Thin Film Transistor: Definition, Structure, and More

Thin-film transistors play an important role in the digital display industry. You’re probably reading this article on a screen that uses thin-film transistor (TFT) technology. These transistors are the driving force behind flat-panel displays on laptops, desktop computers, tablets, smartphones, and high-definition TVs.

However, TFTs have a range of other commercial applications. Understanding these devices, including their structure, history, and usage, can highlight the value of adopting thin film deposition technology in your lab.

In this article, you’ll gain a comprehensive understanding of thin-film transistor devices, including their definition, makeup, history, and more.

What Are Thin-Film Transistors?

Thin film transistors, also known as TFTs or film transistors, are a type of field-effect transistor most commonly used in liquid crystal displays (LCDs). LCD technology uses one thin-film transistor for every pixel within a flat-panel display, and the transistors essentially act as on/off switches for the individual pixels.

TFTs are produced by depositing a semiconductor and a dielectric layer over a non-conducting substrate, such as glass. In flat-panel displays, thin-film transistors are arranged within a matrix pattern, and these devices are the backbones of active-matrix displays.

However, TFTs also have other commercial uses, ranging from digital radiography detectors to head-up displays.

Thin film transistors, as their name implies, are made from thin-film layers, manufactured by a deposition process. More about that below!

The History

Thin-film transistors have changed and improved significantly over the past half-century. Conventional metal-oxide-semiconductor field-effect transistors (MOSFETs) precede them.

In 1957, John Wallmark of the Radio Corporation of America designed a thin-film MOSFET that used germanium monoxide as a gate dielectric. This design allowed Paul K. Weimer, another member of the Radio Corporation of America, to create the first thin-film transistor in 1962. This first thin film transistor (TFT) used a thin layer of cadmium selenide and cadmium sulfide as a semiconductor.

In 1966, Westinghouse Electric employees H. E. Kunig and T.P. Brody created metal-oxide-semiconductor TFTs using indium arsenide as a semiconductor. They developed these transistors in both depletion and enhancement modes, paving the way to utilise TFT technology in on/off modes.

Bernard J. Lechner first conceived the concept of using thin-film transistor devices in liquid-crystal display technology. In 1973, researchers developed a cadmium selenide thin-film transistor (TFT) to use in the first TFT liquid-crystal display. However, the first commercial LCD product using TFT technology was not available until 1984.

In 2012, researchers created the first TFT using indium gallium zinc oxide (IGZO) as a semiconductor [1]. IGZO transistors utilise low power consumption and high refresh rates compared to traditional TFT technology, signifying the next generation of transistor film developments.

TFT research continues to advance the applications and technology behind thin-film transistors.

TFT Structure

Manufacturers produce thin-film transistor (TFT) machinery by layering a semiconductor and dielectric active layer over a substrate. However, manufacturers also include metallic contacts, such as a gate layer, drain metal, and source metal. The above diagram showcases the exact structure of bottom-up (top contact) TFTs.

Here’s a closer look at the components that make up TFTs:

  • Substrate: Commonly glass or plastic
  • Semiconductor: Silicon, cadmium selenide, or metal oxides
  • Dielectric: Inorganic materials such as silicon oxide or silicon nitride
  • Gate electrode: Commonly polysilicon, TiN, TaN, or WN
  • Drain/source metal: Commonly metal silicides

Layering these components in different orders can alter the TFT performance.

Four common TFT structures

Source: Wikipedia.org

Is TFT a Type of MOSFET?

A TFT is a type of metal-oxide-semiconductor field-effect transistor (MOSFET). However, TFTs are essentially improved versions of MOSFETs.

The primary difference between TFTs and conventional MOSFETs is their semiconductors. TFTs utilise distinct substrates and semiconductors — commonly glass and metal oxides, respectively. However, conventional MOSFETs utilise a semiconductor material that doubles as the substrate. For example, a silicon wafer is commonly used in MOSFET manufacturing.

TFTs and MOSFETS have varying threshold voltages [2], making them each suitable for a different transistor operation. However, TFTs offer some benefits over conventional MOSFETs. For example, the tiny nature of TFTs allows for a smaller electric charge, leading to faster re-drawing within an active matrix display.

How Are Film Transistors Made?

Manufacturers can create TFTs using a range of semiconductor materials. But for that, they’ll need thin films, which are made following a deposition process in a vacuum chamber.

There are several techniques for manufacturing thin films, but the most important processes are physical vapour deposition and chemical vapour deposition.

Historically, TFTs used thin films of amorphous silicon or polycrystalline silicon as semiconductors, as silicon is abundant in nature and used in many commercial applications. However, researchers have determined that the low mobility of amorphous silicon [3] makes it suboptimal in TFTs, leading to the usage of other semiconductors.

Today, TFTs can contain one of several materials in their semiconductor layer, including:

  • Zinc oxide
  • Cadmium selenide
  • Organic semiconductors
  • Carbon nanotubes
  • Metal halide perovskites

Note: TFTs that use organic semiconductors are considered organic thin-film transistors or organic transistors.

Each semiconductor material above is suitable for TFT production because of its charge carrier mobility. Carrier mobility determines the speed and high-frequency response of the final TFT.

Thin film transistors are manufactured by laying a thin film of one of the above semiconductors and a dielectric layer onto a substrate.

Glass is the most common substrate material within TFTs. This material is necessary for the traditional application of TFTs in LCDs, which we will discuss later. Glass is also suitable because of its non-conductive nature and excellent optical clarity.

However, some manufacturers use flexible plastic substrates within low-temperature TFT manufacturing processes. Researchers have recently developed a method to create incredibly high-performance TFTs on these flexible substrates [4]. Fully printed TFTs can also utilise flexible substrate materials.

The manufacturing of TFTs requires a highly specialised process to deposit the semiconductor onto the substrate. For example, TFTs are extremely sensitive to process temperature. Manufacturing must occur in low-temperature environments, as high temperatures can melt some typical substrates.

Manufacturers can utilise a few specialised processes to deposit semiconductors. The most common are atomic layer deposition, chemical vapour deposition, printing, and spray coating. Notably, solution-based methods can produce more advanced materials for flexible electronic devices.

Modern Applications of Thin Film Transistors

Thin-film transistors have numerous device applications. Researchers are still determining the various ways TFTs can assist emerging applications ranging from flexible electronics to integrated circuits.

TFTs have recently become common in a wide range of digital detectors. For example, digital radiography detectors in medical radiography utilise these devices within their image receptors. TFTs also have applications in sensors, such as temperature, gas [5], and biochemical sensors.

Recently, researchers have begun developing optically transparent TFT devices using transparent substrates [6]. With further research, transparent TFTs could operate within head-up displays, which are essential for aircraft and automobile operations. Head-up displays allow users to view information without straying from necessary viewpoints.

Printed TFTs also have applications in flexible and printed electronics. Flexible electronic applications include calculators, cameras, personal entertainment devices, medical devices, and more.

However, the most common usage of thin transistor films is within digital displays. For example, active-matrix organic light-emitting diode (AMOLED) screens have a TFT layer that utilises low-cost, low-temperature manufacturing. Since 2013, all high-resolution electronic visual displays have used TFT active matrix technology [7].

While thin-film transistors have several high-performance applications, these transistors carry advantages and disadvantages within materials science. For example, while TFT displays are fast, sharp, and energy-efficient, they must utilise glass panelling, restricting their usage. A TFT display also necessitates disproportionate viewing angles.

Liquid Crystal Display (LCD)

LCD is the most important modern application of TFTs.

Specifically, thin-film transistor active-matrix twisted nematic LCDs are commonly used in digital screens because of their impressive colour capabilities and lightning-fast responses.

LCD panels use TFT to control their pixels, altering the electric field to change the polarisation, and, thus, the colour output. Each TFT is paired with a pixel within the active matrix and includes a capacitor that allows the pixels to retain their charge, eliminating the need for individual charge carriers and enhancing the screen’s responsiveness.

Today, TFT LCD is present in a wide range of appliances, such as:

  • Televisions
  • Smartphones
  • Computer monitors
  • Navigation systems
  • Video game systems
  • Handheld devices

How To Manufacture Thin-Film Transistors

Because thin-film transistors utilise such a specialised manufacturing process, labs must use specialised materials and systems to grow these transistors. For example, labs need reliable, flexible thin film deposition systems to place the semiconductor and dielectric layers onto the substrate.

Korvus’ HEX Series of thin deposition systems offer the versatility, precision, and power necessary to manufacture thin films. The HEX system supports numerous thin-film deposition techniques and methodologies, allowing for versatility within lab or research settings. This system also contains several modules to adapt to specific functionality needs.

the HEX series thin film deposition system from Korvus Technology
The HEX series thin-film deposition system

Using the proper systems ensures that TFTs have the necessary electron mobility, channel width, transfer curves, operational stability, threshold voltage, and electrical performance for their desired applications.

Final Thoughts

Thin-film transistors play an important role in modern digital displays and materials chemistry as a whole. These devices improve upon conventional MOSFETs by offering faster response times and the ability to retain an electrical charge.

TFTs have a wide range of applications, most commonly in liquid crystal displays. Researchers continue to develop new types of and discover new uses for thin-film transistor devices.

Manufacturing TFTs requires specialised equipment and techniques. Utilising a reliable thin-film deposition system is essential to producing durable, functional thin-film transistors.

References

[1] Orland, Kyle (August 8, 2019). “What Sharp’s IGZO display technology will mean for the Nintendo Switch.” Ars Technica.

[2] Matebesi, Unopa & Mogosetso, Gofaone & Lebekwe, Caspar & Ditshego, Nonofo & Khoo, W. & Mohamed Sultan, Suhana. (2019). “IGZO TFT versus the MOSFET.” BIUST Research and Innovation Symposium 2019.

[3] Powell, M.J. (1989). “The physics of amorphous-silicon thin-film transistors.” IEEE Transactions on Electron Devices. 36 (12): 2753–2763. doi:10.1109/16.40933. ISSN 1557-9646.

[4] Shih, C.W., Chin, A. (2017). “Remarkably high mobility thin-film transistor on flexible substrate by novel passivation material.” Sci Rep. 7, (1147. https://doi.org/10.1038/s41598-017-01231-3

[5] Frank Liao, Christopher Chen, Vivek Subramanian, “Organic TFTs as gas sensors for electronic nose applications,” Sensors and Actuators B: Chemical, Volume 107, Issue 2, 2005, Pages 849-855, ISSN 0925-4005, https://doi.org/10.1016/j.snb.2004.12.026.

[6] Nomura, Kenji; Ohta, Hiromichi; Ueda, Kazushige; Kamiya, Toshio; Hirano, Masahiro; Hosono, Hideo (2003-05-23). “Thin-Film Transistor Fabricated in Single-Crystalline Transparent Oxide Semiconductor.” Science. 300 (5623): 1269–1272. doi:10.1126/science.1083212. PMID 12764192. S2CID 20791905.

[7] Brotherton, S. D. (2013). Introduction to Thin Film Transistors: Physics and Technology of TFTs. Springer Science & Business Media. p. 74. ISBN 9783319000022.

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