Thin-film transistor

A thin film transistor ( Thin-Film Transistor English, short TFT ) is a specific field effect insulated gate transistor ( IGFET also MISFET ), large-scale electronic circuit can be manufactured by the. It was developed by Paul K. Weimer early 1960s at RCA Laboratories.

Construction

The structure of a thin film transistor corresponds to the scheme of the prior art metal - oxide semiconductor field effect transistor ( MOSFET), the most commonly used variant IGFET. The main differences are - in addition to the typically used materials - in the subsequently deposited semiconductor layer and the often large areas designed gate, that is, the source and drain and the gate are often above the other.

As for the structure to allow for a thin film transistors in a "bottom- gate" (Ger. bottom-mounted gate, also top -contact, dt overhead source / drain contacts ) and on " top gate" (Ger. overhead gate, also called bottom -contact, dt lying below the source / drain contacts ), on the other hand in "staggered " (Eng. stacked) and " coplanar " (Eng. distinction lying in the same plane ). There are thus four basic variations in the layer structure. In the illustrated version is a "staggered bottom- gate TFT " ie, a " stacked thin film transistor with bottom-gate ". Here, the deposited first gate (bottom- gate) and the source / drain contacts by a layer stack of dielectric and semiconductor isolated ( staggered). In the opposite variant of the " coplanar top -gate TFT ", the gate would last (after Source-/Drain-Kontakt, the semiconductor and the dielectric) have been deposited and the semiconductor material would be only between the source and the drain contact are, that is, the contacts directly touch the dielectric.

An active semiconductor is thereby usually hydrogenated amorphous silicon ( α -Si: H) is used, which can be converted to polycrystalline silicon by means of an excimer laser, for example. Furthermore, ( CdSe ) and metal oxides such as zinc oxide used transparent compound semiconductor such as cadmium selenide. Other materials, such as are used for example in organic field-effect transistors, are the subject of current research (as of 2009).

Application

A common application is the orientation of liquid crystal flat panel displays, in which each point on the screen three transistors are used. This design of display is known as matrix LCD, but is often colloquially referred to as TFT display. PDAs transreflective, TFT -based LCDs are often used, which can also be used outdoors. Also be used in screens based on organic light emitting diodes ( OLED) thin film transistors analogously.