Energy Bands — Conductor, semiconductor, insulator
Semiconductor
11
JEE Qs
8%
Hard
60
min
Focus on understanding the conceptual differences in band structure (overlap, small gap, large gap) and their direct implications for conductivity across conductors, semiconductors, and insulators.
🧮 Key Formulas
✅ Key Points for JEE
- 1In crystalline solids, atomic energy levels broaden into continuous energy bands: Valence Band (VB) and Conduction Band (CB).
- 2Valence Band (VB) is the highest occupied energy band at 0K, containing valence electrons tightly bound to atoms.
- 3Conduction Band (CB) is the lowest unoccupied energy band at 0K, where electrons can move freely and contribute to conduction.
- 4The energy gap (E_g) is the forbidden energy region between the top of the VB and the bottom of the CB; its magnitude primarily determines a material's electrical conductivity.
- 5Conductors have either overlapping VB and CB (effectively E_g ≈ 0) or a partially filled CB, allowing free electrons to move readily, resulting in high conductivity.
- 6Semiconductors have a small but finite energy gap (E_g ≈ 0.1-3 eV). At 0K they behave as insulators, but at room temperature, thermal energy allows some electrons to jump to the CB, increasing conductivity.
- 7Insulators have a large energy gap (E_g > 3 eV), requiring significant energy for electrons to jump from VB to CB, leading to extremely low conductivity even at high temperatures.
⚠️ Common Mistakes
- ✕Confusing the roles of valence and conduction bands or their filling status in different materials.
- ✕Misunderstanding that semiconductors are insulators at 0K and their conductivity increases significantly with temperature, unlike conductors whose conductivity decreases with temperature.
- ✕Incorrectly assuming that all materials with a band gap are semiconductors, overlooking the critical magnitude distinction for insulators.
📝 Practice Questions
See allQ27.Consider the following statements: A. The junction area of solar cell is made very narrow compared to a photo diode. B. Solar cells are not connected with any external bias. C. LED is made of lightly doped p-n junction. D. Increase of forward current results in continuous increase of LED light intensity. E. LEDs have to be connected in forward bias for emission of light. Choose the correct answer from the options given below: (1) B, E Only (2) B, D, E Only (3) A, C Only (4) A, C, E Only
Q43.Which of the following circuits has the same outpur as that of the given circuit? (1) (2) (3) (4)
Q33.Which of the following circuits represents a forward biased diode? Choose the correct answer from the options given below : (1) (A) and (D) only (2) (B), (D) and (E) only (3) (C) and (E) only (4) (B), (C) and (E) only
Q26. A B Y 0 0 1 0 1 1 1 0 0 1 1 1 To obtain the given truth table, following logic gate should be placed at G: 2025 (22 Jan Shift 2) JEE Main Previous Year Paper (1) OR Gate (2) AND Gate (3) NOR Gate (4) NAND Gate
Q28.The output of the circuit is low (zero) for : (A) X = 0, Y = 0 (B) X = 0, Y = 1 (C) X = 1, Y = 0 (D) X = 1, Y = 1 Choose the correct answer from the options given below : (1) (B), (C) and (D) only (2) (A), (B) and (C) only (3) (A), (C) and (D) only (4) (A), (B) and (D) only
Q45. In the circuit shown here, assuming threshold voltage of diode is negligibly small, then voltage VAB is correctly represented by : (1) VAB would be zero at all times (2) (3) (4)
NCERT Chapters
- Class 12 Physics Ch 14: Semiconductor Electronics: Materials, Devices and Simple Circuits