Supposed you have a P+/N junction operating in the region where its current is dominated by recombination current in the depletion region. How would the current change when the doping of the N region is reduced?

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To tackle this question, first consider the physical setup: a P+/N junction where the current is dominated by recombination within the depletion region. The key variable is the doping level of the N region, which affects the depletion width W.
Option 1: increase. When the N-side doping is reduced, the depletion region widens because the space-charge region expands to balance charge. A wider depletion region contains more volume where Shockley-Read-Hokahara (SRH) recombination can occur. Since the recombination current in the depletion region is, to a first approximation, proportional to the volume of that region, enlarging W tends to increase the total recombination current. Thus this option is plausible and aligns with standard PN junction behavior under depletion-region-dominated recombination.
Option 2: decrease. This would imply that making the N doping lighter reduces the recombination current, but the primary effect of lowering N-doping on the N side is to widen the depletion region, not to suppress recombination per unit volume to a lesser extent. The net effect in the depletion-dominated regime is not a decrease, so this option is unlikely.
Option 3: remain unchanged. If recombination dominates in the depletion region, changing the depletion width by altering N-doping should modify the available volume for recombination; therefore, it is unlikely that the current remains exactly unchanged. This option ignores the geometric effect on recombination volume.
Option 4: No answer text provided. This option is simply an invalid or placeholder choice and does not address the physics of the situation, so it should be disregarded in a physical analysis.
In summary, recognizing that reducing N-doping increases the depletion width and that the depletion-region recombination current scales with that volume leads to the conclusion that the current would tend to increase. The other choices misstate the relationship by claiming decrease, no change, or ignoring the option text entirely.

EESM5200 (L1) Part I: Topic 7 - Real PN Junction Characteristics

Supposed you have a P+/N junction operating in the region where its current is dominated by recombination current in the depletion region. How would the current change when the doping of the N region is reduced?

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