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A P-N junction diode is isolated (not connected to any external circuit). Which of the following statements about the junction is correct?

1. The electric potential is uniform throughout the device.
2. The P-type region is at a higher electric potential than the N-type region.
3. An electric field exists at the junction, directed from the N-type side toward the P-type side.
4. An electric field exists at the junction, directed from the P-type side toward the N-type side.

Correct answer: An electric field exists at the junction, directed from the N-type side toward the P-type side.

Solution

When a P-N junction forms, electrons diffuse from N to P and holes diffuse from P to N, leaving behind immobile ionized donor atoms (positive) on the N-side and ionized acceptor atoms (negative) on the P-side. This charge separation creates a built-in electric field directed from the N-type region to the P-type region across the depletion layer. Because positive charge is on the N-side, the N-side is at a higher potential than the P-side (not lower). There is no uniform potential.

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