Why cant extrinsic semiconductors be fabricated from bivalent and hexavalent elements?

In N-type semiconductor, pentavalent impurity is added to contribute one electron.... why cant one add an element with six electrons in the outermost shell to contribute two electrons to increase the conductivity?

similarly,
In P-type semiconductor, trivalent impurity is added to contribute one hole.... why cant one add a bivalent impurity to contribute two holes to increase the conductivity?

 

krishnendukm,

In N-type semiconductor, pentavalent impurity is added to contribute one electron.... why cant one add an element with six electrons in the outermost shell to contribute two electrons to increase the conductivity?

Because it is not the purpose of doping to obtain high conductivity. If that is wanted, just use a thick piece of copper wire. The purpose is to dope the semiconductor, not saturate it. If too many electrons or holes are present, then it cannot be controlled. The ultimate case is a metal wire, where so many electrons are present that no transistor action with low voltage is possible. The doping levels are just high enough in a BJT so that it can be voltage controlled between a moderate level of conduction, and almost no conduction at all.

Ratch

 

could you please explain it with respect to bonding in solids?

 

krishnenduk,

could you please explain it with respect to bonding in solids?

Bonding is present in almost all phases of all matter. Why would semiconductors be any different? Is there anything you don't understand from my previous explanation? Perhaps you could elaborate on your question, because I don't understand it.

Ratch

 

Ratch has the essence of the answer to your question.

We don't want 'good' conductivity, we want controllable conductivity.
I will try to explain, with the help of the attached diagram. This is a very simplified account.

First it is important to realise that we are talking about solids, not individual atoms or molecules.
When large numbers of atoms aggregate to form solids the individual atomic/molecular orbitals (S, P, D etc) are merged into bands. These bands have quantum acceptable energy levels and forbidden levels, just as with orbitals. Similarly you have to supply energy to promote an electron to a higher band.

Now with regard to my diagram. In all cases I am showing two bands,called the valence band and the conduction band. There is an energy gap between these, which is forbidden by quantum mechanics to electrons.
In all cases the electrons are in the valence band in the ground state. We are only talking about the outer bonding electrons and how to promote some of them from the valence band to the conduction band.

I have distinguished three situations, A B and C.

In situation A, the conduction band is very close to or even overlapping the valence band. There is almost no forbidden zone.
This means that electrons require little energy to be promoted from the valence to the conduction band. In fact normal thermal activity is sufficient to achieve this.
We call these solids conductors as there are always some electrons in the conduction band.

In situation C the forbidden zone is large and a great deal of energy needs to be expended to promote electrons. In fact the energy may be greater than the bond energy of the solid so the solid may rupture, rather than conduct.
We call these insulators and the above statement explains their rather abrupt 'breakdown voltage'

In situation B the forbidden zone energy gap is too large for an electron to jump by normal thermal activity. However it is not as large as for an insulator. If we provide a reasonable level of 'bias', thus tilting the playing field, in the form of a suitable voltage we can cause electrons to be promoted to the conduction band in a controllable manner.
This of course was our original goal.
We call these solids semiconductors.

 

thanks for the reply buddies...

I understood whats doping is all about!! If the impurity content in a semiconductor is more than 1 part in 10 million parts of the SC then we can tell that, the SC is doped... anyway we need to add impurities to a pure SC to get it doped... we can add P-type (trivalent) or N-type (pentavalent) impurity...
lets assume i need to add 10 pentavalent atoms to dope a piece of SC to make it N-type.. then my doubt is, if I use a hexavalent atom to dope, then can i dope the SC with 5 atoms?? (because the bonding results in two free electrons).. can the same concept be applied to P-type?

 

I understood whats doping is all about!!

If you understand all this you will be able to calculate the energy gaps in the forbidden zone and answer your own question.

 

lets assume i need to add 10 pentavalent atoms to dope a piece of SC to make it N-type.. then my doubt is, if I use a hexavalent atom to dope, then can i dope the SC with 5 atoms?? (because the bonding results in two free electrons).. can the same concept be applied to P-type?

I'd say maybe (In my not so expertly opinion). Provided the temperature was high enough so that the impurity would ionize 2+, and not so high that the intrinsic material get way to many free carriers(Unless this is wanted in this strange device), it could be possible. If it actually is possible or practical I don't know - it all depends on the energy levels of the elements involved.

If it is commonly done, I highly doubt they do it to save on the n/2 impurity atoms. Probably to adjust some other property of the extrinsic material. E.g. add impurity levels within the bandgap, maybe make a direct band gap to avoid phonons during electroluminescence (contra indirect, but come to think about it, maybe that is mainly a property of the intrinsic material)

I guess at the end of the day I don't know for sure.