Introduction:
In a p-n junction, the movement of charge carriers across the junction is responsible for the flow of electric current. When the p-n junction is unbiased, it means that there is no external voltage applied to the junction. In this scenario, the behavior of the charge carriers and the resulting current can be analyzed.

Explanation:
When zero current is observed in an unbiased p-n junction, it implies that the total current crossing the junction from the p-side to the n-side is equal to the total current crossing the junction from the n-side to the p-side. This can be explained as follows:

1. Equilibrium State:
In an unbiased p-n junction, the junction is in a state of equilibrium. This means that the number of charge carriers, such as electrons and holes, diffusing across the junction in both directions is equal.

2. Diffusion of Charge Carriers:
In a p-n junction, there is a concentration gradient of charge carriers across the junction. The p-side has excess holes (positive charge carriers), while the n-side has excess electrons (negative charge carriers). Due to this concentration gradient, the charge carriers tend to diffuse from the region of higher concentration to the region of lower concentration.

3. Majority and Minority Carriers:
In a p-n junction, the majority carriers are the charge carriers that are abundant in each region. In the p-side, the majority carriers are holes, while in the n-side, the majority carriers are electrons. The minority carriers are the charge carriers that are less abundant in each region.

4. Diffusion Current:
The diffusion of the minority carriers across the junction gives rise to a diffusion current. On the p-side, some electrons (minority carriers) diffuse across the junction and recombine with the holes. Similarly, on the n-side, some holes (minority carriers) diffuse across the junction and recombine with the electrons. This diffusion current is responsible for the movement of charge carriers across the junction.

5. Recombination and Generation of Current:
When the electrons and holes recombine at the junction, they neutralize each other's charge and generate a small amount of current flow. This recombination current, also known as the reverse saturation current, is usually very small compared to the diffusion current.

6. Zero Current Condition:
In an unbiased p-n junction, the diffusion current in both directions is equal. This means that the number of holes diffusing from the n-side to the p-side is equal to the number of electrons diffusing from the p-side to the n-side. As a result, the total current crossing the junction from the p-side to the n-side is equal to the total current crossing the junction from the n-side to the p-side, resulting in zero net current flow.

Conclusion:
In summary, in an unbiased p-n junction, zero current implies that the total current crossing the junction from the p-side to the n-side is equal to the total current crossing the junction from the n-side to the p-side. This is due to the equilibrium state of the junction, where the diffusion of charge carriers in both directions is balanced.