signal - How fast can a Transistor Switch?

An NPN Transistor can be made a switch by applying some power to the Base.

Is there a limit to how fast I can switch the Transistor On and Off? Or I can switch it on and off as fast as i can turn the signal to the base on and off?

As switching, I mean completely off and completely on. Not in between (if there is even any).

Any Answer will be Appreciated. Thanks in Advanced.

Answer

The major limit to BJT switching time is related to the charge carriers and specifically how long it takes to move carriers into the base, and how long it takes to get them out.

The datasheet will include a few parameters that will give you the theoretical maximum switching frequency*. They are

1. Delay time (td) - how long it takes to get out of cutoff


2. Rise time (tr) - how long is the transition from cutoff to saturation


3. Fall time (tf) - how long is the transition from saturation to cutoff


4. Storage time (ts) - how long to get out of saturation

Using the datasheet (these parameters are usually listed), you can figure out how fast a transistor can switch between the two states.

$$ f_{max} = \frac{1}{t_d + t_r + t_f + t_s} $$

* This is what transistor can theoretically do, but there are tricks that can be done to improve the switching speed. Also, if you are switching a square wave, then in order to maintain a nice square waveform, the actual switching frequency will be much less.