🚀 go-pugleaf

raju@bass.bu.edu (rajminder Singh) writes:
>Hi there! I am  taking a course in Solid State Physics but I still 
>haven't been able to answer the basic question of why the voltage drop
>in a Silicon p-n junction is around 0.7 Volts ?

Having dropped that course pretty quickly, I might not be the best person to
answer the question (:-), but the ARRL Handbook provides a clear enough
explanation for me to summarize.  That voltage potential represents the
difference in the energies of the conduction bands of P material and N
material in the assembled diode, and will (I think) be equal to the difference
between the Fermi levels of the two materials when isolated (the energy level
at which half the electrons are in higher states and half are in lower states).
When the two materials are "joined", there is an initial flow of current which
shifts the energy levels in the two materials until the Fermi levels are equal,
which mean the conduction and valence bands in the two materials will shift
by corresponding amounts.  You've probably got a text that will explain it
more clearly than that (no, actually, if your texts are anything like mine,
the explanation will be less clear but far more detailed :-).

>Does this vary significantly
>with the doping level ?

Yes.

>What about the effect of temperature, current and
>the energy band gap (E) of the material used in the junction ?

Temperature and voltage I can answer pretty easily; the Handbook gives the
formula as V(t) = (t / 11 600) * ln(1 + If/(1E-13 * exp((t-300)*ln(2) / 4.5))).
Note that it depends on current as well, but not very much.

To calculate the voltage from the bands will require rereading a text I have
at home that I had trouble getting through the first time, so I won't :-).
However, in practical terms, germanium diodes have a gap of about .3 volts,
ordinary silicon diodes about .7, and LEDs of varying construction range from
1.5V (red, GaAs?) to 5V (blue, SiC).

Interesting tidbit:  common LEDs make moderately good voltage references in
the 1.5 to 3V range, depending on color.

Hope this helps, and I wish you better luck than I had.