Dear Sir, you wanted to tell that use of NPN type transistor increases the operating speed. Does it also mean that use of NPN type power BJT also decreases the turn-ON & turn-OFF time.

Yes, speed is one thing, but a PNP transistor requires 3 times more SI chip surface, so it gets uneconomical when the chip costs are a big part of the component.

But what is not clear why IGBT transistors use internally partly a PNP structure controlled with a N-channel mosfet. Would one not expect some anode gate IGBT for better performance? So combining an NPN structure with a P-channel mos.

Dear Sir,Your thinking is great . I appreciate it. Semiconductor chip designer should design it.

dear sir, i thought it is the effect of current gain is more for NPN compare with PNP....because of this reason regenerative process is more and more which gives very large current rushes from anode to cathode in very less interval of time ..this provides high switching speed ...which is asset of BJT

Also in p-type semiconductor emitter doping will be higher.We have to do this with Boron,generally.Now Boronn has a high misfit factor which will lead to low elecronic activity.Hence we use npn bjt more.

But why IGBT's are so good? They combine in the internal structure a PNP bipolar and an N-channel mosfet. The parasitic NPN in the N-channel mosfet stucture should not conduct. The freewheel diode can indeed be better, as it is not an intrinsic one, but a separated diode.