If we understand the basic theory of the semiconductor, then we know that semiconductors has holes and electrons. But question here arises what are basically holes? Let us discuss:
Most of us know that electrons move and the vacancy electrons left is called hole. But the question again arises here, if hole is just the vacancy then why it has positive charge. Let us discuss this now:
Suppose there are 3 electrons and a vacant space
1. e e e _
Now the third electron moves to the right, that is
2. e e _ e
So now the hole is at third position and we can say that the it moves to the left.
Now the second electron moves to the right, that is
3. e _ e e
so the hole moves to the left
and now first electron moves to the right
4. _ e e e
So the hole again moves to the left.
I know that you all know this. But here the basically only electrons are moving. Holes are just fictitious entity and a mathematical trick. Holes are just introduce to make things easier.
Therefore, it is only a convention that we call hole has a positive charge. In other words, one more reason can be that when an atom loses an electron, we say that it has now positive charge. Similarly, one electron leaves a place, we say that by convention, that vacancy or hole has positive charge.