Reverse Bias on a Junction Diode
Reverse biasing is a condition in which the positive terminal of a voltage source is connected to the n-side of a junction diode and the negative terminal is connected to the p-side of the diode. In this condition, the diode is said to be in reverse bias.

Raising the Junction Potential Barrier
When a junction diode is in reverse bias, the potential barrier at the junction increases. The potential barrier is the energy difference between the conduction band of the n-side and the valence band of the p-side of the diode. In forward bias, this potential barrier is lowered, allowing current to flow through the diode. But in reverse bias, the potential barrier is raised, preventing the flow of current.

Explanation
The potential barrier is created due to the difference in doping concentrations on the n-side and p-side of the diode. The n-side is heavily doped with donor impurities, which provide excess electrons, while the p-side is lightly doped with acceptor impurities, which create holes or absence of electrons.

When a voltage is applied in reverse bias, the positive terminal repels the holes on the p-side, and the negative terminal repels the excess electrons on the n-side. This further increases the width of the depletion region at the junction.

Depletion Region
The depletion region is a region near the junction where there are no free charge carriers. It consists of immobile ionized impurities and creates the potential barrier. In reverse bias, the depletion region expands, increasing the width of the barrier.

Effect on Current Flow
Due to the increased potential barrier, the majority carriers (electrons in the n-side and holes in the p-side) find it difficult to cross the junction. The majority carriers are the dominant carriers in a diode, and their movement constitutes the diode current.

On the contrary, the reverse bias condition greatly increases the resistance to the flow of majority carriers, leading to a very small current, called the reverse saturation current. This current is due to the minority carriers (holes in the n-side and electrons in the p-side) that can still move across the junction due to thermal energy.

Conclusion
In conclusion, reverse biasing a junction diode raises the junction potential barrier. This increases the width of the depletion region, making it difficult for majority carriers to cross the junction and resulting in a small reverse saturation current.