n-p-n transistors are preferred over p-n-p transistors because they have

Since the mobility of electrons is higher in NPN transistors, they are preferred more as it will result in a smooth flow of charge carriers.

Explanation:

Mobility (μ): It is defined as drift velocity per unit electric field applied i.e.

m = effective mass of the charged particle

The effective mass of holes is more than that of electrons. This results in less mobility of holes.

NOTES:

NPN transistor: It is formed by sandwiching a thin layer of P-type semiconductor between two N-type semiconductors. Electrons are the carriers responsible for current flow.

PNP transistor: It is formed by sandwiching a thin layer of N-type semiconductor between two P-type semiconductors. Holes are the carriers responsible for current flow.