well three japanese researchers amano
akasaki and nakamura have won the nobel
prize for their pioneering work on blue
light emitting diet
it's a very interesting one and i think
i haven't had much time to think about
it
a bit surprised well in a semiconductor
unlike a metal
there's a gap
in the allowed
states that an electron can occupy so
we've got electrons
up to here
and then we've got a gap
and electrons can then exist above that
gap okay so we shine some light on it
okay and that gives energy to the
electron which can then rise
above the gap um
and can then gradually thermalize down
to the um
bottom of this circle conduction band if
it then drops back into the valence band
its energy can come out as a
photon or light in in this process we've
used a laser
to lift the electron up from the valence
band to the conduction band and then
allowed it to drop back again to produce
the the light well what's going on in a
light emitting diode is that we want to
have carriers
coming in the conduction band and
carriers coming in the valence band and
carriers coming in the conduction band
and have these
uh interact with one another to form the
light because we don't want to use a
laser because we don't want to use laser
okay
to do that we have to have material that
is doped
p-type here that creates what are called
holes lack of an electron is a hole
and
we have to have n-type material which
has an excess of electrons here and we
drive the current which brings the two
together
they recombine the electron with the
hole and out comes a photon so that's a
light emitting diode in in simple terms
so this energy gap here depends upon the
material so for example the classic
material
classic 3-5 is gallium arsenide
where we combine gallium and arsenic in
equal quantities and this energy gap is
then about 1.4 electron volts so that
means
when
we let the electron combine with the
hole we get an infrared photon
okay that's no good that's no good to us
okay so to make a red led we have to go
to a slightly different material we have
to go from gallium arsenide for example
to gallium arsenide phosphide lighter
material wider band gap and so higher
energy they've been around for a long
time okay but to get
blue light
you need a combination of gallium
plus nitrogen and that gives you
something that emits in the ultraviolet
so actually not blue
but
ultraviolet
so we've got a bigger energy gap here
so we get a photon with higher energy
out when it relaxes but the problem they
had was how do you
create the p-type gallium nitride which
allows you to inject the holes the
n-type galvanite was all right they
could dope that successfully so that was
the big challenge
that they they had it was relatively
easy to dope the material n-type
okay with silicon or
other things that's the top step okay
the silicon-doped n-type material the
difficulty was to produce p-type
gallium nitride and to do that you have
to dope this material with magnesium
and in the process
that they were using magnesium was
combining with hydrogen and so the
material was electrically inert
now what the um
two
of these people discovered amano and
akasaki
was that they put some of this magnesium
doped material into an electron
microscope
and the very act of doing that
broke this bond between the magnesium
and the hydrogen and the material became
electrically active so they
did they do that on purpose nah it was
somewhat accidental i believe yeah what
nakamura did
was to discover
that you could do the same thing by
taking this magnesium doped material and
heating it to high temperature annealing
it as it's called and that again breaks
this bond
and allows the material to be
electrically active and so once we've
got our
n-type and p-type material
we can make
a uv
light emitting diode we can then take
that and combine some of that with a
phosphor to turn that into white light
it's gradually over the last 20 years
you know been developed as a technology
and it's now being used for lighting
which is what everybody dreamed of it's
very exciting the breakthrough really
was in getting this um this p-type
uh
gallium nitride to become electric to
become electrically active yeah and
these two people were working quite
independently in two different places
nakamura working for the company nishir
who
and
amano and nakasaki working in
in the university
you