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Holes are missing electrons. They behave as particles with the same properties as the electrons would have occupying the same states except that they carry a positive charge. Holes are found only in the valence band and because of the positive charge they move in the direction of the electric field applied.
The concepts of holes is introduced based on the notion that it is a whole lot easier to keep track of the missing particles in an "almost-full" band, rather than keeping track of the actual electrons in that band.
Concept of holes was derived mathematically by solving the Scrodinger Wave equations in a semiconductor. Hole is identified as an empty state in the valence band of the semiconductor. Things get confusing when we think hole is an empty state, so how can it have a mass associated with it and how can it drift and act like a positive carrier.
These things can be understood when we realise that the real phenomenon is the movement of electrons in the valence band and this is studied by the schrodinger wave equation, but it is not convenient to always solve things by the wave equation so we equate the results we get using the wave equation to the results we expect using classical newtonian mechanics equations and thus get the equivalent parameters such as the effective mass, the charge, etc. for the hole.
Here is a nice book which explains all this in a simple way in the first few chapters of it:
the concept of holes will known in the simeconductors, when u have a P-N juction and you connect it to a DC voltage, this will lead the electrons near to the gap between the P-N junction from the N type material to move to the P side and the lack of electrons in the N type side will lead to have a holes, so from here we can get the meaning of genration of the current in the P-N junctions (semiconductor)
If everyone here agrees that the holes are just a model used to express the missing electrons .. would someone kindly explain how we have 2 different mobilities for holes and electrons?!
To make my question more clear, in the end the whole motion of particles is just about the electrons, whether we saying moving holes or moving electrons, the actual particles moving throughout the semiconductor are ELECTRONS.
Then why do we say holes are of different mobility ?!
To understand why mobilities of electrons and holes are different you need to understand the concept of effective mass of electrons and holes.
Its true the movement is always of electrons. But in the conduction band of the semiconductor the scenario is that electrons are very few and there are a lot of empty energy states and so these electrons move quite freely. We calculate this Energy movement through Scrodinger Equations and then equate it to the classical F=ma equation and thus get expression for the "effective" mass of the elctron moving in the conduction band. So note the mass we use for electrons in the semiconductor calculation is not even the actual mass of the electron but some "effective" mass, which changes from semiconductor to semiconductor and even worse it also changes from the direction in which the electrons flow.
Now for holes, their movement is always considered in the Valence band, where there are lots of electrons and very few empty states of energy. We call these empty states as holes. So what we do is we consider holes as particles and then for these holes all electron occupied energy states now become empty energy states! But still since the movement of holes is in the valence band which has a different stucture of the available energy states therefore when you do the actual calculation through schrodinger wave equation and equate the result to the equation F=ma, you get a different "effective" mass. And that is the effective mass used for the hole.
So finally back to your question. The mobility of a particle is inversely proportianal to the effective mass as it turns out when you derive the expression for mobility. So since the effective mass of holes is greater than the effective mass of electrons the mobility of holes is less than that of electrons.
Hope this gave you a general idea, whats going on.
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