Crystal Structures

What Is a Crystal?

A crystal is a solid where atoms are arranged in a highly ordered, repeating 3D pattern. This periodic arrangement extends across the entire material — in a silicon wafer, trillions of atoms are arranged in perfect order.

The smallest repeating unit of a crystal is called the unit cell. Common crystal structures include:

• Simple cubic (SC): Atoms at cube corners only — rare in nature
• Body-centered cubic (BCC): Atoms at corners + one in the center — iron, tungsten
• Face-centered cubic (FCC): Atoms at corners + center of each face — copper, aluminum, gold
• Diamond cubic: FCC with 4 additional atoms inside — silicon, germanium, diamond

Why Single Crystals Matter

For semiconductor devices, the silicon must be a single crystal — one continuous crystal lattice across the entire wafer with no grain boundaries.

Why does this matter?

• Uniform electrical properties: Grain boundaries scatter electrons and create unpredictable behavior. A single crystal ensures every transistor on the wafer sees the same material.
• Predictable doping: Dopant atoms substitute into specific lattice sites. In a polycrystalline material, dopants would segregate at grain boundaries.
• Smooth surfaces: Single crystals can be polished to atomic smoothness for lithography.

This is why the Czochralski crystal growth process is so important — it produces large, defect-free single crystals.