BJT(npn) physical operation help!

[jinyong]

I just have a few questions on the physical operations of the BJT.

1) Van Zeghbroeck's book says, The carrier densities vary linearly between the boundary values as expected when using the assumption that no significant recombination takes place in the quasi-neutral regions. Why would the carrier density vary if there is no recombination. What causes the minority carriers to drop down in density as you move across the junction.

2) If the excess minority carrier concentration is the highest at the EBJ and lowest at the CBJ then how does the majority of the electrons get collected at the collector? At the CBJ the electron density is only np0 not np0*e^VBE/VT? How does that work? I thought most of the injected electrons from the emitter is suppose to be collected by the collector?

Thanks.

 

[Miguel Gaspar]

For 1) One of the mechanisms of conduction in the BJT is the result of the width of the base, small, and the aceleration of electrons, if they have enough aceleration to break both barriers of the base conduction is done, and that mean a change of velocity and a change of carrier densities.

2) the answear of 1) gives te answear of this

 

[Ahmed Ragab]

I just have a few questions on the physical operations of the BJT.

1) Van Zeghbroeck's book says, The carrier densities vary linearly between the boundary values as expected when using the assumption that no significant recombination takes place in the quasi-neutral regions. Why would the carrier density vary if there is no recombination. What causes the minority carriers to drop down in density as you move across the junction.

2) If the excess minority carrier concentration is the highest at the EBJ and lowest at the CBJ then how does the majority of the electrons get collected at the collector? At the CBJ the electron density is only np0 not np0*e^VBE/VT? How does that work? I thought most of the injected electrons from the emitter is suppose to be collected by the collector?

Thanks.

Ok first of all concerning your questions, we're talking about the base region of the BJT, which is small in width in order to allow the majority of carriers crossing from the emitter to reach the collecter without having much of a chance to recombine.

Now the carriers coming from the emitter and crossing to the collector are considered minority carriers inside the base region. Since the concentration of carriers in the emitter is much higher than those of the collector, this results in the distribution you mentioned throughout the base for the minority carriers, i.e. high concentrations near the emitter while lower concentration near the collector. This happens in the very beginning when the device is under equilibrium. (no voltage)

Now when biasing, charges move from collector to base, giving a rise to the minority carriers inside the base near the EBJ, while those near the CBJ will fall since the CBJ is reverse biased. The distribution of charges is approximated to a linear distribution since the base is very small in width.
Since the EBJ is forward biased the minority carriers reach a concentration of Npo^(VBE/VT) while the CBJ is reversed and the charge concentration is said to remain at Npo.
The charges moving from the emitter into base, will have a variety of kinetic enegies. Those of the highest will move throughout the base into the collector, while those of lower kinetic energies will move different distances before losing their energy and adding to the minority carrier of base. This explains why the concentration remains linearly varying.

Feel free to ask anything more, and if my reply did help then kindly click on the "helped me" icon.

Salam.