GATE EC 2024 Set 1 — Question 26
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Electronic Devices → Carrier Transport → Diffusion & Drift Current
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Question
The free electron concentration profile in a doped semiconductor at equilibrium is shown in the figure, where the points A, B, and C mark three different positions. Which of the following statements is/are true?
Correct answer
(A) For x between B and C, the electron diffusion current is directed from C to B.; (B) For x between B and A, the electron drift current is directed from B to A.; (C) For x between B and C, the electric field is directed from B to C.
Solution
At equilibrium, the total electron current density is zero: .
1.Electron diffusion current density: (where is the magnitude of electronic charge).
2.Electron drift current density: .
From , we have , which gives .Analysis:- Between B and C: The slope is negative.
- , so the diffusion current is directed in the direction, i.e., from C to B. (Option A is true)
- , so the electric field is directed in the direction, i.e., from B to C. (Option C is true)
- Between B and A: The slope is positive.
- , so the diffusion current is in the direction (from A to B).
- Since , the drift current must be in the direction, i.e., from B to A. (Option B is true)
- , so the electric field is in the direction, i.e., from B to A. (Option D is false)
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