the scan line in the first row is selected by the first and second scan signals, and the scan line in the second row is selected by the first and second scan signals. The scan line in a (2n−1)-th row and the scan line in a 2n-th row can be selected at the same timing. That is, the first scan line 101A and the second scan line 101B connected to one pixel are selected at the same timing as shown in
Note that during the still-image signal writing period of the still-image display period 302, the backlight does not operate.
During the still-image signal writing period in the still-image display period 302, image signals corresponding to the pixels which display a black-and-white grayscale image depending on whether reflected light is transmitted or not are supplied from the first signal line 102A and the second signal line 102B to the pixels in accordance with the first scan signal and the second scan signal which are the signals controlling on/off of the pixel transistors and supplied to the scan lines. Specifically, as shown in
Note that during the still-image signal writing period in the still-image display period 302, the driver-circuit control signals are supplied to the first scan line driver circuit 152A, the second scan line driver circuit 152B, and the signal line driver circuit 153 so as to output the first scan signal, the second scan signal, and the image signal, respectively, at the given timing.
As described, during the still-image signal writing period in the still-image display period 302, the image signal of black-and-white grayscale is supplied to the first to third light-transmitting pixel portions 103 to 105 in addition to the reflective pixel portion 106. Although the backlight does not operate during the still-image signal writing period in the still-image display period 302 in
Next, during the still-image signal holding period in the still-image display period 302, the image signal for displaying a black-and-white grayscale image which has been written is held, so that a still image is displayed. At this time, additional image signals supplied to the first signal line 102A and the second signal line 102B by the first scan signal and the second scan signal are not written, the backlight does not operate, and the driver-circuit control signal is not supplied. Therefore, power consumed by the backlight and the driver-circuit control signal can be reduced; thus, lower power consumption can be achieved. As for the holding of the still image, the image signal written into a pixel is held by a pixel transistor whose off current is extremely small; therefore, the black-and-white grayscale still image can be held for longer than or equal to one minute. In addition, the still image may be held in the following manner: before the level of the image signal held is lowered after a certain period of time, a new still image signal which is the same image signal as the still image signal of the previous period is written and the still image is held again.
During the still-image signal holding period, the frequency of operation such as writing of an image signal can be reduced. When seeing an image formed by writing image signals a plurality of times, the human eyes recognize images switched a plurality of times, which might lead to eyestrain. With a structure where the frequency of writing of image signals is reduced as described in this embodiment, eyestrain can be alleviated.
In the above-described manner, according to an embodiment of the present invention, reduction in contrast due to light scattering in a reflective pixel portion or the like can be suppressed and power consumption can be reduced without making the structure complicated, for example, increase in the number of driver circuits, wirings, and the like.
This embodiment can be implemented in combination with any of the structures described in the other embodiments as appropriate.
In this embodiment, a structure corresponding to the pixel of the liquid crystal display device which is described in Embodiment 1 with
First, an example of the layout of the pixel in the liquid crystal display device is described with reference to
The pixel illustrated in
The conductive layer 851 has regions functioning as a gate electrode and a scan line. The semiconductor layer 852 has regions functioning as a semiconductor layer of the pixel transistors. The conductive layer 853 has regions functioning as wirings and source and drain of the pixel transistors. The transparent conductive layer 854 has regions functioning as pixel electrodes of the liquid crystal elements in the light-transmitting pixel portions. The reflective conductive layer 855 (illustrated in only
As shown in
Note that the reflective conductive layer 855 preferably has an unevenness surface in order to reflect the incident external light diffusely.
In the layouts of the pixel in
Further in the layouts of the pixel in
In the layouts of the pixels in
Next, the structure of the cross-sectional view illustrated in
A transistor 410 illustrated in
The transistor 410 includes, over a substrate 400 having an insulating surface, a gate electrode layer 401, a gate insulating layer 402, an oxide semiconductor layer 403, a source electrode layer 405a, and a drain electrode layer 405b. An insulating layer 407 is provided to cover the transistor 410 and be stacked over the oxide semiconductor layer 403. A protective insulating layer 409 is formed over the insulating layer 407.
In this embodiment, as described above, the oxide semiconductor layer 403 is used as a semiconductor layer. As an oxide semiconductor used for the oxide semiconductor layer 403, an oxide of four metal elements such as an In—Sn—Ga—Zn—O-based metal oxide; an oxide of three metal elements such as an In—Ga—Zn—O-based metal oxide, an In—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metal oxide, a Sn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide, or a Sn—Al—Zn—O-based metal oxide; an oxide of two metal elements such as an In—Zn—O-based metal oxide, a Sn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, a Zn—Mg—O-based metal oxide, a Sn—Mg—O-based metal oxide, or an In—Mg—O-based metal oxide; or an oxide of one metal element such as an In—O-based metal oxide, a Sn—O-based metal oxide, or a Zn—O-based metal oxide can be used. Further, SiO2 may be contained in the above metal oxide. Here, for example, an In—Ga—Zn—O-based metal oxide is an oxide including at least In, Ga, and Zn, and there is no particular limitation on the composition ratio thereof. Further, the In—Ga—Zn—O-based oxide semiconductor may contain an element other than In, Ga, and Zn.
For the oxide semiconductor layer 403, a thin film, represented by the chemical formula, InMO3(ZnO)m (m>0) can be used. Here, M represents one or more metal elements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.
In the transistor 410 including the oxide semiconductor layer 403, the current value in an off state (the off current) can be small. Thus, the time for holding an electric signal such as image data can be extended, and an interval between writings can be extended. Accordingly, the refresh rate can be reduced, which leads to an effect of suppressing power consumption.
Although there is no particular limitation on a substrate used for the substrate 400 having an insulating surface, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like can be used.
In the bottom-gate transistor 410, an insulating layer serving as a base film may be provided between the substrate and the gate electrode layer. The base film has a function of preventing diffusion of an impurity element from the substrate, and can be formed to have a single-layer structure or a stacked structure including any of a silicon nitride layer, a silicon oxide layer, a silicon nitride oxide layer, and a silicon oxynitride layer.
The gate electrode layer 401 can be formed to have a single-layer or stacked-layer structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material which contains any of these materials as its main component.
The gate insulating layer 402 can be formed with a single-layer structure or a stacked structure using any of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a silicon nitride oxide layer, an aluminum oxide layer, an aluminum nitride layer, an aluminum oxynitride layer, an aluminum nitride oxide layer, and a hafnium oxide layer by a plasma CVD method, a sputtering method, or the like. For example, by a plasma CVD method, a silicon nitride layer (SiNy (y>0)) with a thickness of greater than or equal to 50 nm and less than or equal to 200 nm is formed as a first gate insulating layer, and a silicon oxide layer (SiOx (x>0)) with a thickness of greater than or equal to 5 nm and less than or equal to 300 nm is formed as a second gate insulating layer over the first gate insulating layer, so that a gate insulating layer with a total thickness of 200 nm is formed.
For a conductive film used for the source electrode layer 405a and the drain electrode layer 405b, an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, an alloy containing any of these elements, or an alloy film containing a combination of any of these elements can be used, for example. Alternatively, a structure may be employed in which a refractory metal layer of Ti, Mo, W, or the like is stacked over and/or below a metal layer of Al, Cu, or the like. In addition, heat resistance can be improved by using an Al material to which an element (Si, Nd, Sc, or the like) which prevents generation of a hillock or a whisker in an Al film is added.
The conductive film to be the source electrode layer 405a and the drain electrode layer 405b (including a wiring layer formed using the same layer as the source and drain electrode layers) may be formed using a conductive metal oxide. As the conductive metal oxide, indium oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO), indium oxide-tin oxide alloy (In2O3—SnO2, which is abbreviated to ITO), indium oxide-zinc oxide alloy (In2O3—ZnO), or any of these metal oxide materials in which silicon oxide is contained can be used.
As the insulating layer 407, typically, an inorganic insulating film such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, or an aluminum oxynitride film can be used.
As the protective insulating layer 409, an inorganic insulating film such as a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, or an aluminum nitride oxide film can be used.
A planarization insulating film may be formed over the protective insulating layer 409 in order to reduce surface roughness due to the transistor. As the planarization insulating film, an organic material such as polyimide, acrylic, or benzocyclobutene can be used. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material) or the like. The planarization insulating film may be formed by stacking a plurality of insulating films formed from these materials. Note that over the protective insulating layer 409, a necessary component such as a reflective conductive layer or a liquid crystal layer may be formed as appropriate.
This embodiment can be combined with any of the other embodiments as appropriate.
In this embodiment, an example of an electronic device including the liquid crystal display device described in the above embodiments will be described.
When a transflective liquid crystal display device is used as the display portion 9631, the e-book reader having the structure illustrated in
The configuration and operation of the charge and discharge control circuit 9634 illustrated in
First, an example of operation in the case where power is generated by the solar battery 9633 using external light is described. The voltage of power generated by the solar battery is raised or lowered by the converter 9636 so that the power has voltage for charging the battery 9635. Then, when the power from the solar battery 9633 is used for the operation of the display portion 9631, the switch SW1 is turned on and the voltage of the power is raised or lowered by the converter 9637 so as to be voltage needed for the display portion 9631. In addition, when display on the display portion 9631 is not performed, the switch SW1 is turned off and the switch SW2 is turned on, whereby the battery 9635 is charged.
Next, operation in the case where power is not generated by the solar battery 9633 using external light is described. The voltage of power stored in the battery 9635 is raised or lowered by the converter 9637 by turning on the switch SW3. Then, power from the battery 9635 is used for the operation of the display portion 9631.
Note that although the solar battery 9633 is described as an example of a means for charge, the battery 9635 may be charged with another means. In addition, a combination of the solar battery 9633 and another means for charge may be used.
This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.
This application is based on Japanese Patent Application serial no. 2010-019237 filed with Japan Patent Office on Jan. 29, 2010, the entire contents of which are hereby incorporated by reference.
Number | Date | Country | Kind |
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2010-019237 | Jan 2010 | JP | national |
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