All about Diodes, LEDs and the Laser Diode
The fundamental theory behind diode operation can be expressed by way of the diode current-voltage relationship:
Id = Is( e ^ (Vd/nVt) – 1 )
From this 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)..
LEDs (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 their chromaticity.
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.
When 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”.
Each time 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 an LED…
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.
In most 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 tolerate.
A LASER 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.
The two 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.
As the 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.
Most LASER 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.
The increase in operating temperature increases the threshold current of the device—more 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 with temperature.
Practical applications of LASER diodes include: bar code scanners, CD players, fiber optics communication, and LASER printers.
LEDs vs. LASER Diodes
Even though LEDs and LASER diodes are constructed in similar manners, the two technologies exhibit fundamental differences in their output power vs. input current curves.
(a) LED (b) LASER Diode
The LED 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 increases.
The LASER 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 diode’s I-V curves.
In general, LASER diodes output more power than LEDs, and they tend to operate at a higher bandwidth.