At \$ 0K \$ there is no energy available , hence no electrons can flow from valence band to conduction band
\$ \therefore \sigma =0 \$ at \$ T=0K \$
Now as \$ T \$ increases ,energy increases and hence \$ \sigma \$ increases
Now how \$ \sigma \$ decreases as \$ T \$ is further increased from \$ 300K \$ ?please help....
As you keep on increasing the temperature, the concentration of minority carriers goes on increasing due to thermal generation. At a particular temperature, the concentrations of both the carriers are the same, i.e., the material becomes intrinsic again. Beyond 300K, as the semiconductor is going from being extrinsic to becoming intrinsic, the conductivity decreases (conductivity of extrinsic > conductivity of intrinsic).
Beyond Tc, the semiconductor completely behaves as an intrinsic one and it's conductivity increases with temperature.