NOT FROM A WAFER

A transistor deposited on glass

Thin film The semiconductor is deposited as a film a few tens of nanometers thick on glass or plastic

Temperature It has to be processed below what the substrate can survive, so no crystalline silicon

Area Display glass is up to about 3 meters across, so the process covers a huge area cheaply

Where Every active-matrix display, plus X-ray detectors and electronic paper

TFTs trade mobility for area: they are slow, but there are billions of them on a wall-sized sheet.

THE PIXEL

A switch and a storage capacitor

ΔV = I_off · T ÷ C_s

Each pixel has one TFT. A row line turns it on, the data line charges a small capacitor to the pixel voltage, then the TFT turns off and the capacitor must hold the voltage until the next refresh. Everything hinges on the off current: if the TFT leaks, the capacitor droops and the brightness sags. Low-leakage transistors let a display refresh much more slowly and save power.

A display pixel is a leaky capacitor, and the transistor is its valve.

FOUR SEMICONDUCTORS

Mobility, leakage and cost

Amorphous silicon is cheap and uniform but slow. Low-temperature polysilicon is fast enough to build driver circuits on the glass. Oxide semiconductors such as IGZO are in the middle on mobility and extremely tight on leakage. Organic semiconductors can be printed at low temperature on plastic, but are the slowest and least stable.

No single film is best at everything, so every display picks its compromise.

BEYOND DISPLAYS

What else TFTs do

Flexible electronics Polyimide substrates let TFT circuits bend and fold, as in foldable phones

Instability Bias stress shifts the threshold of a TFT, so pixels use compensation circuits to stay uniform

Large-area sensors TFT arrays read out X-ray and fingerprint detectors, one pixel at a time

Biosensing TFTs with electrolyte gates are being developed as low-cost, large-area biosensors

The same field-effect sensing idea works in a film as well as in a wafer.

CHECK THE FILMS

Which thin-film transistor?

Amorphous for cheap and big, poly-Si for fast, oxide for leak-tight, organic for flexible.

UNIT 23 STUDY COMPLETE

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Ready for the Fab Challenge?

You've seen the transistor that is not cut from a wafer, and how its leakage decides the refresh rate of a screen.