the symbol of LED which is similar to p-n junction diode apart from the two arrows
indicating that the device emits the light energy.
Basic Operation
Whenever a p-n junction is forward biased, the electrons cross the p-n junction from
the n type semiconductor material and recombine with the holes in the p type
semiconductor material. The free electrons are in the conduction band while the
holes are present in the valence band. Thus the free electrons are at higher energy
level with respect to the holes When a free electron recombines with hole, it falls
from conduction band to a valence band. Thus the energy level associated with it
changes from higher value to lower value. The energy corresponding to the
difference between higher level and lower level is released by an electron while
traveling from the conduction band to the valence band. In normal diodes, this
energy released is in the form of heat. But LED is made up some special material
which releases this energy in the form of photons which emit the light energy. Hence
such diodes are called light emitting diodes.
This process is called electroluminescence.
The fig shows the basic principle of this process. The energy released in the
form of light depends on the energy corresponding to the forbidden gap. This
determines the wavelength of the emitted light. The wavelength determines the
colour of the light and also determines whether the light is visible or invisible
(infrared). The various impurities are added during the doping process to control the
wavelength and colour of the emitted light. For normal silicon diode, the forbidden
energy gap is 1.1 eV and wavelength of the emitted light energy corresponds to that
of infrared light spectrum hence in normal diodes the light is not visible. The infrared
light is not visible.
LED Voltage and Current:
Consider a source connected to LED and a resistor as shown in the Fig. The outward
arrows associated with a diode indicate that it is LED. The resistor R is the current
limiting resistor. Due to this resistor, the current through the circuit is limited and
prevented from exceeding the maximum current rating of the diode
Let
V = drop across LED
Vs = supply voltage
Applying KVL to the circuit we can write,
When forward biased, the voltage drop across conducting LED is about 2 to 3 V
which is considerably greater than that across a normal silicon or germanium diode.
The current range of commercially available LEDs is 10 to 80 mA. Unless and
otherwise specified, while analyzing the LED circuits, the drop across LED is
considered as VD= 2V. The reverse breakdown voltage of LED is much less than the
normal diode, which is about 3V to 1OV
Spectral Output Curves for LED
The visibility of the light is decided from its wavelength (A). of LED against
the wavelength gives the various curves called LEDs. These are shown in the Fig.
The graph of output light spectral output curves for LED’s.
It can be seen that wavelength is expressed in nm (nanometres). The normalized
output for visible light shows peak at 460 nm for blue, at 540 nm for green, at 590
nm for yellow and at 660 nm for red. The infrared invisible light output shows a peak
at 940 nm.
Advantages of LED
1.The various advantages of LED are,
2.LEDs are small in size, and hence can be regarded as point source of light.
Because of their small size, several thousand LEDs can be packed in one sq.
metre area. The brightness of light emitted by LED depends on the current
flowing through LED. Hence the brightness of light can be smoothly controlled
by varying the current. This makes possible to operate LED displays under
different ambient lighting conditions.
3.LEDs are fast operating devices. They can be turned on and off in time less
than 1 microsecond.
4.The LEDs are light in weight.
5.The LEDs are available in various colours.
6.The LEDs have long life.
7.The LEDs are cheap and readily available.
8.The LEDs are easy to interface with various other electronic circuits.
9.Some LEDs radiate infrared light which is invisible but still useful in some
applications like burglar alarm systems.
Disadvantages of LED:
The various disadvantages of LED are,
1.It draws considerable current requiring frequent replacement of battery in low
power battery operated devices.
2.Luminous efficiency of LEDs is low which is about 1.5 lumen/watt.
3.The characteristics are affected by temperature.
4.Need large power for the operation compared to normal p-n junction diode.
Applications of LED:
Due to the advantages like low voltage, long life, cheap, reliable, fast on-off
switching etc, the LEDs are used in many applications. The various applications of
LED are,
1.All kinds of visual displays i.e. seven segment displays and alpha numeric
displays. Such displays are commonly used in the watches and calculators.
2.In the optical devices such as optocouplers.
3.As on-off indicator in various types of electronic circuits.
4.Some LEDs radiate infrared light which is invisible. But such LEDs are useful
in remote controls and applications like burglar alarm.
Comparison of LED and p-n Junction Diode
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