Why are holes (in a semi conductor) regarded as particle? Can I say that holes in a semiconductors are treated as current-carrying conventional direction ? 
 A: There is a very nice demonstration that you can treat holes as positively charged carriers in the Hall effect.
As you may know, to observe the Hall effect we place a semiconductor in a magnetic field and pass a current through it. We observe a polarization (voltage) at right angles to both the current and the magnetic field as a consequence of the Lorentz force of the magnetic field on the charge carriers.
Assuming that the field is into the page, a conventional current from left to right will result in a force that is pointing up. This is the force that will be experienced by the charge carriers - whether they are positive carriers traveling to the right, or negative ones traveling to the left.
But here is the crazy thing: the observed polarity of the polarization depends on whether the charge carriers are positive or negative. And although it is sometimes said that the holes are really "just electrons moving the other way, with a vacancy", if that was really the case you would see a force displacing the electrons. But you don't: you see a force displacing the positive charge.
If that doesn't surprise you, you haven't been paying attention. There is a nice lab write up at http://labs.physics.berkeley.edu/mediawiki/index.php/Hall_Effect_in_Semiconductor that includes diagrams and more detailed explanation.
A: Holes are technically just vacancies,  but when an electron jumps into this vacancy,  one more hole is created,  but this constant filling up of a hole and creation of a hole appears as if the hole is moving,  that is it appears as if there are these +ve carriers which are moving in the direction of the current (opposite to that of the direction of electron) so we call holes as "particles."
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