Light Emitting Diodes

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≥V₀, 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 E_g.

   

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 R_LED and thus share the voltage input 6V.

The voltage required across R_LED is 1.7V, therefore the voltage needed to be dropped by R is 4.3V.  Also since R and R_LED are in series, the same current is passing through them i.e. 10mA (require for LED bright glow).  

Note that, for V_R (the voltage across R) to be equall 4.3V and I_R (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 (V_cc) 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.

 

Figure 3.12: Using 7-Segment LED to display number 0-9.


See Also:
Photo-diode
Varactor Diode