# Why is n-p-n transistor typically “faster” than p-n-p transistor?

I know that the n-p-n transistor is faster than p-n-p transistor. My book explains it with 'the hole is faster than an electron. But I think the hole is empty space of electron, so these speeds should be perfectly the same.

Why is faster n-p-n transistor than p-n-p transistor?

• In silicon the effective mass of holes in the valence band is indeed less than the effective mass of electrons in the conduction band. – Jon Custer Oct 20 '20 at 13:44
• what do you mean by faster? – Young Kindaichi Oct 20 '20 at 13:48
• The heading to your question "What is difference between...?" is different than the question at the end of the body: "Why is faster...?" Which is the question? (Probably "faster"? You seem to understand the basic difference.) – Brick Oct 20 '20 at 13:58
• I think it is relative to the effective mass. 'faster' means the speed of the electron and hole I think the answer of my question is found in difference between p-n-p and n-p-n. So I write my question this way. I'm sorry to everyone. I am not good at writing to English. – 정우남 Oct 20 '20 at 14:01

## 1 Answer

The mobility of electrons in the conduction band is higher than the mobility of holes on the valence band. Both are indeed using electrons, but one is using the band which is at a higher energy and more "shared" between atoms, while the other is using a lower energy band which is more localized, hampering mobility.

• How is it possible the 'shared' and hampering? – 정우남 Oct 20 '20 at 14:20
• In the conduction band, it takes very little energy to move an electron from one atom to another. So much so that we often think of then as one big shared orbital between all of the atoms. In the valence band, an electron needs to get enough energy to jump into the conduction band, and then it can go to another atom, filling in a hole. – Cort Ammon Oct 21 '20 at 0:12
• Thank you so much! – 정우남 Oct 21 '20 at 1:40