(N/A) An intrinsic semiconductor behaves like an insulator at $T = 0 \,K$. At this temperature, all electrons are bound in the valence band, and there are no electrons in the conduction band. Consequently, no current can flow, and it acts as an insulator.
At temperatures $T > 0 \,K$, thermal energy allows some electrons to gain sufficient energy to jump from the valence band to the conduction band. This process creates free electrons in the conduction band and corresponding holes in the valence band, as shown in the figure.
In the figure, solid dots represent electrons, while empty circles represent holes. The energy gap between the valence band $(E_V)$ and the conduction band $(E_C)$ is denoted by $E_g$. As temperature increases, the number of thermally excited charge carriers increases, thereby increasing the conductivity of the semiconductor.