A high forward voltage far beyond the build-in potential is applied to a P+/N junction (assume the P+/N junction is still functioning properly and not being damaged by the high current). Which of the following action will be the most effective to further increase the current at the same voltage?

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To tackle what happens when a high forward voltage is applied to a P+/N junction, we need to think about which part of the device most strongly governs the current under those conditions.
Option 1: increase the P+ doping. Heavily doping the P+ side already provides a high concentration of majority carriers there, so further increasing P+ doping tends to yield diminishing returns for forward current at a fixed voltage. The main bottleneck under forward bias, especially at large currents, is the resistance through the material rather than the depletion region, so this change would be less effective than altering the N side.
Option 2: reduce the P+ doping. Lowering the P+ doping would decrease the P-side conductivity, increasing overall series resistance and reducing current at the same voltage. This is contrary to the goal of increasing current, so this option is incorrect.
Option 3: increase the N doping. Boosting the N-side doping reduces its resistivity, increasing the availability of majority carriers on that side and enhancing carrier injection across the junction under forward bias. This directly lowers the series resistance and yields a larger current at the same applied voltage, making it the most effective choice among the options.
Option 4: reduce the N doping. Decreasing N-doping raises the N-side resistivity and reduces carrier supply across the junction, which would lower the forward current at the same voltage. This clearly does not help to increase current.
Thus, the reasoning shows that increasing N-doping most effectively lowers resistance and boosts current at a fixed forward voltage, while the other changes either have little effect or reduce current.

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

A high forward voltage far beyond the build-in potential is applied to a P+/N junction (assume the P+/N junction is still functioning properly and not being damaged by the high current). Which of the following action will be the most effective to further increase the current at the same voltage?

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