Amorphous and polysilicon devices

Polysilicon is made up of silicon grains, each grain being a crystal in which the silicon atoms are arranged in a periodic structure. The distance between 0 two silicon atoms in the crystal is 2 – 3 Å, and the typical size of a grain in polysilicon is 1, 000 – 10, 000 Å. Thus each grain contains many millions of atoms. On the other hand, in amorphous silicon the atoms are typically grouped in 4 – 6 atoms, and there is no discerned periodic structure. In order to pacify the dangling bonds some hydrogen is added (approximately 20%); this is called hydrodegenation. Amorphous silicon is relatively easy to make, and it can be deposited on a large area. Polysilicon is harder to deposit on a large area; and pure silicon crystals are still harder to deposit (and more expensive), being more readily susceptible to faults. Amorphous silicon is a poor conductor, and is therefore not used in high-speed computer chips. Thin-film transistors (TFTs) fabricated from hydrogenated amorphous silicon (a — Si) and polycrystalline silicon (polysilicon, poly-Si) are now used in many commercial large-area electronic applications such as in flat panel display, printing and scanning (e.g., in fax machines).