Can someone explain me transistor as switch

[dineshauti]

I want to know about the internal capacitance of transistor c_bc and c_be

 

[MATLABdude]

I don't mean to be snarky, but your title has nothing to do with internal capacitances and the question you ask (unless you're counting switching speed).

If you're asking how you make a BJT as a switch, that depends upon the topology. Usually, you drive the BJT into saturation such that Vce is as low as possible (this requires you to know how much current your load draws). You can also use a base follower, but you need to supply around 5 or 10% of the current from your base (IIRC)

If you're asking what Cbc and Cbe are, well, these are the capacitances formed in the depletion region between the base and collector, and base and emitter, respectively. I'd suggest picking up an introductory text on active devices (Sedra and Smith, Microelectronic Circuits is the book I used many years ago), if you'd like to learn more about the topic though, if you have specific questions, we can definitely try to answer them.

EDIT: And welcome to PhysicsForums!