The energy band gap of semiconductors is small . A little energy (thermal energy) is sufficient to cause charge carriers to be promoted from valence band to conduction band. With more charge carriers in conduction band there is greater flow of current hence increase in conductivity.
Compared with metal , there is no increase in number of free electrons . The rise in temperature only cause the atom to vibrate vigorously hence obstructing the flow of electron. The conductivity decrease or resistance increase.
