Hint:The answer to this question lies in the mobility of electrons and holes. After knowing which one of them – electrons or holes – have higher mobility, we can answer why n-p-n transistors are preferred to p-n-p transistors.
Complete answer:
N-P-N transistors or P-N-P transistors both are different transistors that are being used for different purposes. The transistors in which the p-type is placed in between two n-type to amplify weak signals to enter the base and produce strong amplified signals at the collectors’ end are called n-p-n transistors. Whereas in a p-n-p transistor, the n-type material is placed in between two p-type to make it a current controlling device.
In the case of the p-n-p transistor, they have two $p$ diodes which point to the fact that the majority of its charge carriers are holes. But when it comes to mobility, the mobility of holes is very low when compared to the electrons, which makes p-transistors a controlling device instead of an amplifying device.
But in the case of n-p-n transistors, they have two n diodes which point to the fact that the majority of its charge carriers are electrons. And we already know that the mobility of electrons is far better than the holes. So n-p-n transistors are preferred as they amplify the signals.
So the correct answer is that n-p-n transistors are preferred to p-n-p transistors because electrons have higher mobility than holes and hence high mobility of energy.
Note:There are various uses of p-n-p transistors too, for example, they can be used as a source current, which means the current that flows out of the collector. There are many such uses.The core use of transistors include switching applications or both as amplification and switching. There are a kind of transistors which produce current flow depending on the amount of light shined upon them, those are known as phototransistors.
