A solid which is transparent to visible light and whose conductivity increases with temperature is formed by:

- Conductivity increases with temperature: Characteristic of semiconductors and insulators.Transparent to visible light: This implies a large band gap ($>3\text{eV}$), so photons of visible light do not have enough energy to excite electrons to the conduction band.Many such solids (like diamond or silicon carbide) are formed by covalent binding.