Consider a pure piece of silicon semiconductor. Let’s say we dope it with Phosphorus. The extra electron in the outer most shell of the phosporous is in its conduction band as it is not part of any bond.
My question is regarding the status of the dopant phosporous atom. When is it considered an positive ion? When it’s free electron goes into its conduction band or when an E field is applied and that electron moves?
In the silicon crystal, when the dopant is implanted (or diffused), it loses its electron 'immediately' -- the electron can drift around the crystal easily (with thermal energy).
At very low temperatures, there may not be enough thermal energy to ionize the dopant, and in that case, some fraction of the dopant atoms are not ionized.
When an E field is applied the free electrons can move with the field. Generally they are already 'buzzing' about randomly from thermal energy; the E-field just gives them some additional bias in that direction.
