Q.What is meant by energy band gap in a solid ? Draw the energy band diagrams for a conductor, an insulator and a semiconductor.

Showing the 12 most recent of 24 on this concept.

1. CBSE 2026Set ANNUAL1 markMCQ
   Q.If the energy band gaps for Carbon, Silicon and Germanium are E1, E2 and E3 respectively, then the correct relation between them will be

   (a) E1 > E2 > E3

   (b) E1 = E2 = E3

   (c) E1 < E2 < E3

   (d) E1 > E2 < E3

   ›Reveal solutionSolution

   Going from carbon (diamond) to silicon to germanium in Group 14, the energy band gap decreases, so E1 (carbon) > E2 (silicon) > E3 (germanium).

   Carbon in its diamond form has a very wide energy gap (~5.4 eV), which is why it behaves as an insulator. Silicon has a moderate gap (~1.1 eV) and germanium has a smaller gap still (~0.7 eV), which is why both behave as semiconductors, with germanium being more conductive (thermally more easily excitable) than silicon. So the ordering by band gap is E1 (C) > E2 (Si) > E3 (Ge).

   ✓Final answer

   (a) E1 > E2 > E3.

2. CBSE 2026Set ANNUAL1 markMCQ
   Q.The width of forbidden band is maximum for

   (a) metals

   (b) non-conductors

   (c) semi-conductors

   (d) none of these

   ›Reveal solutionSolution

   The forbidden gap (band gap) between valence and conduction bands is essentially zero for metals, small (~1 eV) for semiconductors, and large (>3 eV) for insulators.

   In the band theory of solids, electrons occupy allowed energy bands, and the 'forbidden band' (band gap) is the energy range between the valence band (filled with electrons) and the conduction band (where electrons can move freely and conduct current).

   • In METALS/conductors, the valence and conduction bands overlap (essentially zero gap), so electrons move into the conduction band with negligible energy input - hence high conductivity.
   • In SEMICONDUCTORS, there is a small forbidden gap (of order 1 eV), so a modest amount of thermal or other energy can promote electrons across it.
   • In INSULATORS (non-conductors), the forbidden gap is large (typically several eV, e.g. >3 eV), so under normal conditions almost no electrons have enough energy to cross into the conduction band - hence they barely conduct.

   So the widest forbidden band belongs to non-conductors (insulators).

   ✓Final answer

   (b) non-conductors (insulators).

3. CBSE 2026Set ANNUAL1 mark
   Q.What do you understand by energy band?

   ›Reveal solutionSolution

   In an isolated atom electrons occupy sharp, discrete energy levels; in a solid, the overlapping of countless such levels from ~10²³ closely packed atoms produces broad, continuous-looking bands of allowed energies.

   In a single isolated atom, electrons can only have certain sharp, discrete energy values. When a huge number of atoms (of order Avogadro's number, ~$10^{23}$ per cm³) are brought close together to form a solid, their outer electrons interact and each originally-discrete energy level splits into a large number of very closely spaced sub-levels — so close together that they appear to form a continuous range, called an energy band. The band formed from the valence electrons is the valence band, the next higher band (which may be empty or partly filled) is the conduction band, and the energy range between them where no allowed states exist is the forbidden energy gap. This band picture explains why solids behave as conductors, insulators, or semiconductors depending on how these bands are filled and how large the gap between them is.

   ✓Final answer

   A closely spaced (near-continuous) range of allowed electron energy levels formed by the merging of discrete atomic levels when many atoms combine to form a solid.

4. CBSE 2025Set 55/4/11 markMCQ
   Q.Which of the following is an electrical conductor at room temperature? (A) Sn (B) Mica (C) Si (D) C

   ›Reveal solutionSolution

   The key idea is to classify each material by its electrical conductivity at room temperature. Sn (tin) is a metal and a good conductor, while mica is an insulator and silicon is a semiconductor. The correct option is (A).

   Concept and Intuition

   Electrical conductivity depends on how freely electrons can move through a material. At room temperature:

   • Metals (like tin) have a "sea" of delocalized electrons that flow easily — they are excellent conductors.
   • Insulators (like mica) have electrons tightly bound to atoms — they barely conduct.
   • Semiconductors (like silicon) have a small band gap; at room temperature, some electrons gain enough energy to jump to the conduction band, making them moderate conductors, but not as good as metals.

   The question asks for an electrical conductor — meaning a material that readily allows current flow. Among the options, only one is a metal.

   Step-by-Step Solution

   1. Identify the nature of each substance

      • Sn (tin): A metal in Group 14 of the periodic table. It has metallic bonding with free electrons.
      • Mica: A silicate mineral — a classic electrical insulator used in capacitors and high-voltage applications.
      • Si (silicon): A metalloid, a semiconductor. Its conductivity is low at room temperature (though it increases with doping or heating).
      • C (carbon): Can exist as graphite (a conductor) or diamond (an insulator). The problem doesn't specify the allotrope, but in standard exam contexts, "C" alone usually refers to the element in its common form — often graphite is considered, but here the intended answer is clearly tin.

   2. Compare conductivities at room temperature

      • Tin has a resistivity of about $1.1 \times 10^{-7} \, \Omega \cdot \text{m}$ — typical of a metal.
      • Mica has resistivity exceeding $10^{12} \, \Omega \cdot \text{m}$ — an insulator.
      • Silicon has resistivity around $2.3 \times 10^{3} \, \Omega \cdot \text{m}$ — a semiconductor, not a good conductor at room temperature.
      • Carbon (graphite) has resistivity ~$1.4 \times 10^{-5} \, \Omega \cdot \text{m}$, which is higher than metals but still conductive. However, in multiple-choice questions for Indian exams (like JEE or NEET), "C" as an element is often treated as a non-metal and not a good conductor unless specified as graphite. The safe and standard answer here is tin.

   3. Eliminate options

      • (B) Mica: clearly an insulator.
      • (C) Si: semiconductor, not a conductor at room temperature.
      • (D) C: ambiguous, but in this context, not the best conductor.
      • (A) Sn: the only metal, hence the only reliable electrical conductor.

   Watch out

   A common mistake is to think silicon conducts well at room temperature. It does not — its conductivity is millions of times lower than a metal's. Also, carbon in the form of graphite conducts, but the problem likely expects the unambiguous metallic conductor.

   Tip

   In such classification questions, remember: metals are always conductors, insulators are non-conductors, and semiconductors are intermediate. If in doubt, pick the metal.

   ✓Final answer

   The correct option is (A) Sn.

5. CBSE 2025Set X11 markMCQ
   Q.The energy gap for silicon is :

   (a) 0.72 eV

   (b) 1.1 eV

   (c) 3 eV

   (d) 5 eV

   ›Reveal solutionSolution

   (b) 1.1 eV The forbidden energy gap between the valence band and conduction band in silicon is about $1.1\ \text{eV}$ at room temperature (for germanium it is about $0.72\ \text{eV}$).

   ✓Final answer

   (b) 1.1 eV

   The forbidden energy gap between the valence band and conduction band in silicon is about $1.1\ \text{eV}$ at room temperature (for germanium it is about $0.72\ \text{eV}$).

6. CBSE 2025Set ANNUAL1 markMCQ
   Q.The conduction band is partially empty in

   (a) non-conductors

   (b) semi-conductors

   (c) metals

   (d) super-conductors

   ›Reveal solutionSolution

   Good conduction requires easily-available empty states just above the highest filled electron levels; in metals the conduction band itself is partially filled (or overlaps the valence band), giving electrons free states to move into under an applied field.

   Band theory picture:

   • Non-conductors (insulators): a large energy gap separates a full valence band from an empty conduction band, so essentially no electrons can be thermally excited across - conduction band stays effectively empty.

   • Semiconductors: a small energy gap allows a few electrons to be thermally excited into an otherwise mostly-empty conduction band.

   • Metals: the conduction band is partially filled with electrons to begin with (or overlaps the valence band), so there are always free electrons AND free states available - this partial filling is exactly why metals conduct so well even without extra excitation.

   • Superconductors are metals/alloys below their critical temperature exhibiting zero resistance, but this is a special low-temperature quantum state, not simply "a partially empty conduction band" as the defining band-structure answer here.

   ✓Final answer

   (c) metals.

7. CBSE 2025Set ANNUAL1 markMCQ
   Q.If temperature of semiconductor is fall, the forbidden energy gap is –

   (a) increased

   (b) remain unchanged

   (c) decreased

   (d) sometimes increased and sometimes decreased

   ›Reveal solutionSolution

   The forbidden energy gap widens slightly as temperature falls.

   For a semiconductor, the width of the forbidden energy gap $E_g$ typically has a small, gradual dependence on temperature: as temperature falls, the band gap increases slightly (e.g. silicon's gap rises from about 1.12 eV at 300 K to about 1.17 eV near 0 K), mainly due to reduced thermal expansion and reduced electron-phonon interaction narrowing the gap less. This is consistent with why a semiconductor's conductivity falls sharply as temperature is lowered — fewer carriers are thermally excited across a wider gap.

   ✓Final answer

   (a) increased.

8. CBSE 2025Set ANNUAL1 markMCQ
   Q.The energy difference between valence band and conduction band of silicon at room temperature is about

   (a) 1.1 eV

   (b) 0.15 eV

   (c) 0.67 eV

   (d) 6.7 eV

   ›Reveal solutionSolution

   Silicon's band gap at room temperature is about 1.1 eV, distinguishing it from germanium (~0.7 eV).

   The energy gap $E_g$ between the valence band and conduction band determines whether a solid behaves as a conductor, insulator, or semiconductor. For silicon at room temperature (300 K), the standard value is $E_g \approx 1.1\ eV$ (germanium's is about 0.72 eV, option c).

   ✓Final answer

   Energy gap of silicon at room temperature ≈ 1.1 eV — option (a).

9. CBSE 2025Set ANNUAL1 mark
   Q.Distinguish between a semiconductor and an insulator based on their energy gap.

   ›Reveal solutionSolution

   The energy gap between the valence and conduction bands is small enough in a semiconductor for thermal energy to promote a few electrons across it, but far too large in an insulator.

   In the band theory of solids, electrical conduction requires electrons in the (otherwise full) valence band to be excited across the energy gap $E_g$ into the empty conduction band.

   • Semiconductor: $E_g$ is small, typically about 1 eV or less (e.g. Si $\approx 1.1$ eV, Ge $\approx 0.7$ eV). At room temperature, thermal energy is enough to excite a small but significant number of electrons across this gap, giving the material a modest, temperature-dependent conductivity.

   • Insulator: $E_g$ is large, typically more than about 3 eV (e.g. diamond $\approx 5.4$ eV). Thermal energy at ordinary temperatures is nowhere near enough to excite electrons across such a wide gap, so the conduction band stays essentially empty and the material does not conduct.

   ✓Final answer

   A semiconductor has a small forbidden energy gap ($\approx$ 1 eV), allowing some thermal excitation of electrons into the conduction band; an insulator has a large energy gap ($\gtrsim$ 3 eV), which thermal energy cannot bridge, so it does not conduct.

10. CBSE 2024Set A11 mark
    Q.The value of energy band in insulators is ———————— than 3 eV. Fill in the blank choosing the appropriate answer from the bracket: (decreasing, interference, helium, greater, diffraction, increasing)

    ›Reveal solutionSolution

    greater (more) than 3 eV.

    ✓Final answer

    greater (more) than 3 eV.

    In insulators the energy gap (forbidden band) between the valence band and the conduction band is large, greater than about $3\ \text{eV}$, so electrons cannot easily be excited into the conduction band and the material does not conduct.

11. CBSE 2024Set ANNUAL1 markMCQ
    Q.Assertion (A): Resistance of conductors is lower than semiconductors. Reason (R): In conductors, energy gap between conduction band and valence band is greater than 3 eV.

    (a) Both A and R are correct and R is the correct explanation of A.

    (b) Both A and R are correct but R is not the correct explanation of A.

    (c) A is correct but R is incorrect.

    (d) Both A and R are incorrect.

    ›Reveal solutionSolution

    Conductors do have lower resistance than semiconductors, but conductors have essentially zero (overlapping) energy gap, not a gap greater than 3 eV.

    Assertion (A) is correct: conductors have much lower resistance (and much higher conductivity) than semiconductors, because conductors have a very large number of free charge carriers. Reason (R) is incorrect: in conductors, the conduction band and valence band actually overlap (energy gap $E_g \approx 0$) -- there is no forbidden gap at all. An energy gap greater than 3 eV is instead characteristic of insulators, not conductors. So A is true but R is false, and R does not explain A.

    ✓Final answer

    (c) A is correct but R is incorrect.

12. CBSE 2024Set A1 markMCQ
    Q.The width of forbidden energy gap in the semiconductor is approximately (A) 1 eV (B) 10 eV (C) 100 eV (D) 0.01 eV

    ›Reveal solutionSolution

    A semiconductor's forbidden energy gap is approximately 1 eV → option (A).

    The energy gap between the top of the valence band and the bottom of the conduction band determines whether a material is a conductor, insulator or semiconductor. For semiconductors this gap is small, of the order of ~1 eV (e.g. ≈1.1 eV for silicon, ≈0.7 eV for germanium). Insulators have much larger gaps (several eV), while conductors have overlapping bands (essentially zero gap).

    ✓Final answer

    (A) 1 eV.