Question

how a transister act as switch which open and close very rapidly.

Answer 1

roughly speaking a transistor is a semiconductor device with charge carriers than can travel between two terminals; you can command the transistor with a third terminal regulating charge carriers quantity: if you have a negligible carriers quantity then the switch is open (OFF) while if you have a big carriers quantity then the switch is closed (ON)

Answer 2

can you describe in detail for a npn transister.?

Answer 3

for the close condition you have to inject a current into the base such that BJT goes into saturation; find the minimum for the current gain from the datasheet and then use the formula:\[I _{B} \ge I _{Cmax}/h _{FEmin}\] For the open condition just give zero input.