Thin Film Transistor : Structure, Working, Fabrication Process, How to connect & Its Applications

The RCA (Radio Corporation of America) had spent many years experimenting & developing transistors. Although the first thin film patent was developed in 1957 by a member of the RCA namely John Wallmar 1957. After that, a series of developments within the microelectronics & semiconductors field, the TFT or Thin Film Transistor emerged in 1962. A TFT is used in liquid-crystal displays to improve image qualities like contrast & addressability. The TFT is an improved version of MOSFET because it uses thin films. This article discusses the introduction to a thin film transistor or TFT – working with applications.

What is a Thin Film Transistor?

A thin film transistor definition is; a type of FET or field-effect transistor which is used in each individual pixel of an LCD (liquid crystal display) to display the screen information at high contrast, high brightness & high speed. The thin film transistor symbol is shown below.

Thin Film Transistor Working Principle

These thin film transistors work like an individual switch that allows the pixels to adjust position very quickly to make them turn on & off much faster. These transistors are the active elements within LCDs which are arranged in a matrix form so that LCD can display information. These are used in commercial display applications like digital radiography detectors, head-up displays, and many more.

Thin Film Transistor Structure

A TFT is a special type of Field Effect Transistor that is made by simply depositing active semiconductor layer thin films, dielectric layer & gate electrode layer on a flexible material known as substrate. The structure of the thin film transistor is shown below.

The TFT includes different layers which are made by using different materials. So, the materials used in every layer are discussed below.

The first layer of TFT is a flexible substrate that is made with little microns thick glass, metals & polymers like Polyethyleneteraphalate. This layer acts as a base where the electronic device is constructed.

The second layer is the gate electrode which is made up of Aluminum, gold, or chromium based on the application. This gate electrode provides a signal to the thin film semiconductor which triggers the contact between the source & drain.

The third layer is an Insulator that is used to avoid the electrical shorting in between the two layers like the semiconductor layer & the gate electrode.

The fourth layer is the electrode layer which is made with different conductors like silver, chromium aluminiumor Gold and is simply deposited over semiconducting surfaces. Even for conducting coating of source & drain electrodes, Indium Tin Oxide (ITO) is used. The whole device is encapsulated within a ceramic or polymer material.

Thin Film Transistor Fabrication Process

The different layers of TFT fabrication are discussed below.

• First, the substrate material is cleaned chemically with the required acid or base to eliminate all the containments that are holding on its surface.
• After that, Metallic gate electrodes are simply deposited on the substrate with a thermal evaporation procedure. Ceramic/ Polymer electrodes are deposited with inkjet printing/ dip coating procedure.
• Insulating coatings are simply deposited on a gate with Chemical Vapour Deposition (CVD) or Plasma Enhanced Chemical Vapour Deposition (PECVD) processes.
• Semiconductor layers are simply deposited with dip coating if it is spray or polymer coating. Both the source & drain are similar to the gate electrode procedure – spray/dip coating or thermal evaporation as required by suitable mask layers.

How to Connect a Thin Film Transistor?

The connection diagram of the thin film transistor is shown below. This example uses p-type semiconductor material. If it uses n-type material, then the polarities will be opposite. The transistor operates, when transistor is biased by applying a negative voltage between drain & source contacts (VDS).

When the transistor is turned off, no charge will be accumulated between the source & drain contacts. So, no current can flow between the source & drain contacts. To turn on the transistor, a negative bias voltage is applied to the gate terminal (VGS). So charge carriers like holes within semiconductors will accumulate to the gate insulation to create a channel that allows the current (ID) to flow from drain to source.

Difference b/w Thin Film Transistor Vs Mosfet

The difference between thin film transistors and mosfet includes the following.

How a Thin Film Transistor is Different from a Normal Transistor?

Thin film transistor is different as compared to normal transistor because; most normal transistors are made with very pure Si (silicon) & Ge (germanium) and sometimes some other semiconductor materials are used. Thin film transistors (TFTs) are made with different kinds of semiconductor materials like silicon, zinc oxide, or cadmium selenide. TFT includes three terminals like source, gate, and drain whereas a normal transistor includes a base, emitter, and collector.

These transistors act as switches by allowing the pixels to adjust state quickly to make them turn on & off very quickly. The normal transistor acts as a switch or an amplifier.

Advantages and Disadvantages

The advantages of thin film transistors include the following.

• They consume less power.
• They have a quicker reaction time.
• TFTs play a key role in the digital display industry.
• Thin-film transistors are key elements of flexible electronics which are implemented on economical substrates
• They have fast, higher & accurate response rates.
• The TFT-based displays have sharp visibility.
• The physical design of TFT-based displays is excellent.
• It reduces eye strain.

The disadvantages of thin film transistors include the following.

• They depend on backlighting to give brightness instead of generating their own light, so, they need in-built LEDs in their backlighting arrangement.
• Restricted utility because of glass paneling.
• The modules of TFTs can only be read once the LEDs are ON.
• TFTs can drain a battery very quickly.
• TFT LCDs are expensive compared to typical monochrome displays.

Applications

The applications of thin film transistors include the following.

• Thin-film-transistor is broadly used in smartphones, computers, flat-panel displays, personal digital assistants & video game systems.
• The best-known thin-film transistor application is in TFT LCDs,
• These transistors play a significant role in present materials chemistry & digital displays.
• TFTs are used in abroad range of applications like organic LEDs, flat panel displays & other electronic devices.
• TFTs are broadly used as sensors within X-ray detectors.
• TFT devices are found in various sensing applications.
• TFT LCDs are utilized in video game systems, projectors, navigation systems, handheld devices, TVs, personal digital assistants & dashboards within automobiles.

Thus, this is an overview of a thin film transistor or TFT which plays a significant role in present digital displays. These are advanced to conventional MOSFETs so it offers fast response times & also capable of retaining an electrical charge. These have a wide range of applications in LCDs and currently researchers concentrating on developing new types of thin-film transistor devices. Here is a question for you, what is FET?