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You ask a lot of elementary questions. If you ask these questions and have the ability to post these questions (internet), then you also have within your abilities or reach (such as a text book) the answers to these questions. Why bother posting such questions when the answers could be found with much easier methods.
Anyways, this will be the last I reply to your quandries. Its as frivolous as some newbie posting to get D/L credit.
Current flow is generally electrons. An electron has a charge of 1.69x10-19 C. Current is the fllow of these charges, which has the units of Coulombs (C). As these charges flow, move, displace there are two factors time and distance. The charge that moves through a small unit area is current density, and you may have guessed that the charge that moves in that unit area with respect to time is an ampere.
With regards to "mechanisms" of current flow in conductors and "semi" conductors, well ahh, its the same. The mechanism for current flow is generally always the same, that is electrons move. Because a conductor conducts and a semiconductor semi-conducts doesn't change this, but what does change is the material which the current must "conduct" in. In 2000 Microsoft, Intel and AOL teamed together to form this material called Si. They created a material which had "trans" electrical properties and a bandgap energy of 1.12 eV. This material has helped to benifit CPU's, memory, Apple Ipod's, and most importantly the geographical valley located below San Fransico but above Los Angeles.
"If you don't know, now you now N!&&@" Notorious BIG, RIP
in conductors, current flow easily b/c its ideal for current flow..
in semi-conductor, u have to do arrangements like u have to dope it to flow the current...and amount of current is less compared to semi-conductor