The PN junction has the very useful property that electrons are only able to flow in one direction. As current consists of a flow of electrons, this means that current is allowed to flow only in one direction across the structure, but it is stopped from flowing in the other direction across the junction.

Why does current flow in the forward-bias direction but not flow in the reverse-bias direction in a pn junction?

In a pn junction, the flow of current depends on the biasing condition applied to the junction. A pn junction can be operated in two biasing conditions: forward bias and reverse bias. In the forward-bias condition, the positive terminal of the battery is connected to the p-region (anode) and the negative terminal is connected to the n-region (cathode). Conversely, in the reverse-bias condition, the positive terminal is connected to the n-region while the negative terminal is connected to the p-region.

The reason why current flows in the forward-bias direction but not in the reverse-bias direction can be explained by the behavior of majority and minority charge carriers in the pn junction.

Majority carriers pushed to the junction:
In a pn junction, there are two types of charge carriers: majority carriers and minority carriers. Majority carriers are the dominant charge carriers in each region, while minority carriers are present in smaller quantities. In the p-region, the majority carriers are holes, and in the n-region, the majority carriers are electrons.

When a pn junction is forward-biased, the positive terminal of the battery repels the holes in the p-region, and the negative terminal repels the electrons in the n-region. This repulsion force pushes the majority carriers towards the junction.

Minority carriers pushed to the junction:
In addition to the majority carriers, there are also minority carriers present in both regions. In the p-region, the minority carriers are electrons, and in the n-region, the minority carriers are holes. In the forward-bias condition, the repulsion force applied by the external battery also pushes the minority carriers towards the junction.

Combining the effect of majority and minority carriers, both types of charge carriers are pushed towards the pn junction in the forward-bias condition. This results in an accumulation of charge carriers near the junction, creating a region of high charge density.

The intrinsic carriers are removed:
On the other hand, in the reverse-bias condition, the positive terminal of the battery attracts the electrons in the n-region, and the negative terminal attracts the holes in the p-region. This attraction force pulls the majority carriers away from the junction, creating a depletion region with a low charge density. Additionally, the reverse-bias voltage also increases the width of the depletion region.

In this condition, the minority carriers are not actively pushed towards the junction. Instead, they are effectively removed from the junction region due to the widening of the depletion region. As a result, there is a lack of charge carriers near the junction, preventing the flow of current.

Therefore, the correct answer is option C: Majority carriers are pushed to the junction.