What role does doping play in creating n-type and p-type semiconductors?

[PainterGuy]

hi everyone,

when antimony is added to germanium, antimony's atoms make covalent bonds with germanium. antimony has five valence electrons and germanium has four. there would be total 4 covalent bonds each consisting of 2 electrons. this would mean that there would be one extra from each antimony's atom hence only four electrons are used to make covalent bonds with germanium atoms. that's what n-type semiconductor is.

now suppose indium, with three valence electrons, is added to germanium atoms. before indium was added to germanium, atoms of germanium were covalently bonded to each other. each atom of germanium making 4 covalent bonds with other germanium atoms. now when indium is added, three out four covalent bonds of each germanium atom will break to form new covalent bonds with indium atoms because indium has only three valence electrons to form three covalent bonds. you see there is still one covalent bond remaining for each germanium atoms, which means all the electrons are used up. no electron is free and neither there is any deficiency of electrons. in other words no holes are there. where am i going wrong? any idea. any help would be welcome.

cheers

 

[nasu]

The Indium atom is surrounded by 4 Ge atoms. You say that 3 of them will form covalent bonds with the In atom. Good. Then you say that the 4-th Ge atom will have 4 covalent bonds. Now with whom will form this 4-th germanium atoms the fourth covalent bond?
Imagine that Germanium atom surrounded by 3 other Ge and one In. The In is already bound to its maximum capacity.

 

[PainterGuy]

thank you.

now suppose indium, with three valence electrons, is added to germanium atoms. before indium was added to germanium, atoms of germanium were covalently bonded to each other. each atom of germanium making 4 covalent bonds with other germanium atoms. now when indium is added, three out four covalent bonds of each germanium atom will break to form new covalent bonds with indium atoms because indium has only three valence electrons to form three covalent bonds. you see there is still one covalent bond remaining for each germanium atom, which means all the electrons are used up. no electron is free and neither there is any deficiency of electrons. in other words no holes are there. where am i going wrong? any idea. any help would be welcome.

i have boldfaced the parts which are important in my view. you see indium has formed as many bonds as it can. and germanium still has four covalent bonds (three with indium atoms, and one with other germanium atom). so where is any deficiency of electrons? please show me the light.

 

[nasu]

thank you.

now suppose indium, with three valence electrons, is added to germanium atoms. before indium was added to germanium, atoms of germanium were covalently bonded to each other. each atom of germanium making 4 covalent bonds with other germanium atoms. now when indium is added, three out four covalent bonds of each germanium atom will break to form new covalent bonds with indium atoms because indium has only three valence electrons to form three covalent bonds. you see there is still one covalent bond remaining for each germanium atom, which means all the electrons are used up. no electron is free and neither there is any deficiency of electrons. in other words no holes are there. where am i going wrong? any idea. any help would be welcome.

i have boldfaced the parts which are important in my view. you see indium has formed as many bonds as it can. and germanium still has four covalent bonds (three with indium atoms, and one with other germanium atom). so where is any deficiency of electrons? please show me the light.

In order to add the indium atom you have to remove one Ge atom. This will break 4 covalent bonds - the ones between this Ge atom and its neighbors. Then you put the In and restore 3 bonds. If you try to draw a diagram you'll see that one Ge atom has nobody to bind with through his 4-th electron. Not because there is a deficiency of electrons but because there is no neighbor in the right position do do it.


In reality this is not done in this order. More likely that the material is grown as a mixture of Ge and In. But I hope you get the idea.

The solid is an ordered structure. The atoms have to occupy specific places in a lattice.
There are cases when you can have interstitial impurities - the foreign atoms go in the spaces between the host atoms. In impurities in Ge is not such a case from what I know.