Answer :
In most simplified way, metals are good conductors of eletricity due to overlapping of valence and conduction bands and plenty of conduction electrons are available.
while in the case of insulators, there will be a large gap between valence and conductions bands(>5 ev) as such there is almost no conduction electrons are available, hence they are not able to conduct electricy.
finally in the case of semiconductors, the gap between valence and condudtion bands is very samll( nearly 1 ev) as such at room temperature only, some electroins manage to become conduction electrons and show small electrical conductiity. however, their conductivity is increased by adding impurities known as doping.
Conductors have an overlapping of valence and conduction bands. So the electrons can easily flow across the bands and hence conduct easily.Semiconductors have a small gap between the valence and conduction bands and hence a little of the electrons flow across the bands . But on increasing temperature , semiconductor shows increased conductivity , opposite to the metals.
Insulators have a very large energy difference between the valence and conduction bands . No electron flows across these bands . hence no conductivity.
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