We know that, cut-in voltage of a transistor is less than 0.3v only,then what is the need to give higher voltages(5v or 12v) as power supply.
And if we give DC supply to a transistor, will it be work? or not?

 

The transistor is used to control other devices. These devices need an appropriate voltage to work properly. Transistors are DC devices and not AC devices, therefore they work with DC power supplies.

 

We know that, cut-in voltage of a transistor is less than 0.3v only,then what is the need to give higher voltages(5v or 12v) as power supply.
And if we give DC supply to a transistor, will it be work? or not?

Your post does not make much sense.

A typical NPN transistors' cutoff voltage is when Vbe is around 0.5v; the collector current is then quite small, perhaps 250uA. As Vbe increases, Ic increases. However, the response is very non-linear when measuring Vbe rather than Ib vs Ic, and is also subject to wide variations over the full temperature range.

Ib and Ic must always be limited by some means, whether resistors or otherwise. Without current limiting, your transistor will have the magic smoke let out of it, and will no longer function.

 

I wonder if the OP means the collector emitter saturation voltage?