What limits maximum forward current in a junction diode?

• A. The peak inverse voltage (PIV)
• B. The junction temperature
• C. The forward voltage
• D. The back EMF

Answer: (B)

When a diode conducts forward, it converts part of the current into heat at the junction. The amount of heat equals the forward voltage drop times the current, and this heat raises the junction temperature. The device has a maximum allowable junction temperature, so the maximum forward current is limited by how much power the junction can safely dissipate without overheating. If the current (and thus power dissipation) would push the junction temperature above that limit, the diode could be damaged or its performance degraded.

The peak inverse voltage is about how much reverse voltage the diode can block, not about forward current. The forward voltage is the voltage drop at a given current, not the limit on how much current can flow. Back EMF relates to inductive loads and switching transients, not the forward current rating of a diode.