Re: gEDA-user: LED in reverse

["Samuel A. Falvo II" <sam.falvo@xxxxxxxxx>]

On 11/20/06, Karel Kulhavy <clock@xxxxxxxxxxxxx> wrote:
> But the breakdown means that electron go through the diode just the other way.
> Why do the atoms stay the same when hit by electron from one side and something
> permanently changes in them when hit from the other? Are they asymmetric?

The reason is that P-doped silicon has a net absence of electrons
(hence, it's positively charged), while N-doped silicon has a net
overage of electrons (hence, it's negatively charged).

When applying current in the correct direction, the electrons coming
in on the cathode repel the electrons in the N-region and into the
P-region.  This results in a drop in their energy level, which is
usually dissipated as heat, but in an LED, is optimized to radiate
photons instead.

The reverse, however, results in an interesting effect.  When you
reverse-bias a diode, the electrons on the anode results in an
attraction of the holes towards the electrode (in other words, the
electrons on the anode *repel* the electrons in the crystal lattice,
leaving the positive holes near the terminal).  Likewise, the positive
charge on the cathode causes the supplement of electrons in the
N-region to accumulate near the cathode terminal.  The result is a
widening of the gap between the N and P regions, turning the existing
silicon more into a virtual slab of intrinsic (non-conductive)
silicon.

This, BTW, is why varactors (and ALL diodes, for that matter) have a
variable, voltage-dependent capacitance.  The stronger the voltage,
the more attraction between unlike charges, and thus, the widening of
the intrinsic gap.  The net result is that as the reverse bias voltage
increases, the net capacitance *decreases*, as the virtual plates
become effectively farther apart.

Since all insulators have a breakdown voltage (given enough electric
potential, even cinder blocks will conduct electricity!), and since
voltage depends on distance between two charges, and the LED is so
darn small, it follows that diodes have a relatively small reverse
breakdown voltage.  Larger, more powerful diodes, have either multiple
smaller diodes in parallel, or a physically larger slab of silicon.
Or both.  :-)

--
Samuel A. Falvo II


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