The forbidden energy gaps in conductors, semiconductors and insulators are \( (E_g)_c \), \( (E_g)_s \), and \( (E_g)_I \) respectively. What is the relation among them?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( (E_g)_c \lt (E_g)_s \lt (E_g)_I \)
The forbidden energy gap separates the valence band from the conduction band, and its size decides how easily a solid conducts.
\( \text{Conductor: bands overlap, } (E_g)_c \approx 0 \)
\( \text{Semiconductor: } (E_g)_s \lt 3\,\mathrm{eV} \)
\( \text{Insulator: } (E_g)_I \gt 3\,\mathrm{eV} \)
So the gap is smallest in a conductor and largest in an insulator:
\( (E_g)_c \lt (E_g)_s \lt (E_g)_I \)
Note: A similar question was also asked in CUET (UG) 2023 on 20-06-2023 (Slot 3).
NCERT (2026-27):
Classification of Metals, Conductors and Semiconductors – “… Case I: … One can have a metal either when the conduction band is partially filled and the balanced band is partially empty or when the conduction and valance bands overlap. …
Case II: In this case, as shown in Fig. 14.2(b), a large band gap Eg exists (Eg > 3 eV). … This is the case of insulators.
Case III: This situation is shown in Fig. 14.2(c). Here a finite but small band gap (Eg < 3 eV) exists.”