n-type sample of silicon

[PZ]

One question,

"An n-type sample of silicon has a uniform density Nd= 10(power 16)
atomes cm(-3) of arsenic. Find out the temperature at which half the
impurity atoms are ionized. Assume that all mobile electrons and holes
come from dopant impurities".


don't understand the question at all. If assume all electrons come from
dopant, that means I should I consider the intrinsic carrier? So, as
temp goes up, some dopant get ionized, half which is 0.5 X 10(power 16)
is the current. if intrinsic is not allowed to consider, can I still
use p = Ni(power 2) / n to find p?


thanks, I guess I don't get the meaning of this question. Any comments
is appreicated.

 

[Jonathan Kirwan]

One question,

"An n-type sample of silicon has a uniform density Nd= 10(power 16)
atomes cm(-3) of arsenic. Find out the temperature at which half the
impurity atoms are ionized. Assume that all mobile electrons and holes
come from dopant impurities".

don't understand the question at all. If assume all electrons come from
dopant, that means I should I consider the intrinsic carrier? So, as
temp goes up, some dopant get ionized, half which is 0.5 X 10(power 16)
is the current. if intrinsic is not allowed to consider, can I still
use p = Ni(power 2) / n to find p?

For materials in equilibrium, I think it holds.

thanks, I guess I don't get the meaning of this question. Any comments
is appreicated.

My general understanding is that at room temperature nearly all of the
extrinsic dopant electrons are thermally ionized into the intrinsic's
conduction band and that almost none of the intrinsic's electrons are
ionized. This is seen on a plot of electron density versus inverse-
Kelvins.

But that isn't the question, really. The question is about at what
temperature it would be the case that there would .5e16 be ionized.

Perhaps this will help:
http://ece-www.colorado.edu/~bart/book/book/chapter2/ch2_6.htm

I'm thinking of figure 2.6.8 and the material both above and below it.

Jon

 

[Rich Grise]

One question,

"An n-type sample of silicon has a uniform density Nd= 10(power 16)
atomes cm(-3) of arsenic. Find out the temperature at which half the
impurity atoms are ionized. Assume that all mobile electrons and holes
come from dopant impurities".

don't understand the question at all.

Me neither. The way semiconductors were explained to me, a millennium or
so ago, all of the dopant atoms are "ionized" all of the time, no matter
what - that's what makes it an N-type semiconductor. They have no choice
but to "ionize", because the valence electron won't fit into the crystal
lattice.

When a copper wire is carrying current, do the copper atoms get "ionized?"

But, I'm confident that someone who knows their elbow from a hole in the
ground will help clear this up for both of us.

Thanks!
Rich

 

[Jonathan Kirwan]

Me neither. The way semiconductors were explained to me, a millennium or
so ago, all of the dopant atoms are "ionized" all of the time, no matter
what - that's what makes it an N-type semiconductor. They have no choice
but to "ionize", because the valence electron won't fit into the crystal
lattice.

They can 'freeze out' if the temperature is low enough.

You might think about this in terms of positive and negative
attraction forces versus thermal energy. The extrinsic materials, if
the temperature were at absolute zero or close to it, would keep their
electrons very near their nucleus by + and - attractions. The
probability of such an electron being buffeted away would be extremely
low and near 0K temperatures. But since that net attraction isn't
very strong in the case of extrinsic materials embedded in a silicon
lattice, it doesn't take a lot of thermal jostling to bump them "up"
into the conduction band of the silicon. At room temperature, none of
them are able to settle for any length of time with an extrinsic
nucleus. They keep getting kicked back up. So it appears that "all"
of them are in the conduction band. But if you lower the temperature,
there is less and less thermal energy and the probabilities shift. But
we are talking "very cold" to reach 50%, even -- approximately the
boiling point of liquid nitrogen at 1 atmosphere pressure, I think.

Jon