Given a metallic compound, for example NiO, how can you tell if it's an i-, n- or p-type semiconductor?

Answer 1

Nickel Oxide (as with most binary metal oxides) is a nonstoichometric compound meaning that the ratio of nickel to oxide is usually not 1:1. That means the material will contain missing atoms which changes the conducting band/valence band electron energy states.

If the Ni:O ratio is <1 then there are more electron rich O atoms than electron poor Ni atoms, thus the material would be n-type.

If the ratio is >1 then more electron poor Ni atoms means the material is p-type

If the ratio is =1 (or very close) it will be an i-type, due to the vacancies or Ni and O being equal.