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GATE 2024 EC – Question 26

Electronic Devices · Carrier Transport · 1 mark · Multiple select

The free electron concentration profile $n(x)$ 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?

Diagram for GATE 2024 EC question 26
  1. For $x$ between B and C, the electron diffusion current is directed from C to B.
  2. For $x$ between B and A, the electron drift current is directed from B to A.
  3. For $x$ between B and C, the electric field is directed from B to C.
  4. For $x$ between B and A, the electric field is directed from A to B.

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Show answer and explanation

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.

Explanation

Electrons diffuse from high to low concentration, so between B and C they move towards C and the (conventional) electron diffusion current points from C to B: (A) is true. Between A and B, $n$ rises towards B, so electrons diffuse towards A and the diffusion current points from A to B. At equilibrium the net current is zero, so the drift current is opposite: from B to A: (B) is true. Drift current $qn\mu_nE$ flows along $E$, so between B and A the field points from B to A (D is false). Between B and C the diffusion current points C to B, so the drift current and hence the field point from B to C: (C) is true.