Difference between semi conductor and semi insulator?

Answer 1

some semiconductors have comparatively low resistivity, and this is due to their containing electrically active impurities with energy-levels near a band edge. Others have very high resistivity and this is due to their containing a predominance of impurities with levels far from the band edges, that is, far compared with the thermal energy kT. These latter materials are often referred to as insulators, but since they can carry electronic currents we prefer to introduce the term 'semi-insulator' to describe them.

Answer 2

A semiconductor is a material that has electrical conductivity between that of a conductor and an insulator, allowing it to be used in electronic devices. A semi-insulator, on the other hand, is a material that has very high electrical resistance, similar to an insulator, but can be modified to become conductive under certain conditions. Semiconductors are widely used in electronics, while semi-insulators are less common and typically used in specialized applications.