A neutral copper atom has only one electron in its outer orbit. Since the single electron can be easily dislodged from its atom, it is called a free electron.
Silicon is the most widely used semiconductor material. The number of electrons in the valence orbit is the key to conductivity. Conductors have one valence electron, semiconductors have four valence electrons, and insulators have eight valence electrons.
Each silicon atom in a crystal has its four valence electrons plus four more electrons that are shared by the neighboring atoms. At room temperature, a pure silicon crystal has only a few thermally-produced free electrons and holes.
An intrinsic semiconductor is a pure semiconductor. Intrinsic silicon acts as an insulator at room temperature.
Two Types of Flow
Flow of free electrons, flow of holes
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