In a p - n junction, the diffusion current exceeds the drift...

In an unbiased $p$-$n$ junction, the diffusion current and drift current are equal in magnitude and opposite in direction, making the net current zero.

When the junction is reverse-biased, the width of the depletion region and the potential barrier increase. This opposes the diffusion of majority carriers, making the diffusion current negligible. The drift current, due to minority carriers, dominates.

When the junction is forward-biased, the applied voltage opposes the built-in potential, reducing the potential barrier. This allows a large number of majority carriers to diffuse across the junction. The drift current remains small and relatively unchanged. Thus, the diffusion current significantly exceeds the drift current in magnitude.