Re: gEDA-user: LED in reverse

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Mon, Nov 20, 2006 at 02:13:26PM -0800, Samuel A. Falvo II wrote:
> 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.

So the principal factor that makes the rectifying action is some kind of
intrinsic charge that is nailed down within the volume of the material and
encourages flow of electricity one way, and cause the flow in the other way to
jam?

Would it be possible to create the same device using an electret?

I read a lot of explanations how this works. I am able to learn any explanation
like a poem, where each verse is a logical deduction I can understand. But I
wasn't able to understand the diode at once.

CL<
> 
> 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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