My book says:
A pure semiconductor at room temperature possesses free electrons and holes but their number is so small that conductivity offered by the pure semiconductor cannot be made of any practical use. By addition of impurities to the pure semiconductor in a very small ratio ($1:10^6$), the conductivity of a $Si$-crystal (or $Ge$ crystal) can be remarkably improved.
The process of adding impurity to a pure semiconductor crystal so as to improve its conductivity is called doping. During doping, the impurity atoms are added to the silicon crystal in a small ratio, its atoms replace the silicon atoms here and there.
Fig (left): Substituting a phosphorus atom (with five valence electrons) for a silicon atom in a silicon crystal leaves an extra, unbonded electron that is relatively free to move around the crystal
Fig (right): Substituting a boron atom (with three valence electrons) for a silicon atom in a silicon crystal leaves a hole (a bond missing an electron) that is relatively free to move around the crystal
I got few questions here, how are silicon atoms replaced, did impurity atoms exerted force to make position?
In chemistry we are taught about defects i.e some of the atoms might be missing from the crystal. So, I assumed some of silicon atoms to be missing, thus impurity atoms can be thought to go and fit into those vacancies. But, considering this to be true I got few troubles,
- Leaving those vacancies (without adding impurity atom) will be far better because, we will have $4$ unpaired bonding electrons and thus $4$ holes (considering coordination no. to be $4$), this is greater than having impurity atom in that vacancy which produces single hole from trivalent impurity. Absolutely there might be some thing wrong going here, because in reality adding impurity increases conductivity than staying quite without adding. Then is it that impurity atoms didn't go into vacancies, did they actually exerted force on silicon atoms to make position?
Adding pentavalent impurity i.e, adding single pentavalent impurity atom to $10^6$ silicon atoms would fetch only one free electron. How could this increase conductivity remarkably? We already have free electrons at room temperature, with out the one pentavalent impurity atom also conductivity should have remained same. But it is not the case. Then how could this single impurity atom make such a difference in conductivity (in a crystal of $10^6$ silicon atoms)?
I don't know whether I have misunderstood anywhere, if so pardon me and explain.
Sometimes I might not be communicating with you better, if so please comment on the part where explanation on the problem is to be extended.