If I take your question at face value, then the answer is very simple...so simple that I don't think it's what you meant to ask.

THe reason they dope instrinsic silicon to turn it into extrinsic silicon is not to "change it into a conductor". In fact, it does not actually do this at all. Doping silicon changes it into a semi-conductor which turns it into a material that can change between conductor and insulator depending on the application of an electrical signal. It does not turn it into a conductor (or an insulator). That is why it's called a semi-conductor (I suppose you could also called it a semi-insulator). This allows you to make dynamic solid state devices like transistor switches, etc.

If you used only a conductor, it would always be a conductor (which is more of a static solid state device. Sure it has no moving parts, but it also has really static properties. You would never have anything more than a wire.

That is the reason (if I understood your question correctly).

There's a lot to a full answer as you need to understand the properties of conductors, insulators, electron bonding/affinity etc etc.

In a conductor the electrons are only loosely bound to their host atoms. So if you apply a potential difference (PD) to a conductor the electrons are easily moved along.

In an insulator the electrons are tightly bound to their host atoms, so it takes a large PD to break the bonds, so at small PD's no current flows because no electrons move.

Pure silicon is an insulator which is why it needs doping, and the amount and type of doping determines the type (p or n) and ability to conduct when a PD is applied. Silicon has a valency of 4 so it forms four tight covalent bonds with its neighbours in a silicon crystal.

Doping with phosphorous (valency 5) creates an n-type semiconductor as it creates an excess of electrons. Doping with boron (valency 3) creates a p-type semiconductor as it creates a shortage of electrons.

On their own I'm not sure that p-type and n-type silicon are much use, but when you put them together you get a p-n junction between the two and at equilibrium the electrons from the n-type will flow into the p-type and the holes vice versa, so the n-type will take on a positive charge and the p-type a negative charge.

Connecting a battery to this setup will have differing effects then, depending on which way round you connect it. If you connect the positive to the p-type and the negative to the n-type then electrons from the negative terminal will flow into the positively charged n-type, then in the p-type you can look at it two ways: (a) holes from the positive battery terminal will flow into the negatively charged p-type, or (b) electrons from the p-type will flow into the battery's positive terminal, and so current will flow (and there's more to it than that, but that's basically how it works).

If the battery is connected the other way round then any electrons left in the positively charged n-type, and any holes left in the negatively charged p-type, will be pulled towards the battery and current won't flow.

The Wikipedia articles explain it in more depth and much more clearly than this:
Semiconductor - Wikipedia, the free encyclopedia
p-n junction - Wikipedia, the free encyclopedia

Have another look at what your book says. It's definitely wrong if it describes pure silicon as a semiconductor - it isn't, it must be doped first.

Originally Posted by Electronman

The book which I am reading does not tell me so!
According to it the builders add impurity to the semiconductors like silicone to heighten its conductivity properties.
The book says adding more impurity is equal to more conductivity.

It is silicon, not silicone. Silicone is a compound, not an element and is a soft rubbery material.

Doped silicon does increase the conductivity, but not enough to make it a conductor. It is still very insulating. But the doping introduces defects into the silicon crystal lattice which adds in weak holes or weakly held electrons.
http://en.wikipedia.org/wiki/Donor_(semiconductors)
http://en.wikipedia.org/wiki/Acceptor_(semiconductors)

Normally, in a compound a negative atom will share it's excess electrons with a positive atom to balance out their charges. This makes the atoms bond together and the electrons are held in a valence bond between both atoms. Weak holes and weak electrons are holes and electrons that are not shared between the atoms, but are there because they came along with the dopant atom. They are not shared between atoms in a valence bond because the dopant atoms in the crystal lattice do not perfectly balance out the silicon atoms. Since the extra hole/eletrons is being held by only the one atom that brought them along, they are not in a valence bond where both atoms are holding onto it. The result is that they held more weakly and can be more easily knocked around and moved by an electric field which means they become much more conductive when you apply an electric field.

Take water, H2O, for example. Oxygen has two excess elecrons and hydrogen only has one. So one oxygen atom can share it's electrons with two hydrogen atoms in a valence bond to balance out their charges. But a dopant atom is "similar enough" to fit into a crystal lattice, but it is not the same so it does not fit in perfectly. Normally it might do what H2O and other compounds do by bonding with the right number of other atoms to share ALL of it's excess electrons in valence bonds, but a crystal lattice is very regular and does not allow this to happen. So there are gaps (holes) or extras pieces (electrons) lying around inside the lattice

Originally Posted by Electronman

is not it possible to use a natural conductor and heighten the holes of one conductor and the connect them together?
Is it possible to add more holes into a conductor by adding another matter to it anyway?
is there any conductor with more holes than electrons in it in this world?
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An electron is just a negative charge and a hole is a positive charge. Positive charges come from protons in the nucleus. So holes are just the absence of electrons in the presence of protons. So really, a hole is just a proton in the nucleus that does not have an electron paired up with it. If you add more protons to try to increase the number of holes, you get a different element. You can also try and strip it of it's electrons to produce holes but it will try and attract more electrons to fill in the holes (static electricity is the atom attracting more electrons to refill the holes). If you took enough electrons away, eventually you would have more holes than electrons (you would have more protons that do not have hold of an electron than protons that do).

I don't know the physics behind what makes an element capable of doping or not. It might be possible with any crystal lattice of any element. I am not sure. I am also not sure if it would increase conductivity. It might decrease it for all I know.

I think this answers all 4 of your questions. By the way, adding protons to a nucleus (which changes the element of the atom) is called fusion. It's not easy and releases a lot of energy. Stripping electrons away from an atom produces ions.

It doesn't roll off your tongue as nicely though. Variconductor would be a better name.

I think semi conductor is perfect.