To understand this, consider the behaviour of a p-n junction diode under reverse bias. When a reverse voltage is applied, the depletion region widens and majority carriers are pulled away from the junction, effectively blocking the majority-carrier current. However, a very small current still flows due to minority carriers — electrons present in the p-region and holes present in the n-region.

These minority carriers are not created by the applied voltage; rather, they are thermally generated within the semiconductor material due to the breaking of covalent bonds at ordinary operating temperature. Once generated near the depletion region, the existing electric field of the junction (reinforced by the reverse bias) simply sweeps these minority carriers across the junction, contributing to what is called the reverse saturation current (I₀), sometimes also called leakage current.

The key insight is this: since I₀ depends on the rate of thermal generation of minority carriers, and not on the magnitude of the reverse voltage, increasing the reverse voltage (before breakdown) does not significantly change I₀. All the applied reverse voltage does is ensure that essentially all of the already-available minority carriers are swept across — once that "saturation" condition is reached, adding more voltage cannot pull across more carriers than are being thermally generated. This is why I₀ is termed a saturation current, and it explains why the reverse current-voltage curve is nearly flat (constant) over a wide range of reverse voltages, right up until the breakdown voltage is reached.

Only when the reverse voltage reaches the breakdown voltage does the current increase sharply, through mechanisms such as avalanche breakdown (carriers gaining enough kinetic energy to knock out additional electron-hole pairs in a chain reaction) or Zener breakdown (direct field-induced tunnelling of electrons across the depletion region in heavily doped junctions).

The other options do not hold up: the current does not become exactly zero, since minority carrier generation guarantees a small but non-zero I₀ at all temperatures above absolute zero; it does not increase rapidly from the very beginning, since that rapid rise is characteristic only of the breakdown region, not the region before it; and reverse current does not spontaneously convert into forward current merely due to increasing reverse voltage — the current direction is set by the bias polarity, not by its magnitude. It is worth noting, though, that I₀ does increase with temperature (roughly doubling for every 10°C rise), since higher temperature increases the thermal generation rate of minority carriers — but that is a separate dependency from the voltage dependency being tested here.