To distinguish between Avalanche breakdown and Zener breakdown in a PN junction diode, we must understand the conditions and characteristics of each type of breakdown:

1. Zener Breakdown:
   • Occurs in heavily doped diodes, resulting in a narrow depletion region.
   • Prominent in diodes with low breakdown voltage (less than 5V).
   • Happens at relatively low reverse voltages, where the electric field across the junction becomes extremely strong.
   • The strong electric field causes electrons to tunnel through the depletion region, leading to a sudden increase in reverse current.
2. Avalanche Breakdown:
   • Occurs in lightly doped diodes, which have a wider depletion region.
   • Prominent in diodes with higher breakdown voltage (typically greater than 5V).
   • Happens when the reverse voltage is high enough to impart sufficient kinetic energy to free electrons, allowing them to collide with atoms and generate additional charge carriers through impact ionization.
   • This process results in a gradual and significant increase in current once the breakdown voltage is reached.

Now let's analyze the options:

• Zener breakdown occurs only under forward bias conditions. - This is incorrect as Zener breakdown occurs under reverse bias conditions.
• Avalanche breakdown does not result in a sharp increase in reverse current. - This is incorrect because avalanche breakdown does lead to a significant increase in current, although it may not be as sharp as Zener breakdown.
• Zener breakdown voltage is typically higher than the avalanche breakdown voltage. - This is incorrect. Zener breakdown voltages are usually lower than avalanche breakdown voltages.
• Avalanche breakdown is the predominant mechanism in lightly doped junctions. - This is correct because the wider depletion region in lightly doped junctions favors avalanche breakdown.

Therefore, the correct statement is: "Avalanche breakdown is the predominant mechanism in lightly doped junctions."