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3. Classify the solids into conductor, semiconductor and insulators based on band theory.
Conductors
In a conductor there are no band gaps between the valence and conduction bands. In some metas the
conduction and valence bands partially overlap. This mcans that clectrons can move freely between
the valence band and the conduction band. The conduction band is only partially fillcd. This means
there are spaces for electrons to move into. When electrons for the valence band move into the
conduction band, they are free to move. This allows conduction.
Partially filled conduction band
Overlap (no band gap)
Valence band
Insulators
An insulator has a large gap betwcen the valence band and the conduction band. The valence band is
fiull as no elect can mote nt conduction hand As a result the conduetion bond ie ennty
Semiconductors
In a semiconductor, the gap between the valence band and conduction band is smaller. At room
temperature there is sufficient encrgy available to move some electrons from the valence band into
the conduction band. This allows some conduction to take place. An increase in temperature increases
the conductivity of a scmiconductor because more clectrons will have enough energy to move into the
conduction band. The difference betwcen insulators and semiconductors is due to a small amount of
impurity added to a semiconductor which affects the energy bands. This process is called doping.
Electrons can move
to conduction band
Smaller band gap
Semiconductor
4 What is intrinsic semiconductor, explain the formation extrinsic semiconductors through
doping and derive the expression for intrinsic carrier concentration.
Semiconductors that are chemically purc, in other words, frece from impurities are termed as intrinsic
aloud 3 of 14
Valence band
Insulators
An insulator has a large gap between the valence band and the conduction band. The valence band is
full as no clectrons can move up to the conduction band. As a result, the conduction band is empty.
Only the clectrons in a conduction band can move casily, so because there aren't any clectrons in an
insulator's conduction band, the material can't conduct.
Conduction band (empty)
Valence band
