Light Emitting Diodes or LED´s, are
among the most widely used of all the different types of semiconductor
diodes available today. They are the most visible type of diode, that
emit a fairly narrow bandwidth of either visible light at different
coloured wavelengths, invisible infra-red light for remote controls or
laser type light when a forward current is passed through them.
A Light Emitting Diode or LED as it is more commonly called, is basically just a specialised type of PN Junction diode, made from a very thin layer of fairly heavily doped semiconductor material.
When the
diode is forward biased, electrons from the semiconductors conduction
band recombine with holes from the valence band releasing sufficient
energy to produce photons which emit a monochromatic (single colour) of
light. Because of this thin layer a reasonable number of these photons
can leave the junction and radiate away producing a coloured light
output.
LED Construction
Then we can say that when operated in a forward biased direction Light Emitting Diodes are semiconductor devices that convert electrical energy into light energy.
The construction of a Light Emitting Diode is very different from that of a normal signal diode. The PN Junction
of an LED is surrounded by a transparent, hard plastic epoxy resin
hemispherical shaped shell or body which protects the LED from both
vibration and shock.
Surprisingly,
an LED junction does not actually emit that much light so the epoxy
resin body is constructed in such a way that the photons of light
emitted by the junction are reflected away from the surrounding
substrate base to which the diode is attached and are focused upwards
through the domed top of the LED, which itself acts like a lens
concentrating the amount of light. This is why the emitted light appears
to be brightest at the top of the LED.
However,
not all LEDs are made with a hemispherical shaped dome for their epoxy
shell. Some indication LEDs have a rectangular or cylindrical shaped
construction that has a flat surface on top or their body is shaped into
a bar or arrow. Also, nearly all LEDs have their cathode, (K) terminal
identified by either a notch or flat spot on the body, or by one of the
leads being shorter than the other, (the Anode, A).
Unlike
normal incandescent lamps and bulbs which generate large amounts of heat
when illuminated, the light emitting diode produces a “cold” generation
of light which leads to high efficiencies than the normal “light bulb”
because most of the generated energy radiates away within the visible
spectrum. Because LEDs are solid-state devices, they can be extremely
small and durable and provide much longer lamp life than normal light
sources.
Light Emitting Diode Colours
So how
does a light emitting diode get its colour. Unlike normal signal diodes
which are made for detection or power rectification, and which are made
from either Germanium or Silicon semiconductor materials, Light Emitting Diodes
are made from exotic semiconductor compounds such as Gallium Arsenide
(GaAs), Gallium Phosphide (GaP), Gallium Arsenide Phosphide (GaAsP),
Silicon Carbide (SiC) or Gallium Indium Nitride (GaInN) all mixed
together at different ratios to produce a distinct wavelength of colour.
Different
LED compounds emit light in specific regions of the visible light
spectrum and therefore produce different intensity levels. The exact
choice of the semiconductor material used will determine the overall
wavelength of the photon light emissions and therefore the resulting
colour of the light emitted.
Light Emitting Diode colors
Thus, the
actual colour of a light emitting diode is determined by the wavelength
of the light emitted, which in turn is determined by the actual
semiconductor compound used in forming the PN junction during
manufacture.
Therefore
the colour of the light emitted by an LED is NOT determined by the
colouring of the LED’s plastic body although these are slightly coloured
to both enhance the light output and to indicate its colour when its
not being illuminated by an electrical supply.
Light emitting diodes are available in a wide range of colours with the most common being red, amber, yellow and green and are thus widely used as visual indicators and as moving light displays.
Recently
developed blue and white coloured LEDs are also available but these tend
to be much more expensive than the normal standard colours due to the
production costs of mixing together two or more complementary colours at
an exact ratio within the semiconductor compound and also by injecting
nitrogen atoms into the crystal structure during the doping process.
From the table above we can see that the main P-type dopant used in the manufacture of Light Emitting Diodes
is Gallium (Ga, atomic number 31) and that the main N-type dopant used
is Arsenic (As, atomic number 33) giving the resulting compound of
Gallium Arsenide (GaAs) crystalline structure.
The
problem with using Gallium Arsenide on its own as the semiconductor
compound is that it radiates large amounts of low brightness infra-red
radiation (850nm-940nm approx.) from its junction when a forward current
is flowing through it.
The
amount of infra-red light it produces is okay for television remote
controls but not very useful if we want to use the LED as an indicating
light. But by adding Phosphorus (P, atomic number 15), as a third dopant
the overall wavelength of the emitted radiation is reduced to below
680nm giving visible red light to the human eye. Further refinements in
the doping process of the PN junction have resulted in a range of
colours spanning the spectrum of visible light as we have seen above as
well as infra-red and ultra-violet wavelengths.
By mixing together a variety of semiconductor, metal and gas compounds the following list of LEDs can be produced.
Types of Light Emitting Diode
-
Gallium Arsenide (GaAs) - infra-red
-
Gallium Arsenide Phosphide (GaAsP) - red to infra-red, orange
-
Aluminium Gallium Arsenide Phosphide (AlGaAsP) - high-brightness red, orange-red, orange, and yellow
-
Gallium Phosphide (GaP) - red, yellow and green
-
Aluminium Gallium Phosphide (AlGaP) - green
-
Gallium Nitride (GaN) - green, emerald green
-
Gallium Indium Nitride (GaInN) - near ultraviolet, bluish-green and blue
-
Silicon Carbide (SiC) - blue as a substrate
-
Zinc Selenide (ZnSe) - blue
-
Aluminium Gallium Nitride (AlGaN) - ultraviolet
Like conventional PN junction diodes, light emitting diodes are current-dependent devices with its forward voltage drop VF,
depending on the semiconductor compound (its light colour) and on the
forward biased LED current. The point where conduction begins and light
is produced is about 1.2V for a standard red LED to about 3.6V for a
blue LED.
The exact
voltage drop will of course depend on the manufacturer because of the
different dopant materials and wavelengths used. The voltage drop across
the LED at a particular current value, for example 20mA, will also
depend on the initial conduction VF point. As an LED is
effectively a diode, its forward current to voltage characteristics
curves can be plotted for each diode colour as shown below.
Light Emitting Diodes I-V Characteristics
Light Emitting Diode (LED) Schematic symbol and I-V Characteristics Curves
showing the different colours available.
Before a
light emitting diode can “emit” any form of light it needs a current to
flow through it, as it is a current dependant device with their light
output intensity being directly proportional to the forward current
flowing through the LED.
As the LED is to be connected in a forward bias condition across a power supply it should be current limited
using a series resistor to protect it from excessive current flow.
Never connect an LED directly to a battery or power supply as it will be
destroyed almost instantly because too much current will pass through
and burn it out.
From the
table above we can see that each LED has its own forward voltage drop
across the PN junction and this parameter which is determined by the
semiconductor material used, is the forward voltage drop for a specified
amount of forward conduction current, typically for a forward current
of 20mA.
In most cases LEDs are operated from a low voltage DC supply, with a series resistor, RS
used to limit the forward current to a safe value from say 5mA for a
simple LED indicator to 30mA or more where a high brightness light
output is needed.
LED Series Resistance
The series resistor value RS is calculated by simply using Ohm's Law, by knowing the required forward current IF of the LED, the supply voltage VS across the combination and the expected forward voltage drop of the LED, VF at the required current level, the current limiting resistor is calculated as:
LED Series Resistor Circuit
Light Emitting Diode Example
An amber
coloured LED with a forward volt drop of 2 volts is to be connected to a
5.0v stabilised DC power supply. Using the circuit above calculate the
value of the series resistor required to limit the forward current to
less than 10mA. Also calculate the current flowing through the diode if a
100Ω series resistor is used instead of the calculated first.
1. series resistor required at 10mA.
2. with a 100Ω series resistor.
We remember from the Resistors
tutorials, that resistors come in standard preferred values. Our first
calculation above shows that to limit the current flowing through the
LED to 10mA exactly, we would require a 300Ω resistor. In the E12 series
of resistors there is no 300Ω resistor so we would need to choose the
next highest value, which is 330Ω. A quick re-calculation shows the new
forward current value is now 9.1mA, and this is ok.
Connecting LEDs Together in Series
We can
connect LED’s together in series to increase the number required or to
increase the light level when used in displays. As with series
resistors, LED’s connected in series all have the same forward current, IF
flowing through them as just one. As all the LEDs connected in series
pass the same current it is generally best if they are all of the same
colour or type.
LED’s in Series
Although
the LED series chain has the same current flowing through it, the series
voltage drop across them needs to be considered when calculating the
required resistance of the current limiting resistor, RS. If
we assume that each LED has a voltage drop across it when illuminated of
1.2 volts, then the voltage drop across all three will be 3 x 1.2v =
3.6 volts.
If we
also assume that the three LEDs are to be illuminated from the same 5
volt logic device or supply with a forward current of about 10mA, the
same as above. Then the voltage drop across the resistor, RS and its resistance value will be calculated as:
Again, in
the E12 (10% tolerance) series of resistors there is no 140Ω resistor
so we would need to choose the next highest value, which is 150Ω.