PN junctions and diodes
Which correctly describes the depletion approximation used in analyzing PN junctions?
- AThe impurity concentrations on both sides of the junction are assumed to be equal
- BThe depletion layer is assumed to contain no carriers at all, and the regions outside it are assumed to be charge-neutral
- CMajority carriers are assumed to exist in the depletion layer at the same density as the impurities
Show answer and explanation
Answer: B. The depletion layer is assumed to contain no carriers at all, and the regions outside it are assumed to be charge-neutral
The depletion approximation assumes that the depletion layer contains no free carriers, only the charges of ionized impurities, and that the regions outside it are electrically neutral. This simplification with sharp boundaries allows the depletion width, field and capacitance to be obtained with simple formulas.
- A. The doping can differ on the two sides; it is not assumed to be equal.
- B. Correct. Zero carriers inside the depletion layer and neutrality outside.
- C. The depletion approximation assumes almost no carriers in the depletion layer.