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!

 

[oswaler]

Sure, I'd be happy to explain the concept of a transistor as a switch and the internal capacitance of the transistor.

A transistor is a semiconductor device that can be used as an electronic switch. It has three terminals: the collector, the base, and the emitter. In its simplest form, a transistor can be thought of as a switch that can be turned on or off by applying a small current or voltage to the base terminal.

When a small current or voltage is applied to the base, it creates a larger current flow between the collector and emitter terminals. This is because the base and emitter are doped with different types of semiconductors, creating a forward-biased junction. This allows current to flow from the collector to the emitter, completing the circuit and turning the transistor "on."

On the other hand, when no current or a very small current is applied to the base, the collector and emitter are not connected, and the transistor remains "off."

Now, let's talk about the internal capacitance of the transistor. There are two types of capacitance that are important in transistors: the junction capacitance and the Miller capacitance.

The junction capacitance, also known as the CBC (collector-base capacitance) and CBE (collector-emitter capacitance), refers to the capacitance between the collector and base, and between the collector and emitter, respectively. This capacitance is caused by the depletion region that forms at the junction between the different doped regions in the transistor.

The Miller capacitance, on the other hand, refers to the capacitance between the input and output terminals of the transistor. It is caused by the amplification of the input signal by the transistor and can affect the high-frequency performance of the device.

In summary, a transistor can act as a switch by using a small current or voltage to control a larger current flow between its collector and emitter terminals. The internal capacitance of the transistor, such as the CBC and CBE, can affect its performance and must be taken into consideration when designing electronic circuits. I hope this explanation helps!