36.

3 LIGHT EMITTING DIODES (LEDS)

The phenomenon of light emission by electrical excitation of a solid was first
observed in 1907 by H.J. Round using Silicon Carbide (SiC). This phenomenon is
called electro luminescence and is the inverse of Einstein's well known photoelec
ric effect. The cffect can be seen in the light emitting diodes.
LEDS or the 'Light Emiting Diodes' are special p-n junction diodes that
emit radiation in visible or infrared region when a suitable forward voltage is
applied across them. LEDs that emit visible light are widely used in instrument
display indicators, digital watches, calculators etc. LEDs that emit invisible infrared
ight fínd applications in remote controlschemes, object detectors, burglar alarm
systems etc.

526 B.SC Prctical Physics

Construction
A ransparent or a coloured enoxv case encloses a semiconductor chip. The
two wires cxtending below ihe Denox enelosure or the bulb indicate how the
LED should be conneted in the cireuit (Eie. 36.3). The negative side of an LED is
indicated in two ways:
I. by the flat sidc of the bulb and
2. by the shorter ofihe two wires extending from the LED.
Epoxy lens -

Glass window

Metal can
LED chip Glass Insulator

T| Fig. 36.3
The negative lead should be connected to the negative terminal of the batery.
The LEDs operate at relatively low voltages of about l lo 3 volts, and draw current
in the range of 5 to 20 milliamperes. Recombination
Voltages and currents substantially Emitted Light
above these valuces can melt the LED
chip.
A cross-sectional view of a
P
typical LED is shown in Fig. 36.4
When the p-n junction is forward
biased, recombination of charge
carriers occurs in the p-region. This
region is therefore requircd to be kept Metal film cathode
at the top. Thus the p-region becomes Fig, 36.4
the device surface. The metal anode
connections are made at the outer edge ofp-layer so as to allow more surface area for
the light to come out. A metal film is applied to the bottom of n-layer which acts as
cathode and reflects back any light coming from this side.
Advantages
LEDs have a number of advantages over ordinary incandescent lamps. They
operate at low voltages (0-3 V) and currents (5 to 20 mA) and thus consume very
less power (10-150 mW). They require no heating, no warm-up time and hence are
very fast in action (response time 10 nanosecond). They are rugged in construction
and can withstand shock and vibrations and have long life.

527
Opto-Electronic Devices
Working
The most important part of an IED is the seniconductor chip located mm
centre of the bulb as shown
Fig. (36.3). The chin is a )-H junction dode. ike n
normal diodes, the junction acts as abarrier lo tho flow ectrons belwCen me
 Working
The most important part of an IED is he soniconductor chip located m ne
centre of the bulb as shown in Fip. (36 3) The chin is a D-t junction die. .kC m
normal diodes, the junction acts as abarrier to the low olclectrons hetween the p and
nregions. When sutfhcient forward volare is applied lo the chip across the lcads of
the LED, electrons can move easily in only one direction across thc junction beiween
the p and n regions. Once an clectron crosses the iunction, it is immediately attracted
to the holes in the p region and recombines, Each timc an cleetron recombincs with a
positive hole, elecric potential energyis converted into electromagnetic energV. ror
each such recombination, a quantum of electromagnetic energy is emitted in the torm
ofa photon of light with a frequency characteristic of the semiconductor material of
the p-n diode.
Suitable Materials for LEDs
The probability of radiative recombination process responsible for he light
emission by LEDs depends primarily on the band structure of the semiconductor. A
material, suitable for visible LEDs, should primarily fulfill two requirements:
1. Is band gap (E,) should be greater than I.8 eV.
2. It should have a 'direcr' band gap i.e. the maximum of the valence band and
the minimum of the conduction band should occur at the same value of the
quantum mechanical wave vector k.
The first condition is essential because the human eye is sensitive only to light
with a photon energy hv equal to or greater than 1.8 eV (s 0.7 um) and the energy
of the light emited is equal to the energy of the band-gap. So a semiconductor of
smaller gap would emit light of wavelength higher than the visible range. This is
one of the reasons that Si (1.1 eV) and Ge (0.7 eV) are not used for LEDs.
In a direct band-gap semiconductor (eg. GaAs), momentum and energy remain
constant during the process of emission of photon. In contrast, in an indirect band-gap
semiconductor such as silicon (Si) and germanium (Ge) momentum conservation is
not possible in photon emission and so nonradiative transitions dominate and most
of the energy is released in the form of heat. So a normal diode made of Si or Ge
emits invisible far-infrared light but the materials like gallium arsenide phosphide
(GaAs P) and gallium phosphide (GaP) cmit light in the visible range and hence
are used in LEDs.
Almost all the currently available LEDs and semiconductor lascrs are based
on GaAs, whose band-gap of 143 eV corresponds to an emission in the intrared
region near 900 nm. When GaAs is mixed with GaP (2.24 eV), the resulting struc
ture has a direct band gap of about 2 eV and gives light in the red-orange spectral
region. Other important materials used in LEDs are ZnN(1.9 eV), ZnSe (2.6 eV).
and GaN (3.4 eV). Afew LED materials and the colour of ight gmitted by them
are listed in Table 36.1.

S28 BS: Praetieul Phrsics

TABLE 36.1
Material Colour oflight emitted
Allumnium gallium Arsenide (Al GaAs) Rcd and intrared
Gallum Alluminium phosphalc (AlGal') Grcen
Gallum ANenide Plosphde (iaAs) Red, Orarnge,
(iallum Nitride (GaN) Green, Blue
Galliunn Phosphide (GaP) Yellow, Green
Zine Selenide (ZnSe) Bluc
|Indium GalliunNitride (n GaN) Bluish green, Blue

Experiment: 36.2 To determine the value of Planck's Constant by using

light-cmitting-diodes (LEDs)
Apparatus: Visible LEDS capable of cmiting light of different wave-lengths, a
battery (5.volts), a rheostat, a volmeter (0-3V),a millammeter, connecting wires, et.
Theory: Just as it takes energy to generate an clectron-hole pair, energy is re
leased when an electron recombines with a hole. This principle is the basic cause of
emission of light by an LED. As discussed earlier, LEDs are pn-junction diodes made
of materials which have suitable value of thedirect band gap (> 1.8eV) necessary for
the emission of visible light.
In such semiconductors, when the p-n junction is suficicntly forward biased.
electrons which get an cxtra energy, eV, flow across the junction from n-conduction
band to p-conduction band. Thesc excited electrons recombine with a hole in that
region (or fall to D-valence band) and nhotons are emitted havino an eneruv annroxi
 -Valence band) and photons are emitted having an energy approx1
mately cqual to the band gap energy. If Vo is the minimum voltage required for the
emission of light, we may wrtie,
eV, =R+ hy (36.4)
where Ris the energy lost in non-radiative recombination and v is the frequency of
the photon emitted. As R << hv it may be neglected.
eV hv
hc
eV, = (36.5)
Hence, ifa minimum vollage V, (also called the 'threshold voltage' or the rurn
on' voltage) given to an LED causes it io glow with light of wavelength 2, then
hc
aplot
of V, along y-axis and along x-axis is a straight line with slope equal to from
which the Planck's constant h cun be deternmined.
Prucedure:
1. Conncet the circuit of Fig. 36.5. A rheostat is used as a potential-divider to
vary the voltage across the LED. Avoltmeter and millammeter connected
in the circuit give respeetively the voltage across the LED and the current
through it. Diffierent LEDs capable of emitting light of different colour are
connccted as shown in figure and can be incorporated into the circuit one
by onc by connccting the point O to l, 2, 3,4, or 5 ele
2. Keep the sliding contact of the rheostat towards the P cnd. Switeh on the
power supply and connect Owith 1. Increase the voltage across the diode
by slowly moving the sliding contact towards Q. Note down the volage I

P
mA

5V DC

Fig. 36.5
across the diode and current l through it. Plot the V-f characteristics and note
down the turn on voltage V, by extrapolating the lincar portion of the curve
as shown in Fig. (36.6). This is the minimum voltage at which the LED just
starts to glow. Note also the colour of the light emitted'
Infrared
Red

elue

-(mA)

V(volts)

Fig. 36.6
3. Disconnect I and connect O with 2, 3,4,5, one by one and repeat the step
2 and note down the turn-on voltage V, and the colour of light emitted.
4. Find the maximum wavelength (minimum frequency) 2. of the light enit
ted by the different LEDs by spectrometer, or simply take thc values trom
Table 36.2.
5. Plot a graph with 1/a along r-axis and the turn-on voltage V, along y-axis.
It will be a straight linc.
hc
6. The slope of the above graph is equal to where c is the speed of light
and e is the clectronic charge. Calculate h from the slope.
1. The tum on voltage Vcan be found without plotting the I-V curve just by looking at the
diode carefully. But this would cause error as it would be diffhcult to find the voltuge I'at
which LED just begins to glow.
*Perform the experiment in a dark room to determine the wavelength with the help of :a
spectrometer. A band will be observed whose maximum wavclength gives A. The reusou
for choosing this Ais that when V> V, higher frequencies or lower wavelengl1s are also
cmitted so the light emitted by an LED may span a range of wavelengths. lowever. the
naximum wavelength corresponds to he threshold vollage -
 S30 BS'cical PhsA

TABLE 36.2

Light cmitcd by LED

Infratel |000
Red 695
Yellow 590

Light green 570

Dark green 505
Bluc 472
Violet 432

Observations:
For V-lcharacteristics
LED Irifrared Red Yellow Green Blue
S. No.
(Vol) (mA)(ol) (mA) 0(Voly (mA) (Valy (mA)(Volt) (mA)

For 1/2 vs. Vo'

S. No Colour of Light
emitted (nm) (m') (Volts)
Red 695 1.44 x 10
2

Calculations: The slope of the straight line in 'Vo vs. I/À' graph (Fig. 36.7) is
hc
Slope = =... Vm

h= xslope (HOA) A

= ... J.sec
where e=1.6022 x10-9c
and c=2.998 × 10 m/s.
Result: The experimentally determincd value 1/A (m-1)
of the Planck's constant .. J sec
Fig. 36.7

Actual value 6.626 x10 " see

' . Frror

Precautions and Sources of Error:

1. The LEDs slhoukd aluavs beceele in fowrd has
Z. The curTentthrouglh the LED shoull ol exceed the prescrihed lmt
3. The turn on voltage , should be notcd very cartfully
. The volmeter and the millianneter shoutd be free from any errors
Weak Points
It isdificult to obtain the exact value of the lun-on voltage. Also the energy lost
duc to non-radiation recombinations is diferent for different LEDs. An error can als he
introdueed in the dtemination of à. Becausc of thesc reasons. the valuc of the Planck
constant as determined by this nmethod is not expectcd to be very accurate one.
Experiment: 36.3 To draw the V-/ characteristics of a LED.
Apparatus: Same as in Experiment 36.2
Procedure: Conncct the circuit of Fig. 36.5 and Perform steps -1 and 2 ot