6. Give examples of point defects in a crystal. Explain what kind of defect is a dopant atom.

Defects in semiconductor crystals are:

  1. Point defects
  2. Line defects
  3. Surface defects
  4. Volume defects

Of point defects, there are native defects (happening without the addition of other atoms) and non-native defects due to contaminants. Native defects are vacancies and self-interstitials. Non-native point defects are substitutions or interstitials. Dopants must be substitution defects.

1. Describe the crystal structure of Si and GaAs. Explain the difference between the diamond and zinc-blende lattice cells.

Both Silicon and GaAs are cubic semiconductors. Silicon is a diamond crystal, while GaAs is a zinc-blende crystal. Diamond differs from zinc-blende lattice in the composition of the atoms in the lattice.

Since Si is a pure substance, only Si atoms are present in the crystal lattice. A diamond crystal has 8 atoms, which include the corner atoms, atoms on faces, and internal atoms.

In the GaAs semiconductor compound, the corner and face atoms will be of a different atom than the internal atoms. For instance, the corner and face atoms can be the metal Gallium while the internal atoms are Arsenic.