HEARTY WELCOME

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PRESENTATION

FLEXIBLE DISPLAYS USING TFT’S

What is a TFT:

A special kind of FET-made by deposition of thin films of diff.

materials.

What is a FET:

A semiconductor device in which current is controllled by electric field.

Consists of 3 regions:

> Source

> Drain

> Gate

> 2 types of FET:n channel,p channel.

Operation of FET

Source and Drain doped with charge carriers - provide excess holes (p-doped) or electrons (n-doped).

Body - between Source and Drain; is semiconductor that is either non-doped or oppositely doped from source and drain.

The gate is placed near the body region, but separated by an electrical insulator, (Often oxides).

In n-channel FET, applying positive voltage to (a negative voltage in the case of a hole-doped p-channel, or pnp, transistor) to the gate, the gate provides and electric field that causes electrons to be mobile in the body of the transistor, which becomes conductive and even current amplifying. This electric field affect coins the name FET.

The most prevalent current use of TFT’s In Active Matrix LCD(AMLCD)

• The AMLCD uses TFT’s to store the state of a cell temporarily with a capacitor- until the next refresh.

• For this use, the most important characteristics of the TFT are a low leakage current and a fast response time

Main method of manufacturing TFT’s :

• sputtering/deposition processes • photolithography • chemical and dry etching

.

AMLCD

• In TFT’s may be made in staggered or planar structures.

• The substrate most often used in the creation of a TFT is glass.

• Glass has a very low transmission of liquid water and water vapor

• The creation of a top-gate staggered n-channel TFT, the next step will be the deposition by Plasma Enhanced Chemical Vapor Deposition Process.

Reasons for the use of hydrogenated amorphous silicon:

• Readily available for electronic companies • It has a good charge carrier mobility once hydrogenated. • charge carrier mobility can be enhanced through the excitation of hydrogenated amorphous silicon into hydrogenated polycrystalline silicon

• Excitation is performed with laser annealing by an excimer laser. • Photolithography will next take place.

• By masking the desired layout of the body with a positive resist, the exposure of the martial to UV light will result in a change of the chemical structure

Requirements for Flexible Electronics

Must be thin to be able to bend without enough physical stress to break adhesion between device layers.

In addition, flexible electronic devices will need to be able to withstand many temperature cycles.

In different environments, including those with a high relative humidity, semiconductors and circuits, such as OLED’s, must be isolated from the climate changes.

The current method of making polymer films more water resistant is by depositing a barrier layer upon the component side of the film

Current Research & Development in TFT’s & Displays

• The most widely used organic semiconductors, such as pentacene, thiophene oligomers, and polythiophene, reached ”maturity” .

DIFFERENCE BETWEEN OTFT&NORMAL SEMI CONDUCTORS:

• In silicon semiconductors, the atoms are connected with strong covalent bonds and charge carriers move as delocalized waves and thus have a high mobility. • In organic semiconductors, carrier transport takes place by hopping between localized states.

• Polymer materials reduce cost of flexis.

CHALLENGES TO OVERCOME :•Low shrinkage from heating• High enough processing temperature for semiconductor excitation •Surface smoothness•Solvent & moisture resistance•Clarity.

currently investigated or accessible Polymers : •polyimide(PI) •polyethylene terephthalate, (PET) • polyethylene naphthalate (PEN).

•defects can be reduced on smoothening beforehand.

•Clarity in polymer films may reduce due to heat dissipation .

•The use of OLED’s as the luminescent component reduces the above effect.

• A polymer or organic semiconductor to have necessary carrier mobility than silicon.

Faster•Software development (eg:games)•Material development

Colour•Colour filter definition•Image processing

Pen input•Handwriting recognition

High durability.

• cost is still too large for widespread commercialization.

• LED’s replaced by OLED’s-larger scope for flexies.

• Advances in OLED’s-replace florescent lights in wide-area lighting,

•To make high performance OLED, TFT driven, flexible displays production and discovery cost to be reduced.

[1] E.A. Al-Nuaimy and J.M. Marshall. Excimer laser crystallization and doping of source and drain regions in high quality amorphous silicon thin film transistors. Applied Physics Letters, 69(25):3857–3859, December 1996.

[2] C.D. Dimitrakopoulos and D.J. Mascaro. Organic thin film transistors: A review of recent advances.

IBM Journal of Research & Development, 45(1):11–27, January 2001.

[3] Emmanuel P. Giannelis. Ubiquitous electronics: Why now?, March 2006. A presentation in the Cornell