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QPages 416-417: Q4: HC Verma: Concepts of Physics - Volume 2 Semiconductors and Semiconductor Devices Pages 416-417: Q4 Solution: In semiconductors, thermal collisions are responsible for…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often assume thermal collisions only promote electrons to the conduction band, overlooking that electrons simultaneously recombine with holes; the steady‑state carrier concentration is set by the balance of excitation and recombination rates, not by continual accumulation.

In semiconductors, thermal collisions are responsible for taking a valence electron to the conduction band. Why does the number of conduction electrons not go on increasing with time as thermal collisions continuously take place?

Step-by-Step Verified Solution

**Answer:** The number of conduction electrons does not increase indefinitely because electrons also fall back from the conduction band to the valence band, recombining with holes. An equilibrium is established where the rate of excitation equals the rate of recombination. **Step-by-Step Explanation:** Thermal collisions continuously excite electrons from the valence band to the conduction band, creating electron-hole pairs. However, conduction electrons also continuously recombine with holes, returning to the valence band. At a given temperature, a dynamic equilibrium is reached, and the number of conduction electrons remains constant .

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