The Evolution of TFT Displays

Views: 232     Author: Reshine Display     Publish Time: 2023-09-28      Origin: Site


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The Evolution of TFT Displays

As our society becomes increasingly technological, screens appear to appear almost everywhere. Hundreds of thousands of complex, tiny devices control the pixels that comprise the overall image we see behind those glass displays or flat panel displays. These are known as thin film transistors or TFTs for short.

When and who invented the TFT?

The TFT appeared in 1962, following a series of developments in the field of semiconductors and microelectronics. The Radio Corporation of America (RCA) had spent years experimenting with and developing transistors in the hopes of expanding their applications. Though John Wallmark (a member of the RCA) received the first thin film patent in 1957, it was Paul K. Weimer, also of the RCA, who developed the TFT.

The Evolution of TFTs

1. Before the advent of the TFT, there was the Field Effect Transistor (FET).

The FET is a type of semiconductor device that enables the transistor to amplify, control, or generate electric signals. This transistor was designed to control the flow of current within devices. FETs are typically constructed with the source, drain, and gate, as well as electrodes that allow for contact and conduction with the semiconductor. This device can control the applied voltage through the gate by increasing or decreasing the movement of charge carriers such as electrons or holes (the absence of an electron causes a charged pull) in a process known as carrier mobility, or field-effect mobility for FETs. Charges can be amplified, controlled, or generated more easily with high-mobility semiconductors. The FET can then change the signal strength (from the source) and send it to the destination (the drain and the designated signal recipient).

The FET was successfully built for the first time in 1945, years after the idea was first patented in 1925. However, it was not until many years later, when experimentation yielded the Metal Oxide Semiconductor Field Energy Transistor (MOSFET), that the FET became much more usable. Scientists discovered that they could make a gate insulator for the device, allowing for controlled oxidation (the forced diffusion of the oxide layer into another surface) of the semiconductor piece, which was previously made of silicon. This new layer is known as the MOSFET's dielectric layer or gate dielectric. This advancement enabled the incorporation of FETs into a wide range of applications, most notably display technology.

display technology

2. The TFT evolved from the MOSFET.

The TFT differs from standard MOSFETs or bulk MOSFETs in that it employs thin films, as the name implies. The TFT signaled the start of a new era in electronics. Bernard J. Lechner of RCA shared his idea of the TFT Liquid Crystal Display (LCD) in 1968, just six years after the first TFT development, something that would boom in popularity in our modern times. The TFT LCD was invented at Westinghouse Research Laboratories in 1973. These LCDs were made up of pixels that were controlled by transistors. Substrates in FETs were simply the semiconductor material, but in the production of TFT LCDs, glass substrates were used so that the pixels could be displayed.

But TFT development did not stop there. T. Peter Brody, one of the TFT LCD's developers, and Fang-Chen Luo created the first active-matrix LCD (AM LCD) in 1974. An active matrix controls each pixel individually, which means that the signal of each pixel's respective TFT is actively preserved. As displays became more complex, this enabled better performance and speed.

matrix LCD

A comparison of the signaling structures of an active matrix (left) and a passive matrix (right) is shown above.

3. The TFT Display's primary material

Though TFTs can use a variety of semiconductor layers, silicon has become the most popular, resulting in the silicon-based TFT, abbreviated as Si TFT. The TFT, like all FETs, is a semiconductor device that uses solid-state electronics, which means that electricity flows through the structure of the semiconductor layer rather than through vacuum tubes.

The Si TFT's characteristics can vary due to the various silicon structures that can be used. The most common form is amorphous silicon (A-Si), which is deposited onto the substrate at low temperatures during the first step of the semiconductor fabrication process. It is most useful when hydrogenated into A-Si: H. This changes the properties of A-Si significantly; without the hydrogen, the material struggles with doping (the introduction of impurities to increase charge mobility); in the form of A-Si: H. The semiconductor layer, on the other hand, becomes much more photoconductive and dopable. The A-Si: H TFT was invented in 1979 and is room-temperature stable. It quickly became the best option for AM LCDs, which grew in popularity as a result of this breakthrough.

Microcrystalline silicon is a potential second form of silicon. Though it has a similar shape to A-Si, this type of silicon also has grains with crystalline structures. Amorphous structures have more random, less geometric network-like structures, whereas crystalline structures are more structured and organized. Microcrystalline silicon, when grown properly, has better electron mobility than A-Si: H and greater stability because it contains less hydrogen. It is deposited in the same manner as A-Si.

Finally, polycrystalline silicon is also known as polysilicon and poly-Si. Microcrystalline silicon is an intermediate between A-Si and polycrystalline silicon due to the polycrystalline structure of polycrystalline silicon. This specific form is created by annealing the silicon material, which means adding heat to change the structure's properties. When poly-Si is heated, the atoms in the crystal lattice shift and move, and when cooled, the structure recrystallizes.

structure recrystallizes

The main difference between these forms, particularly A-Si, and poly-Si, is that charge carriers in poly-Si are much more mobile and the material is much more stable than in A-Si. Poly-Si's properties enable the creation of complex and high-speed TFT-based displays. Nonetheless, A-Si is very important because of its low leakage nature, which means that leakage current is not lost as much when a dielectric insulator is not non-conductive.

Hitachi demonstrated the first low-temperature poly-Si (LTPS) in 1986. Because the glass substrate is not as resistant to high temperatures as LTPS, lower temperatures are used to anneal the poly-Si.

Several years later, indium gallium zinc oxide (IGZO) was developed, allowing for a more powerful display in terms of refresh rates and greater efficiency in terms of power consumption. As the name implies, this semiconducting material contains indium, gallium, zinc, and oxygen. Despite being a type of zinc oxide (ZnO), the addition of indium and gallium enables this material to be deposited in a uniform amorphous phase while retaining the oxide's high carrier mobility.

Transparent semiconductors and electrodes became more appealing to manufacturers as TFTs became more prevalent in display technology. Indium tin oxide (ITO) is a popular transparent oxide due to its attractive appearance, good conductivity, and ease of deposition.

TFT with different materials

TFT research with various materials has resulted in the application of threshold voltage, or how much voltage is required to turn on the device. This value is highly dependent on the thickness and type of oxide used. This is related to the concept of leakage current when it comes to oxide. The leakage current may be higher with thinner layers and certain types of oxide, but this may lower the threshold voltage as leakage into the device increases. To capitalize on the TFT's potential for low power consumption, the lower the threshold voltage, the more appealing the device.

Organic TFTs (OTFT) are another branch of development that arose from TFT. OTFTs, which were first developed in 1986, are typically made by solution-casting polymers or macromolecules. People were wary of this device because it had slow carrier mobility, which meant slow response times. However, researchers have experimented with the OTFT because it has the potential to be used in displays other than those for which traditional TFTs are used, such as flexible, plastic displays. This investigation is still ongoing. The OTFT, with its simpler processing than traditional silicon technology, has a lot of potential for modern and future technologies.

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