A very simple question...yet confusing... so plz give me ans...

[immadi.jagadish]

Q. If a tetra-valent impurity is added to a pure silicon crystal at room temperature, it becomes

(A) a p-type semiconductor (B) an n-type semiconductor

(C) an npn-type semiconductor (D) an insulator

what is the answer and justify it

 

[ckshivaram]

consider a pure silicon crystal with an impurity say gallium added to it
We know that silicon atom has 4 valence electrons and Gallium has 3 electrons. When Gallium is added as impurity to silicon, the 3 valence electrons of gallium make 3 covalent bonds with 3 valence electrons of silicon.

The 4th valence electrons of silicon cannot make a covalent bond with that of Gallium because of short of one electron as shown above. This absence of electron is called a hole. Therefore for each gallium atom added one hole is created, a small amount of Gallium provides millions of holes.
Due to thermal energy, still hole-electron pairs are generated but the number of holes are very large compared to the number of electrons. Therefore, in a p-type semiconductor holes are majority carriers and electrons are minority carriers. Since the current conduction is predominantly by hole( + charges) it is called as p-type semiconductor( p means +ve)

 

[immadi.jagadish]

if i add a material having 4 electrons in its valency to si at room temparature what happens?

 

[yadavvlsi]

If added atom sit in silicon as interstitial impurity i.e empty space between two silicon atoms. Than there is no sharing of electrons among silicon and impurity atom. Sharing of electrons takes place only when a silicon atom is replaced by impurity atom. Replacement of silicon atom can be done by another atom having nearly same size as silicon atom. It may be not be the case that atom having four electrons replace silicon atom.

 

[jpanhalt]

Perhaps the correct answer is not included in the four options you give, nor is the question completely clear. Two groups of elements commonly have a valence of +4. Historically, they were considered groups IVa and IVb. Unfortunately, Group IVb in CAS (commonly used in the USA) nomenclature (i.e., carbon, silicon, germanium, tin, and lead) were considered Group IVa in IUPAC nomenclature. People usually knew which group was being discussed by context. However, to avoid that ambiguity, current IUPAC recongnizes Group 4 (titanium, zirconium, hafnium) and Group 14 (carbon, silicon, germanium, tin, and lead). In other words, the IUPAC Group IVa (IVb in CAS) has been renamed Group 14.

If your question assumes doping with what are now Group 14 elements (carbon, germanium, tin, lead), such doping woud appear to give a semiconductor substrate with some potentially desirable qualities:

Silicon-germanium - Wikipedia, the free encyclopedia
Silicon carbide - Wikipedia, the free encyclopedia
Doping (semiconductor) - Wikipedia, the free encyclopedia

Results from doping with current Group 4 (IUPAC) elements (i.e., titanium, zirconium, hafnium)is less clear:

IEEE Xplore - Magnetoresistance in silicon-based semiconductor-metal hybrid structures
Properties of Silicon Crystals Doped with Zirconium or Hafnium - Lemke - 2006 - physica status solidi (a) - Wiley Online Library
**broken link removed**

Unfortunately, I do not have access to those articles.

John

 

[FvM]

Perhaps the correct answer is not included in the four options you give, nor is the question completely clear.

I think, you shouldn't put too much into the simple question. Because A to B are obviously wrong and C absurd, only D can be meant. Furthermore, the answer is correct at least for some dopants.

 

[immadi.jagadish]

as i know every semi conductor behaves as an insulator at room temparature.
one doubt is if i add anything suchas trivalent,tetravalent and pentavalent will the semi conductor behave like an insulator at room temparature?

 

[FvM]

as i know every semi conductor behaves as an insulator at room temparature.

Applies to the pure (undoped) material.

one doubt is if i add anything suchas trivalent,tetravalent and pentavalent will the semi conductor behave like an insulator at room temparature?

The result will be different, as previously discussed.

 

[immadi.jagadish]

What is the conclusion for my question?,still i am not clear.

 

[FvM]

Although jpanhalt has mentioned, that doping with tetra-valent elements can have different purposes, it basically doesn't change the "i" nature of pure Si.

 

[immadi.jagadish]

thank you got the point