Existence of minority carriers

[Aranion]

why do minority carriers exist? in a p-type material, why don't the minority carriers recombine (and get annihlated) with excess holes?

 

[reddvoid]

in P type if there are any minority carriers (electrons) in Valence band where majority charge carriers (holes) are present then they will recombine but minority carriers in p type which are electrons will be in conduction band while holes will be in valence band

 

[uart]

why don't the minority carriers recombine with excess holes?

They do. But you have to understand that this is an equilibrium situation where electron-hole pairs are also continuously being thermally generated. So yes, the concentration of minority carries DOES indeed get seriously reduced in a doped semiconductor, but not reduced to zero.

For example the "intrinsic" carrier concentration in Si is about $10^{10}$ $cm^{-3}$ at about 27C. This means that electrons and holes are being thermally generated (continuously) and also recombining (continuously) such that the steady state concentration of each is approx $10^{10}$ $cm^{-3}$. The product of electron times hole concentration will be approx $n p = 10^{20}$.

Now if we dope the Si with say $10^{16}$ $cm^{-3}$ donor ions then the electron concentration will increase to approx $10^{16}$. As a result the amount of recombination will increase whereas the amount of thermal generation will remain largely unchanged and the product will still be $n p = 10^{20}$. So this means we'll now have $p = 10^4$ $cm^{-3}$, which is one million times lower concentration of holes (minority carriers) than in the undoped silicon.

 

[Aranion]

thank you guys... i get it now..