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LEDS & LASER DIODESLight-emitting diodes or LEDs are now very widely used inalmost every area of electronics, mainly as indicator anddisplay devices in effect, solid state lamps. Theyre verywell suited for such uses, because they are physically quiterugged and hence much more reliable than filament-typeincandescent lamps. They also run much cooler and aremuch more efficient, requiring far less electrical power

input for the same amount of light output.Other common uses for LEDs are as a source of eithervisible or infra-red light, as a carrier for data and otherinformation over short line of sight distances.

A LED is basically just a specialised type of P-N junctiondiode, made from a thin chip of fairly heavily dopedsemiconductor material. When it is forward biased toreduce the potential barrier provided by the junctionsnarrow depletion layer, electrons from the semiconductorsconduction band can combine with holes from the valenceband, releasing sufficient energy to produce photons of light. Because of the thin chip a reasonable number of thesephotons can leave it and radiate away as its light output.

Unlike diodes made for detection and power rectification,which are generally made from either germanium or silicon,

LEDs are made from compound semiconductor materialssuch as gallium arsenide (GaAs), gallium phosphide (GaP),gallium arsenide-phosphide (GaAsP), silicon carbide (SiC)and gallium indium nitride (GaInN). The exact choice of semiconductor determines thewavelength of peak emission of photons and hence thecolour of the light emitted, inthe case of visible light LEDs. Itcan also determine the electro-optical conversion efficiency, andhence the light output.

Another parameter which isdetermined by thesemiconductor used is the

forward voltage drop for a givenamount of forward conductioncurrent.

Table 1 shows the peak emissionwavelength for the most common types of LED, with theapproximate colour indicated and also the typical forwardvoltage drop for a forward current of 20mA.

The construction of a typical LED is shown in Fig.1. Thesemiconductor chip is glued via electrically conductivecement to the inner end of the cathode lead, which is oftenformed into a tiny dish-shaped reflector to direct as muchas possible of the emitted light upward. A small bondingwire is used to connect the anode lead to an electrodedeposited on the top of the chip, and the completeassembly is then encapsulated in transparent epoxy resin

plastic either water clear, or tinted to match thecolour of the LEDs peak emission.

Leaded LEDs like that shown are made in a variety of package shapes and sizes, of which the 3mm and 5mm

diameter bullet type with a spherical front lens are themost common. Other much smaller packages are used for

surface-mount LEDs, including naked LED chips.

In most cases LEDs are operated from a low voltage DCsupply, with a series resistor (Fig.2) to limit the forwardcurrent to a suitable value from say 5-6mA for a simple

pilot lamp or status indicatorapplication to 20mA or morewhere more light output isneeded. The series resistor valueRs is calculated using Ohms law,knowing the required operatingcurrent IF, the supply voltage VS

and the expected forwardvoltage drop of the LED at thiscurrent level, VF:

Rs = (VS - VF)/IFThe LEDs forward voltage dropVF can usually be estimated fromthe figure usually given in the

data books for a current of 20mA, although the actualvoltage drop will be a bit lower for very low current levels.

LEDs are intended to operate only  in forward conductionmode, and should not be subjected to reverse voltage. Inmost cases they have a reverse voltage rating of 5V or evenless, so they can be damaged by accidental reverseconnection at even quite low supply voltages. If the LED

Electus Distribution Reference Data Sheet: LEDLASER.PDF (1)

Fig.1:  The  construction  used  for  most  common leaded  LEDs.

Fig.2:  When  a LED is  operated  from DC, a simple series  resistor  is used  to  set  the for ward  current level.

Fig.3:  Two  w ays of  running  aLED  fr om  an AC  supply.

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needs to be operated from an AC supply, or from a signalsource which cannot be relied upon not to reverse itspolarity, it can be protected by one of the methods shownin Fig.3. The simplest approach is shown in A, where areverse connected silicon diode is connected directlyacross the LED to limit any reverse voltage to 0.6V. Thisprotects the LED, but of course no light is emitted for thenegative half-cycles of the AC waveform even thoughcurrent is still drawn from the supply. So the light outputand efficiency are both effectively halved.

The alternate method shown in B is more efficient, and alsomaintains the LED light output. Here a bridge of fourdiodes is used to ensure that the current always flowsthrough the LED in the forward direction, regardless of supply polarity. (Note that the voltage drop of two diodes  about 1.2V needs to betaken into account when the valueof Rs is being calculated.)

The maximum light output from aLED (usually measured and rated inmillicandelas) is essentially limitedby the maximum average forwardcurrent which it can handle, whichis determined mainly by the LEDchips power dissipation rating  typically less than 100mW, forplastic encapsulated devices.

When higher light output isrequired, the usual approach is tooperate the LED not from a steadyDC supply, but from a pulsedcurrent with a fairly short dutycycle (on-off ratio). This allows the current and hence thelight output to be increased significantly during the actualpulses, while still keeping the LEDs average current leveland power issipation within its ratings (Fig.4).

Why does this pulsed output give an advantage? Partlybecause the electro-optical efficiency of LEDs actually tends

to increase with current level. So short pulses of significantly higher output separated by periods of nooutput actually result in a higher average light output, forthe same average current .

In addition, the human eyes persistence of vision tends tofill in the gaps between the light pulses, providing thepulse repetition frequency is significantly higher than theeyes critical fusion frequency (CFF). So pulses at afrequency of 100Hz or more actually appear brighter thancontinuous light of the same average intensity.

A simple low cost pulse generator using a 555 or similardevice can be used to produce pulses with a duty cycle of say 20%, and can be used to drive the LED either directlyor via a power MOSFET. Either way a series resistor isagain used to limit the LED current but in this case tothe right peak value (say 100mA, for a 20% duty cycle).

Laser diodesLaser diodes (also called injection lasers) are in effect aspecialised form of LED. Just like a LED, theyre a form of P-N junction diode with a thin depletion layer whereelectrons and holes collide to create light photons, whenthe diode is forward biased.

The difference is that in this case the active part of thedepletion layer (i.e., where most of the current flows) is

made quite narrow, to concentrate the carriers. The ends

of this narrow active region are also highly polished, orcoated with multiple very thin reflective layers to act asmirrors, so it forms a resonant optical cavity.

The forward current level is also increased, to the pointwhere the current density reaches a critical level wherecarrier population inversion occurs. This means there aremore holes than electrons in the conduction band, andmore electrons than holes in the valence band or inother words, a very large excess population of electronsand holes which can potentially combine to releasephotons. And when this happens, the creation of newphotons can be triggered not just by random collisions of electrons and holes, but also by the influence of passingphotons.

Passing photons are then able to stimulate the productionof more photons, without themselves being absorbed. Solaser action is able to occur: Light Amplification byStimulated Emission of  Radiation. And the importantthing to realise is that the photons that are triggered byother passing photons have the same wavelength, and arealso in phase with them. In other words, they end up insync and forming continuous-wave coherent radiation.

Because of the resonant cavity, photons are thus able totravel back and forth from one end of the active region tothe other, triggering the production of more and morephotons in sync with themselves. So quite a lot of coherentlight energy is generated.

Electus Distribution Reference Data Sheet: LEDLASER.PDF (2)

Fig.4:  To  get more  light output  from a LED, it  can be  run  from a higher  but pulsed current...

Fig.5:  The  two  main  types  of  laser  diode  chip    horizontal  (left)  and  vertical. Fig.6:  How  a typical  horizontal  laser  chip  is  mounted  in its  pack age, with  the  built-in  monitoring  diode  able  to sense  its  rear  output...

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