[sdiy] Charge diffusion conceptual confusion

[Ian Fritz]

At 01:57 PM 3/31/2011, David wrote:

>Now, my problem. In an N type, there are literally free electrons which can
>move and drift and that's easy for me to conceptualize. But, in a P type,
>the holes are derived from the doping element which is bound in the lattice.
>These holes can be filled/emptied, but they can't move. They are tied to the
>doping element which is bound in the lattice. What holes are moving? Do the
>bonds in Si to Si atoms in the lattice break to allow an electron to move
>into a doped hole?  If so, I can see that. The holes associated with the
>doping element get filled from electrons from adjacent Si to Si bonds. The
>hole moves through the entire lattice, not just where a doping atom is.
>Maybe I just answered the question...

Yes, basically you did.  First you must remember to think of energy levels 
in a crystal as actually being energy bands.  A small energy band gap 
between the (empty) conduction band and (full) valence band in an 
"intrinsic" semiconductor allows electrons to be thermally excited from the 
valence to the conduction band, where they are mobile (as in a metal). The 
holes left behind are also mobile.  This results in "bipolar conductivity".

In an "exstrinsic" semiconductor such as Si the free carriers come from 
dopants.  In an n-type material the dopant has an energy level just below 
the conduction band.  Its "extra" electron is thermally promoted (or 
"ionized") to the conduction band where it is mobile.

In a p-type material a complementary process occurs.  The acceptor levels 
are just above the valence band.  An electron thermally excited into the 
acceptor state leave behind a hole in the valence band.  This hole is 
mobile because the electrons in the valence band do not fill the band and 
therefore are free to move around.

This is the simple picture.  In reality the Coulombic attraction between 
the dopants and the carriers must be taken into account.  This leads to the 
formation of atomic-like "exciton" states.  These are easily ionized, but 
may be observed, for example in low-temperature photoluminescence 
experiments, where a series of spectral lines are seen.


>Lastly, I can grasp the concept of electrons flowing in a conductor or
>semi-conductor. But, holes don't "flow" in a conductor, right? So, don't the
>holes just stay in the semiconductor?

Really, there is no such thing as a hole. A hole is just a space where an 
electron is missing.  So really, only the electrons are actual physical 
entities that move.

Hope I haven't just confused you more!

Ian