HI,
Is transistor a back to back connection of diodes......If so why do we need the transistor device.Can we directly replace a transistor by connecting the two diodes...

 

A transistor is fabricated using two junctions, P-N and N-P as in a PNP transistor where there is a commonly shared n-region.
In an NPN transistor the polarity is reversed, i.e., N-P and P-N junction with a shared p-region.

In some respects this can be modeled as two diodes back-to-back. However, two diodes placed back-to-back do not make a transistor. Why?

Firstly, because the junctions share a common base region, the current flowing through one junction (base-emitter) has a huge influence on the current flowing through the other junction (collector-base).

Secondly, the impurity doping concentrations at the two junctions are very different.

You really need to look at the internal physics of the device where the density of electron-hole pairs play a critical role in the conductivity at the junction.

If you examine how a transistor is biased you will observe that the collector-base junction is reversed biased thus impeding the flow of current across this junction.
When the flow of charge carriers across the base-emitter junction increases, the change in charge density in the base region spreads to the collector junction eventually overcoming the reverse bias at the collector. Collector current then begins to flow, joining up with the base current towards the emitter. Thus the transistor begins to conduct from collector to emitter across the base region.

This phenomenon can never happen with two diodes connected back-to-back.

 

Two diodes would indeed form a transistor if you could get them close enough together.

The point is that the electric fields controlling the flow of charge carriers, both holes and electrons, overlap in space if you can bring the junctions close enough together.

That is how the can as Mr Chips says, affect each other.

Since these fields are short range they need to be closed than you can connect two discreet diodes, closer even than just two diodes formed on the same bar of semiconductor.

 

Funny, I had the same question many many years ago.

There is some funny (meaning I and a lot of opther people only think we understand this) quantum mechanical process where electrons jump across a relatively very thin base region. If you have two separate devices (two diodes) you do not have this thin base region and it will not ever exhibit and transistor action.

The reverse is also (almost) true too: a transistor makes a very poor pair of diodes (though it can make one very good diode).