This is a general term to describe thin and flat display-devices that have replaced cathode-ray tubes.

Compared to the previously used CRT displays, flat panel displays offer the benefit of less distortion of the image and no depth requirement. Therefore, even a small space can accommodate a large screen display.

During earlier days of FPD’s application, we encountered several technical issues, such as power consumption, brightness, coloring, field of view, number of reproducible colors, and response time. However, these challenges have been overcome one after another.

Flat panel displays include liquid crystal displays, organic EL displays, and electronic paper. Plasma displays, field emission displays, and other display devices that have been developed and sold in the past are also flat panel displays. FPD are also used in TVs, mobile phones, PC displays, laptop computers, mobile music players, etc. and installed in daily-use equipment. It is expected that the application of FPD will be further expanded in the future. Therefore, technological development is always needed to meet the demands for larger and thinner screens, lower costs, higher definition, higher brightness, higher color reproducibility, etc. 

➀Liquid Crystal Display
By applying voltage, the direction of liquid crystal molecules will be changed to control the amount of light passing through and display the image. Since the liquid crystal itself does not emit light, a backlight unit and a device making use of the reflected light of ambient light are placed behind the display unit. (Refer to the section on optical films.)
In general, two types of LCD panels are available: In-Plane Switching (IPS) and Vertical Alignment (VA). IPS panels have wide viewing angles and little change in color and brightness. In other words, the screen can be seen clearly even from a wide angle.
The downside is that the brightness of the black part is slightly higher than that of the VA panels, so low contrast makes it difficult to show the difference in brightness between white and black.
On the other hand, most computers now use TN panels, which have quick response and speed. Furthermore, because of its relatively low cost, TN panels help to lower the whole price of the device. The demerit is that it has a narrow viewing angle and obvious color variation.

②Organic EL
Electro-Luminescence (EL) is a kind of light-emitting diode. The light emitter, which is a thin film, will emit light when a voltage is applied to it. An organic EL uses organic materials like diamines that is a compound used in gray hair dyeing and hair coloring as the light emitter. In contrast, inorganic EL uses inorganic materials such as zinc sulfide.
A major feature of organic EL is that it can emit light when connected to DC current at a much lower voltage than inorganic EL. Thanks to great R&D efforts, it is being commercialized as a third kind of display after CRTs and LCDs,
Characterized by low power supply, high brightness, and sharp and clear images, Organic EL also enjoys rapid response. Furthermore, unlike LCDs, it does not need backlights, color filters, polarizing plates, etc., so it is expected to be thin and low in power consumption and cost. 
However, compared with LCDs, organic EL is weak in durability and moisture resistance. Therefore, research is being conducted in this area to identify new materials and technologies to solve these problems.

③Electronic paper
Electronic paper is a sheet display that combines the merits of paper and electronic display. Among various types of electronic paper, electrophoretic display and twisting‐ball display are two reprehensive ways.  
In the electrophoretic display, particles of colored oil and pigments are placed in the microcapsules, and when a voltage is applied, the particles of pigments move to display an image.
The twisting-ball display shows white-and-black images by controlling the orientation of spherical particles with an electric field, which is painted white and black for each hemisphere. Either way, electronic paper feels more like paper than digital terminals. Since it is simple and thin and works on the power of dry cells, it can be rolled into a cylinder or attached to other objects.

④Plasma display
The light emission principle of this display is similar to fluorescent lights. It works by sandwiching a neon/xenon gas mixture between two glass plates to discharge plasmas and generate ultraviolet radiation. Then the UV radiation irradiates phosphors of different colors painted on the glass plates to emit light.

Compared to other methods, this display has high contrast and a wide viewing angle, so it is used for large-sized TV monitors and indoor display panels. However, it is not suitable for portable mobile devices because of its requirement for high voltage and difficulty in miniaturization. 

【Supplementary explanations】　～Plasma ～
In general, when the temperature of the substance is raised, the state of the substance will change from solid to liquid then from liquid to gas, but when the temperature of the gas is further raised or when an electric field is applied, a phenomenon called ionization will occur. It is a highly activated state in which neutral molecules, positive ions, and electrons are mixed. This fourth state of the substance is called plasma.
The existence of plasma is quite common. For example, it can be found in the sun, aurora, lightning, and others in the natural world, and in fluorescent lights, candle flames, plasma television, etc. around us. Plasma comes in two types: high-temperature plasma where the temperature of all particles is high, and low-temperature plasma where only the temperature of electrons is high. High-temperature plasma is used to generate energy in fusion and arcing processes (welding, cutting, thermal spraying), while low-temperature plasma is used in semiconductor fine processing, synthesis of thin films, surface processing, sterilization, and other industrial applications.

【Supplementary explanations】　～Contrast～
Contrast refers to the difference between darkness and brightness when the screen or image is displayed. It is expressed that the larger the difference between light and dark, the higher the contrast, and the higher the contrast, that is, the sharp and sharp state. The larger the difference, the higher the contrast. High contrast means that the image is sharp and clear. 

⑤Field Emission Display
The light emitting principle here is the same as CRTs. Light will come out when the electron beam emitted from the cathode activates the phosphor on the other side. This display is expected to be thinner.
In recent years, carbon nanotubes that can take a large amount of current are attracting a lot of attention. As the name suggests, carbon nanotubes comprise carbon atoms linked like a mesh and arranged in tube structure. Its diameter is measured in nanometer units and is 50,000 times thinner than human hair.

Thin film transistors used in flat panel displays, especially liquid crystal displays, will be discussed here. 
A thin film transistor is a thin transistor on a glass substrate composed of amorphous silicon or the like. It is used in liquid crystal panels, etc. As for amorphous silicon, it is a semiconductor and has disordered atomic arrangement. 
Thin film transistor liquid crystal uses a thin film transistor processed by the active matrix method and is mostly applied in the LCDs of personal computers. Currently, it is used as the mainstream liquid crystal panel.
The transistor is used to control the display of each dot that makes up the screen, making a uniformly displayed image possible. In addition to high response speed and high contrast, it is also easy to adjust screen brightness by changing the voltage of the transistor.
Even if used in a large-size display, the image quality will not deteriorate, and the viewing angle is extremely wide. 
Amorphous silicon is often used as a component of liquid crystals, but recently polycrystalline silicon has also come into use as a material with a higher quality. Amorphous silicon is easily manufactured in large quantities, making it possible to produce panels of various sizes. However, compared to amorphous silicon, polycrystalline silicon boasts a moving speed of electrons that is 100 times faster. Therefore, it can have excellent performance with regard to response speed and contrast.