Transistor saturation

Thread Starter


Joined Nov 13, 2006
what is all this about transistor haveing a saturation delay hen switching between cutoff and saturaton.

I dont see why such a things should happen. Can someone explain it based on the water analogy of the transistor.


Joined Oct 14, 2005
Every transistor, I assume we are talking about BJT here, has base capacitances. When the transistor in cut-off mode, the base is at the same potential as the emitter. The base-emitter capacitance is therefore is empty of charge, while the base-collector capacitance is fully charged.

When base current is applied, the current has to charge the base-emitter capacitance and discharge the base-collector capacitance. With finite value of maximum base current, the time taken to charge/discharge these capacitances are also finite. It is this charging/discharging time that causes the turn-on delay from cut-off to saturation.

The same mechanism exists when the transistor is moving from saturation to cut-off, except the charging/discharging are now reversed. However, the amount of charges might be a little bit more in this case, causing a longer turn-off delay compared to turn-on delay.

Water analogy? Hmm... let's just say it takes time to turn the faucet on and off :d