Amorphous silicon (a-Si) is disordered, non-crystalline silicon deposited at lower temperatures than polysilicon, used for various semiconductor applications. Deposition: LPCVD or PECVD using SiH4 at 300-550 C. Lower temperature prevents crystallization. Structure: No long-range atomic order. Random network of Si-Si bonds with some dangling bonds. Hydrogen content: PECVD a-Si:H contains 5-20% hydrogen which passivates dangling bonds, improving electrical properties. Crystallization: Can be converted to polysilicon by annealing at 550-700 C. Solid-phase crystallization (SPC) or laser crystallization (ELA). Applications: Hard masks in patterning, sacrificial layers in process integration, TFT displays (a-Si:H), solar cells. Etch selectivity: Different etch rates compared to crystalline Si and poly-Si, useful for selective processing. Optical properties: Higher absorption than crystalline Si. Used as anti-reflective layer in some lithography applications. Surface smoothness: Smoother than polysilicon due to absence of grain structure. Beneficial for some interface requirements. Density: Lower density than crystalline silicon due to disorder. Depends on deposition conditions. Transition: As-deposited a-Si begins crystallizing during subsequent thermal steps. Must account for in process integration.