a-Si, LTPS, Oxide: The Backplane Decides
The transistor layer you never see controls the resolution, brightness, and price of every panel above it.
Three Materials, Three Trade-offs
Every active-matrix display — LCD or OLED — is built on a backplane of thin-film transistors that switch each pixel on and off. The semiconductor material in those transistors sets the ceiling for everything else.
Amorphous silicon (a-Si) is the industry workhorse: cheap to deposit uniformly across Gen 10.5 glass sheets, tolerant of large substrates, and well understood after four decades of production. Its weakness is low electron mobility — roughly 0.5–1 cm²/V·s — which limits how fast each transistor can charge its pixel. That rules out high-refresh, high-resolution panels and means OLED is nearly impossible on a-Si: the material degrades under the continuous current OLED pixels demand.
Low-temperature polysilicon (LTPS) crystallises the silicon with excimer-laser annealing, lifting mobility to 50–150 cm²/V·s. Smaller transistors, higher pixel density, faster switching, and enough current drive for OLED. The price is uniformity: the laser crystallisation process leaves grain boundaries whose electrical properties vary across the panel, so LTPS suits smaller substrates — smartphones, tablets — where compensation circuits can correct mura. Scaling to large-format glass remains economically brutal.
Oxide semiconductors, principally indium gallium zinc oxide (IGZO, commercialised by Sharp and licensed broadly), sit between the two. Mobility reaches 10–50 cm²/V·s — far above a-Si — and the amorphous deposition process yields the uniformity LTPS cannot. IGZO holds charge leakage extremely low, enabling very low refresh rates for power saving and supporting the large pixel counts of 4K and 8K LCD panels. It cannot match LTPS current density for high-brightness OLED drive, which is why premium OLED phones stay on LTPS or hybrid LTPS/oxide stacks.
The backplane is not a footnote in the spec sheet. It is the spec sheet.