On the basis of the energy-band picture, materials are classified by the size of the energy gap $E_g$ between the valence band and the conduction band. Which choice gives the typical order for METALS, SEMICONDUCTORS and INSULATORS?

1. NCERT §14.2 classifies materials by their valence-band/conduction-band relationship. 2. METALS: the conduction and valence bands OVERLAP (or the conduction band is partly filled), so $E_g \approx 0$. Even at $T = 0\,\text{K}$ there are mobile charge carriers. 3. SEMICONDUCTORS: $E_g$ is small ($< 3\,\text{eV}$). At $T = 0\,\text{K}$ they behave like insulators; at room temperature thermal excitations create some carriers ($\sim 10^{10}\,\text{cm}^{-3}$ in Si). 4. INSULATORS: $E_g$ is large ($> 3\,\text{eV}$, typically $\sim 5\text{ to }10\,\text{eV}$). At normal temperatures essentially no electrons reach the conduction band. 5. Sanity: Si has $E_g \approx 1.1\,\text{eV}$, Ge has $E_g \approx 0.67\,\text{eV}$ — both semiconductors. Diamond has $E_g \approx 5.5\,\text{eV}$ — an insulator. Copper has overlapping bands — a metal. 6. Options B–D reverse or invent the ordering. _Source: NCERT Class 12 Physics Part 2, Ch 14 "Semiconductor Electronics", §14.2 (Classification on the basis of energy bands), p. 2–3._