Transistor and Back to Back Diode Operation?

[varma844]

back to back diode

Hi

The Ebbers Moll model says that a transistor can be treated as two diodes connected in back to back fashion .....
Still , if u observe the internal operation of these 2 , then the operation of the diode circuit doesnot give the same characteristics as that of transistor.....

Why is this so ??

 

[itsthepip]

back diode operation

Ebbers Moll model says that these 2 are equivalent..This means that while analysing their behaviour thru equations, the BJT can be replaced with this model

However , when go into the internal operation , the conept of a short and long base diode creeps in .
The 2 diodes in back to back give an effectively long base width and the width will be larger than the diffusion length of the holes .So effectively no current flows from the first diode to the second diode ..But for amplification , u need that current should flow from the emitter to collector ..
Also in BJT, u use a short base diode so that the current that recombines in base is less .

 

[sengyee88]

why two diodes back to bcak

I agree. Short base and long base make the difference.
That's why we can never form a transistor by using two diodes back to back.

 

[zenisle]

two diodes back to back

diode and diode back to back is not same a transistor, because the base with is too large.

 

[Grig]

back to back diode representation of bjt

Hi
back to back diodes and for ex npn bjt is only schematicaly the same
The physics is defferent
Back to back diodes have not only np and pn janctions but p to contact and contact to p junctions too.
So carrier transfer is not the same realy

 

[Gundam001]

diode operation of a transistor

Back to back diode is just a representation but it is not what really happening..

 

[raduga_in]

+two diodes+transistor

The thing is like this, the process of amplification
happens because of the carriers reach collector
of the transistors from the emitter. If we keep the
arrangement of the back to back diodes this phenomenon will not occur and hence the one you
are refering is a MODEL not the physical reality.