|What Is A Diode / What Does A
Diode Do / LEDs and Laser Diodes?...
The fundamental theory behind
Diode operation can be expressed by way of the diode
Id = Is( e ^ (Vd/nVt) - 1 )
equation we can see that with a large enough forward voltage
applied, the diode begins to conduct with magnitude
according to the eVd/nVt limiting term. In the reverse
situation, we see that the eVd/nVt approaches a very small
value and the current can be approximated according to the
Is value (which is usually small). This relationship holds
true until the point of reverse breakdown is reached, at
which time the Diode fails to function according to the
fundamental theory. At reverse breakdown voltage the diode
will abandon its non-linear behavior and allow for a rapid
increase in current versus incremental voltages. The two
breakdown mechanisms in semiconductor diodes are Avalanche
and Zener breakdowns. Each occurs as a result of different
mechanisms coming into play, yet they are oftentimes
mistakenly confused. In fact, in many cases both Avalanche
and Zener breakdown mechanisms can be observed together in
causing this device 'failure'.
LEDs (Light Emitting Diodes)..
(Light Emitting Diodes) become very important light
sources in many applications such as traffic signals,
flashlights, information boards, general lighting, etc. They
have also become very popular for use as instrumental radiation sources. LEDs
emit in narrow emission bands and, as in the case of every semiconductor,
their optical characteristics are temperature dependent. In
outdoor applications they are exposed to extreme
temperatures, influencing both their absolute intensity and
LEDs have become very practical
in modern electronics because they fit easily into different circuits
and can be sized as needed. Unlike
ordinary incandescent bulbs, they don't have a filament that
will burn out and take up a great deal of space. They
also don't release as much heat energy and work solely by the movement of electrons in a
semiconductor material. LEDs also last just as long as a
standard transistor - oftentimes 5-10 times the length of a
similarly performing incandescent bulb.
sufficient voltage is applied to the chip across the leads
of the LED, electrons can move easily in only one direction
across the junction between the P and N regions.
In the P
region there are many more positive than negative charges.
In the N region the electrons are more numerous than the
positive electric charges. When a voltage is applied and the
current starts to flow, electrons in the N region have
sufficient energy to move across the junction into the P
region. Once in the P region the electrons are immediately
attracted to the positive charges due to the mutual Coulomb
forces of attraction between opposite electric charges. When
an electron moves sufficiently close to a positive charge in
the P region, the two charges "re-combine".
an electron recombines with a positive charge, electric
potential energy is converted into electromagnetic energy.
For each recombination of a negative and a positive charge,
a quantum of electromagnetic energy is emitted in the form
of a photon of light with a frequency characteristic of the
semi-conductor material (usually a combination of the
chemical elements gallium, arsenic and phosphorus). Only
photons in a very narrow frequency range can be emitted by
any material. LED's that emit different colors are made of
different semi-conductor materials, and require different
energies to light them.
How To Measure
One way to
measure LED power output is to place the LED very near to a
large photodetector. In addition, the best way to pulse
module LEDs is to drive them with square waves.
This idea is related to the use
of tuned receiver circuits and the observation that, of all
rectangular waveshapes, the squarewave has the most energy
at the fundamental. However, for LEDs, much higher
SNR is usually possible with smaller DF. This has to do with
the nature of the LEDs themselves (we can achieve higher
peak power by reducing DF) and with the nature of the
dominant noise sources (shot noise in photodetector
junctions), op-amp input noise, and ambient light
fluctuations, which add up to a noise spectrum that tilts up
at low frequency.
cases, however, we just double the rated continuous current
for the LED and we get our best results with an AlGaAs LED
with as much as directionally narrow beam as we can
diode is a semiconductor device that emits coherent light
(as opposed to a LED which emits incoherent light) when
forward biased. Its active medium is a p-n junction and is
constructed in a similar fashion to the LED. The recombination
processes in the p-n junction produce photons.
ends of the slab of a LASER diode are made to be reflective
edges to form a resonator (Fabry-Perot Cavity). One end is
fully reflective while the other is partially reflective. A
wave-guide is constructed on the piece of slab.
photons travel along the wave-guide, they are reflected
several times between the reflective edges before they are
emitted. Each time the photons are reflected, they
amplify. At a certain current threshold, the photons would
initiate LASER action. The length of the slab controls the
operating wavelength of the LASER diode.
diodes are sensitive to temperature fluctuations. Operating
temperatures change the threshold current. In general, as
temperature increases, the threshold current increases.
Also, as operating temperature increases, the efficiency of
the LASER diode decreases as seen by the decrease in slope
of the output power vs. input current curves after the
threshold current is reached.
increase in operating temperature increases the threshold
current of the deviceómore drive current is needed to turn
on the LASER diode. This phenomenon is attributed to the
fact that as temperature increases, the nonradiative
processes in the device would increase and compete with the
system for photons. Thus, the threshold current increases
applications of LASER diodes include: bar code scanners, CD
players, fiber optics communication, and LASER printers.
though LEDs and LASER diodes are constructed in similar
manners, the two technologies exhibit
fundamental differences in their output power vs. input
LED (b) LASER Diode
light output vs. drive current curve is linear at low drive
currents. As current increases, the light output starts to
saturate. This saturation effect could be attributed to the
self-heating of the chip in the LED as drive current
diode exhibits minimal light output at low drive currents.
However, once the drive current reaches a certain threshold,
the light output increases significantly with the drive
current. This characteristic is very similar to that of a
diodeís I-V curves.
general, LASER diodes output more power than LEDs, and they
tend to operate at a higher bandwidth.