Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
In an isolated atom, an electron has one exact energy for each orbit (like a single step). When billions of atoms come very close in a solid, their outer orbits overlap. By the exclusion rule no two electrons can share the exact same energy, so each level splits into a huge number of very closely spaced levels. Because there are so many atoms, these split levels lie so close together that they look like one continuous range of energy. That continuous range is an energy band. So a single sharp step becomes a solid ramp of allowed energies.
The valence band is the band that contains the energy levels of the outer (valence) electrons of the atoms. With no extra energy, all valence electrons sit here and stay bound to atoms. The conduction band is the next band above it. An electron in the conduction band is free to move through the whole solid and carry electric current. So valence band = bound electrons, conduction band = free electrons that give current. NCERT writes the top of the valence band as E_V and the bottom of the conduction band as E_C.
The gap between the top of the valence band (E_V) and the bottom of the conduction band (E_C) is called the energy band gap or forbidden energy gap, Eg. Formula: Eg = E_C minus E_V. It is called forbidden because no electron is allowed to have an energy inside this gap. An electron must sit either in the valence band or in the conduction band, never in between. To move up, an electron must gain at least Eg of energy in one jump so it lands inside the conduction band.
No. In a metal either the conduction and valence bands overlap, or the conduction band is only partly filled. So Eg is about 0. Because there is no gap to cross, a huge number of electrons are already free to move even at room temperature. That is why metals have very low resistance and high conductivity. This is the key difference from semiconductors and insulators, which do have a real gap.
It is decided by the size of the band gap Eg. Metal: Eg is about 0 (bands overlap or conduction band is partly filled) so many free electrons, high conduction. Insulator: Eg is large, greater than 3 eV, so thermal energy at room temperature cannot push electrons across, almost no conduction. Semiconductor: Eg is small, less than 3 eV (NCERT range 0.2 eV to 3 eV), so at room temperature a few electrons cross the gap and give weak conduction. Examples from NCERT: C (diamond) Eg = 5.4 eV insulator, Si Eg = 1.1 eV semiconductor, Ge Eg = 0.7 eV semiconductor, Sn Eg = 0 eV metal.
At 0 K there is no thermal energy. So every electron stays in the valence band and the conduction band is completely empty. With no electrons in the conduction band, no current can flow, exactly like an insulator. When temperature rises above 0 K, thermal energy lifts a few electrons across the small gap into the conduction band, and only then does the semiconductor start to conduct. This is why a semiconductor at 0 K behaves like an insulator, but not at room temperature.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
In a pure semiconductor or insulator at 0 K the valence band is completely filled and the conduction band is empty. In a metal the valence band may be only partly filled or may overlap the conduction band, which is why metals conduct so well.
NCERT gives Eg = 1.1 eV for silicon (Si) and Eg = 0.7 eV for germanium (Ge). Both are small, so both are semiconductors. Diamond (C) has Eg = 5.4 eV (insulator) and tin (Sn) has Eg = 0 eV (metal).
Because no electron is allowed to have an energy value that lies inside the gap. An electron must be either in the valence band or in the conduction band. There are no allowed energy states between E_V and E_C, so that region is forbidden.
The full reason (why levels split) comes from quantum physics and the exclusion rule, but for NEET you only need the simple picture: atomic levels split into bands, the valence and conduction bands are separated by the gap Eg, and the size of Eg fixes whether the solid is a metal, semiconductor or insulator.
It depends on the material: its atomic structure and the way its atoms are arranged (lattice). NCERT notes that lattice structure and atomic structure of the constituent element decide whether a material is a metal, semiconductor or insulator, which is why Si, Ge and diamond (all group IV) have very different gaps.
