The crystal of Germinium or silicon in its pure form at absolute zero acts as:

Explanation

In silicon, the crystalline form is a silvery, metallic-looking substance. In a silicon lattice, all silicon atoms bond perfectly to four neighbors, leaving no free electrons to conduct electric current. This makes a silicon crystal an insulator rather than a conductor.In semiconductors i.e. germanium the energy gap is small. Hence at room temperatures some of the valence electrons gain enough thermal energy and jump to the conduction band. More the thermal energy available, the more electrons will get excited and jump to the conduction band. According to the Boltzmann law, the number of thermally excited electrons (n )is given by,n∝e−Eg/KT where Eg is the energy gap for a particular substance, T is the temperature of the substance and K is the Boltzmann constant.If we consider the above equation the number of thermally excited electrons keeps on increasing as we increase the temperature T. If the temperature of the semiconductor is brought down to 0K then the number of thermally excited electrons will literally be nil. Hence there won't be any electrons in the valence band. Hence we can conclude that at 0K, germanium will behave as an insulator.