What goes on in a semiconductor....

Discussion in 'Homework Help' started by Urmi, Aug 2, 2010.

  1. Urmi

    Thread Starter New Member

    Jun 5, 2010
    Hi everyone! I need help with a few conceptual questions in regard to semoconductor diodes please bear with me..I have only 4 questions to ask,any help will be greatly appreciated,especially as I have my exams starting next week.

    1. Why does the diffusion of carriers during the formation of depletion region (when the p and n type crystals are just put together) stop after a while?---is it because the stationary charges (e.g negatively charged ions/kernels in the p-side) repel the incoming charges?

    2.When we say that in reverse bias,the holes in the P-side are attracted to the negative terminal of battery in the P-side,it is equivalent to saying that the minority electrons (that reside in the conduction band of the host semiconductor atoms) move toward the junction...so it's exactly similar to the situation when the electrons from the n-side move in (diffusion current) during depletion layer formation,isn't it?

    3.In forward bias, electrons and holes from close to the junction are migrating a short distance and recombining. Since this continually happens, the external circuit appears like a steady current is flowing....this is what it says in my book.
    Does this mean that the electrons in the conduction band (i.e the minority carriers in p-side )moving away from the junction ?
    Again,there'll be some of these conduction band electrons that cross over into the n side to fill in the vacancies created by the electrons of the n side,which move away from the junction in respone to the applied voltage.....:confused:can anyone tell me what exactly the line from my book says?

    4.After hole-electron pair generation,in the p-side,does the electron generated cross the potential barrier and enter n-side...I mean if there are accumulation of electrons,there will be an accumulation of negative charges on the p-side,but at the same tome,I don't see how the electrons so generated could move accross the depletion layer into the n-side.
    4.After hole-electron pair generation,in the p-side,does the electron generated cross the potential barrier and enter n-side?​

    Please do help.
  2. Urmi

    Thread Starter New Member

    Jun 5, 2010
    It's been a long long time! Won't I get any answers..atleast to some of the questions?
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    Maybe, most of as don't study the physics of semiconductor devices.
  4. kunalpat

    New Member

    Aug 7, 2014
    its been a long long time after this post was pasted, but i found this a really exciting question considering the fact that i am deeply passionate about the semiconductor physics..
    1.In the first question, yes, u are correct, the electrons(or holes), experience forces arising because of two factors, one being the density/concentration gradient across the junction, and two, because of any electric field within the junction.the electric field because of the electrons themselves can be assumed to add up to zero, in diffused state.Now wen diffusion starts the concentration gradient starts decreasing and an electric field starts developing simultaneously because of the uncovered ions,implies ,the force on the electron which was initially only because of the concentration gradient starts decreasing and now, at some point when the force from the built in electric field exactly balances out the force from the concentration gradient, the diffusion process stops.
    2.one needs to know that the electrons which drift or diffuse are in the conduction band, and the holes are just vacant quantum states in the valence band.it is violation when u associate motion in the valence band with tht in a conduction band because both are entirely different,analogically speaking, its like electronic motion is similar to traffic on a flyover, and hole motion is like traffic on the road beneath it, when u associate hole motion with electron motion , its essentially true only that the electrons which move(giving us the illusion of holes moving) belong to the valence band, not the conduction band, so the shift occurs because of both the conduction AND valence electrons.It would be good if u read about mesomeric effect in organic chemistry because that is exactly how the holes move.i guess i have answered ur question :)