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.\(\mu = \frac{{{v_d}}}{E} = \frac{{q\tau }}{m}\)m = effective MASS of the charged particleThe 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.