Light Emitting Diode
(LED)
LEDs are common semiconductor diodes used in many
applications today. They emit a fairly narrow bandwidth of light, which can be
either visible light at different coloured wavelengths, invisible infra-red light
used in remote controls or laser light when they are in forward biased
connection in a circuit. It is important to study the working principles of
this device because of its various found applications in electronics.
LEDs Applications
-
They are used in displaying numbers in digital clock
and some digital instruments.
-
Transmission of information from remote controls
-
As power indicator in appliances
-
There collections form images on jumbo Television screen and illumination of traffic lights.
-
Tiny LEDs are been used to replace the tubes that light
up Liquid Crystal Display (LCD) High Definition Television (HDTs) to make very
slim televisions.
Construction and
Principle of Operation
Instead of the vacuum emitting light source like incandescent bulb or emission of light
from gas like the compact fluorescent
light bulb, an LED emits light from
a piece of semiconductor material.
The historical discovery
of light emission capabilities of semiconductor
material, dates back to 1907, when electrical engineer Henry J. Round touched a crystal of silicon carbide found near Niagara Falls with two
wires connected to a battery. He reported that on applying 10V between two
points on the silicon carbide crystal,
the crystal gave out a yellowish light. Though not realised at that time Henry J. Round had operated the first
crude solid-state lamp, the light
emitting diode.
The light emission mechanism
of this semiconductor material is of
paramount importance in sense that, most electrical light sources such as
incandescence of a hot electrode, luminescence of a glowing plasma, or
fluorescence of a phosphor coating, are capable of producing exceedingly high
luminous outputs, but each of the conventional light sources is unsuited to a
broad range of potential applications because of their slow response time, inherent
fragility, and short lifetime.
The LED eliminates these set backs of conventional lamps with
its solid-state reliability, speed,
and compact size.
Figure 3.6: Symbol
of a Light Emitting Diode (LED)
Theory of LED’s
operation
The LED is essentially a P-N junction diode. To realise the light emitting property in most
commercial LEDs, they use a heavily doped
n and slightly doped p junction.
Figure 3.7: Unbiased pn+ junction diode
(the superscript + on the n indicates and heavily doped n-side).
For better understanding of the principle behind LEDs, let’s
consider and unbiased pn+ junction shown in figure 3.7. The
depletion region extends mainly into the p-side because it contains lesser
charge carrier due to the fact that it was slightly doped as explained earlier.
The potential barrier V0 prevents the excess free electron on the n+
side form diffusing into the p-side.
Now, if the pn+ junction is forward biased, as in figure
3.8, at V≥V
0, electron from n+ side gets injected into the p-side. But
the holes injection from the p side to the n+ side is very less and so the
current is primarily due to the flow of electrons into the p-side. These
electrons injected into the p-side recombine with the holes. This recombination
results in spontaneous emission of photons (light). Noting that the energy of
emitted photons is given by; E
g =
E
c-E
v, Ec is the energy
of electron in the conduction band, E
v energy of electron the
valence band, E
g energy required for an electron to jump form the
valence band to the conduction band, (See;
Basics of Semiconductor devices)
.This effect is called
injection electroluminescence.
These photons are allowed to escape from the device without being reabsorbed,
thus gives the illumination observed in LEDs.
Figure 3.8: Forward
biased pn+ junction
The wavelength of the photons of light emitted is derived
from the relation shown in the figure 3.8 i.e. Eg = hv,
Eg is the Energy gap, h, is planck’s
constant and v, the frequency of the
emitted light.
v
= c/λ c is the speed of light and λ is the wavelength.
Therefore;
The wavelength λ of
the emitted light determines the colour of light emitted by the LEDs. Thus given
the value of planck’s constant h = 4.135
×10-15 eV and speed of light c
=3×108m/s, we can determine the wavelength and thus the colour of
light emitted by a semiconductor, through its energy gap Eg.
For example a semiconductor with band-gap of 3.5eV will
emits light of;
Given an approximate wavelength of different colours of
light as in table 3.1 bellow;
Table 3.1
Color
|
Wavelength (nm)
|
Red
|
780 - 622
|
Red
|
780 - 622
|
Orange
|
622 - 597
|
Yellow
|
597 - 577
|
Green
|
577 - 492
|
Blue
|
492 - 455
|
Violet
|
455 - 390
|
Using table 3.1, then the light emitted by the semiconductor
in our example is violet colour.
LEDs in Electronic Circuit
Using an LED in a circuit, it normally
requires about 10mA of current for a bright glow and has about 1.7V across its
terminals. Hence, when we connect a 6V power supply to it, we must include a
resistor in series to drop the excess 4.3V i.e. (6-1.7)V.
For example in the circuit bellow;
Figure 3.9: LED in a circuit.
To determine the value of R to be
connected in the circuit above, we can see that R and the resistance of the LED
RLED and thus share the voltage input 6V.
The voltage required across RLED
is 1.7V, therefore the voltage needed to be dropped by R is 4.3V. Also since R and RLED are in
series, the same current is passing through them i.e. 10mA (require for LED
bright glow).
Note that, for VR (the voltage
across R) to be equall 4.3V and IR (current through R) equal 10mA,
we need R value given by;
The value of R required in the
circuit in figure 3.9 is 430Ω.
If you reverse the connection of
the 6V battery in the connection of the circuit in figure 3.9, you’ll realise
that the LED will not glow at all, this proves that the LED is a diode, i.e. allows
the flow of current in only one direction.
Seven Segment Display
A very common application of LEDs in electronic equipments and devices is in Seven Segment Display. The technique used in applying LED in a seven segment display is to arrange the LED in form of figure 8, as
shown in figure 3.10 bellow;
Figure 3.10: Seven Segment LED
display
All the 7-segments in figure 3.10
are individual LEDs labelled a – g. They com in two connections, either in a
common anode or common cathode connection see figure 3.11a (common anode) and
3.11b (common cathode).
Figure 3.11: LED connections in
7-segment display
In the common anode, all the anodes of each LED are connected together and
to the positive pole of the battery (Vcc), while each of the LED is switched on/off
individually by connecting to the negative pole of the battery (GND) through
the 430Ω resistor with their corresponding switch
a to g.
While in the common cathode, the cathode of all the LEDs are connected together
and to the negative pole of the battery (GND) through 430Ω resistor for each of the LED. The LEDs are switched on/off
individually by connecting to their anode to the positive pole of the battery
(Vcc) with their corresponding switch
a to g.
The idea behind the 7-segment
display is that, by selectively glowing (switched ON) the LEDs a to g, we can
generate all the numbers we want from 0-9. See figure 3.12.
