why do we have free electrons in metal and not in insulators...what makes them to be an insultaor/semiconductor/metal?

there is a gap called energy band gap denoted by Eg which is the gap between conduction band & valence band.since Eg for metals is approx. equal to 0.e- can easily jump from valence band(vb) to the conduction band(cb) & hence we have free e-.on the other hand Eg for semiconductors is < 3ev.hence e- can make a jump but at higher temp.and for insulators Eg is > 3ev making no free e- to jump.

for further info. refer NCERT,part 2,ch-14(pg-469)