Light Emitting Diode (LED)

Light Emitting Diode (LED)

• A diode which emits light when forward biased is called a Light Emitting Diode (LED).

1. Construction of LED

• In LED three semiconductor layers on the substrate are used as shown in Fig. 6.6.1 (a). In between p type and n type region, there exists a region called active region. This region is responsible for the emission of the light.

• The LED emits light all the way around the layered structure. This layered structure is placed in a tiny reflective cup so that the light gets reflected towards the desired exit direction. This cup type structure is shown in Fig. 6.6.1 (b) while the symbol of the LED is shown in Fig. 6.6.1 (c).

2. Working Principle of LED

• The LED works on the principle of electroluminescence.

• When a p‒n junction is forward biased, the electrons in n region cross the junction and recombine with holes in p region.

• The free electrons exist in the conduction band while the holes exist in the valence band.

• The energy level of free electrons is higher than the energy level of the holes.

• When electrons recombine with the holes, they move from conduction band to valence band which is at lower energy level.

• While moving, the additional energy is released by the free electrons which appears in the form of light due to the special material used in the LED.

• Practically when LED is forward biased, the holes from p region and electrons from n region enter the active region between p and n regions. In the active region recombination of electrons and holes take place and the energy is released in the form of a light.

• The energy released depends on the forbidden gap energy which determines the wavelength and the colour of the emitted light.

• Fig. 6.6.2 shows the principle of working of LED.

3. Internal Quantum Efficiency

• For an ideal LED, the active region emits one photon for every electron injected. Hence ideally the quantum efficiency of an LED is unity.

• Practically the internal quantum efficiency of an LED is defined as the fraction of the electrons that are injected to the depletion region which results into photon getting produced.

• In practical LED, all the power emitted from the active region is not emitted to the free space. Some photons remain in the active region and never leave the semiconductor.

• The internal quantum efficiency is mathematically expressed as,

where

η_int = Number of photons emitted from active region per second / Number of electrons injected into LED per second

 η_int = (P_int/ hν) / (I/e)

P_int = Optical power emitted from the active region

I = The injection current

e = The charge on each electron

h = Planck's constant

 ν = Frequency of photon

4. External Quantum Efficiency

• The photons must escape from the active region to the free space. But due to physical structure and geometry of the material layers, the total internal reflection occurs, which causes obstruction to escaping photons. Thus extraction efficiency of an LED is defined as,

 η_extraction = Number of photons emitted into free space per second / Number of photons emitted from active region per second

 η_extraction = P/hν / Pint/hν

where P = Optical power emitted into free space

• Due to this, the external quantum efficiency of an LED is defined as the ratio of photons emitted into free space to the electrons injected into LED.

 η_ext = Number of photons emitted into free space per second / Number of electrons injected into LED per second

 η_ext = (P/hv) / (I/e)

 = η_int × η_extraction

• Thus external quantum efficiency gives the indication of number of useful light particles to the number of injected charge particles.

5. Materials of LED

• The materials which are the mixtures of gallium, arsenic and phosphorus, are used in LED to obtain different colour of light.

• The colour of light depends on wavelength which depends on the forbidden energy gap value.

• The various materials and colour obtained are given in Table 6.6.1.

6. LED Voltage and Current

• Consider a source connected to LED and resistor as shown in Fig. 6.6.3.

• Due to R_S the current I_S gets limited.

• If V_D is the drop across LED then, applying KVL,

V_S = I_SR_S + V_D

i.e.

I_S = [ V_S‒V_D ] / R_S

         ……….LED current

• When forward biased, V_D is about 1.5 V to 2.5 V.

• Practically current range of I_S is 10 to 70 mA.

• If not provided, V_D is assumed as 2 V.

7. Output Characteristics of LED

• The amount of power output translated into light is directly proportional to the forward current I_f. More the forward current I_f, the greater is the output for of forward current light. The graph of forward current and output light and output light in mW is shown in Fig. 6.6.4. This is called output characteristics for LED.

8. Advantages of LED

• The various advantages of LED are,

1. 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.

2. 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.

9. Applications of LED

• 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.

10. 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.

11. Comparison of LED with P‒N Junction Diode

LED

1. It emits light, when forward biased.

2. It uses materials like gallium, arsenide phosphide and gallium phosphide.

3. The drop across forward blased LED is about 2 V.

4. Reverse breakdown voltage is low, about 3 V to 10 V.

5. Needs large power for the operation.

6. Draws considerable current from battery.

7. Symbol is 

8. The applications are optocouplers, seven segment displays, alpha numeric displays.

P‒N junction diode

1. It does not emit light.

2. It uses materials like silicon and germanium.

3. The drop across forward biased diode is about 0.7 V much less than that of LED.

4. Reverse breakdown voltage is high, about 50 V and more.

5. Needs less power for the operation.

6. Draws less current.

7. Symbol is 

8. The applications are rectifiers, clippers, clampers, voltage multipliers and many other electronic circuits.

Ex. 6.6.1: What is the current through LED shown in Fig. 6.6.5.

Solution: :

V_S = 15 V, R_S = 2.2 kΩ

Assume LED voltage drop as V_D = 2 V

 I_S = (V_S‒V_D) / R_S

  = (15‒2) / 2.2×10³

= 5.91 mA

            …………. LED current

Review Questions

1. Enumerate the construction and operation of LED.

2. Explain the working principle of LED.

3. State the materials used in LED.

4. Explain the output characteristics of LED.

5. State the advantages of LED.

6. State the applications of LED.

7. State the disadvantages of LED.

8. Compare LED with P‒N junction diode.