Physics Electronics PMDC Conceptual Practice
PMDC Verified Question 189 of 494
What is the physical direction of the built-in electric field (\( E_0 \)) established across the depletion region of a PN junction at equilibrium?
A
From the P-region toward the N-region
B
From the N-region toward the P-region
C
Parallel to the flat junction surface plane
D
Alternating periodically in direction
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: From the N-region toward the P-region
Concept:

Electric field lines point from positive charges to negative charges. In the depletion layer, positive donor ions are exposed on the N-side and negative acceptor ions on the P-side, creating an electric field directed from N to P.

Formula:

$$\vec{E}_0 = -\frac{d V}{d x} \hat{i} \quad (\text{pointing from } +N_D^+ \text{ on N-side to } -N_A^- \text{ on P-side})$$

Solution:

  • The N-side of the depletion layer contains fixed positive donor ions (\( N_D^+ \)).


  • The P-side of the depletion layer contains fixed negative acceptor ions (\( N_A^- \)).


  • Therefore, the built-in electric field is directed from the positive N-region to the negative P-region.


Why other options are incorrect:

  • Option A: The P-side contains negative ions, so the field points toward it, not away from it.
  • Option C: The electric field is perpendicular to the metallurgical junction interface plane.
  • Option D: In thermal equilibrium, the built-in electric field is static and unidirectional.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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