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Chapter: 12th Physics : UNIT 10a : Semiconductor Electronics

Optoelectronic devices using Diodes

Diode Optoelectronic devices : 1. Light Emitting Diode (LED) 2. Photodiodes 3. Solar cell.

Optoelectronic devices

Optoelectronics deals with devices which convert electrical energy into light and light into electrical energy through semiconductors. Optoelectronic device is an electronic device which utilizes light for useful applications. We will discuss some important optoelectronic devices namely, light emitting diodes, photo diodes and solar cells.

 

1. Light Emitting Diode (LED)

LED is a p-n junction diode which emits visible or invisible light when it is forward biased. Since, electrical energy is converted into light energy, this process is also called electroluminescence. The circuit symbol of LED is shown in Figure 9.22(a).

The cross-sectional view of a commercial LED is shown in Figure 9.22(b). It consists of a p-layer, n-layer and a substrate. A transparent window is used to allow light to travel in the desired direction. An external resistance in series with the biasing source is required to limit the forward current through the LED. In addition, it has two leads; anode and cathode.


When the p-n junction is forward biased, the conduction band electrons on n-side and valence band holes on p-side diffuse across the junction. When they cross the junction, they become excess minority carriers (electrons in p-side and holes in n-side). These excess minority carriers recombine with oppositely charged majority carriers in the respective regions, i.e. the electrons in the conduction band recombine with holes in the valence band as shown in the Figure 9.22(c).

During recombination process, energy is released in the form of light (radiative) or heat (non-radiative). For radiative recombination, a photon of energy hv is emitted. For non-radiative recombination, energy is liberated in the form of heat.

The colour of the light is determined by the energy band gap of the material. Therefore, LEDs are available in a wide range of colours such as blue (SiC), green (AlGaP) and red (GaAsP). Now a days, LED which emits white light (GaInN) is also available.

Applications

• Indicator lamps on the front panel of the scientific and laboratory equipments.

• Seven-segment displays.

• Traffic signals, emergency vehicle lighting etc.

• Remote control of television, airconditioner etc.

 

EXAMPLE 9.4

Determine the wavelength of light emitted from LED which is made up of GaAsP semiconductor whose forbidden energy gap is 1.875 eV. Mention the colour of the light emitted (Take h = 6.6 × 10-34 Js).

Solution

Eg hc / λ

Therefore

λ= hc / Eg  = 6.6×10−34 ×3×108  / 1.875×1.6×10−19

= 660 nm

The wavelength 660 nm corresponds to red colour light.

 

2. Photodiodes

A p-n junction diode which converts an optical signal into electric signal is known as photodiode. Thus, the operation of photodiode is exactly inverse to that of an LED. Photo diode works in reverse bias. Its circuit symbol is shown in Figure 9.23(a). The direction of arrows indicates that the light is incident on the photo diode.

The device consists of a p-n junction semiconductor made of photosensitive material kept safely inside a plastic case as shown in Figure 9.23(b). It has a small transparent window that allows light to be incident on the p-n junction. Photodiodes can generate current when the p-n junction is exposed to light and hence are called as light sensors.


When a photon of sufficient energy () strikes the depletion region of the diode, some of the valence band electrons are elevated into conduction band, in turn holes are developed in the valence band. This creates electron-hole pairs. The amount of electron- hole pairs generated depends on the intensity of light incident on the p-n junction.

These electrons and holes are swept across the p-n junction by the electric field created by reverse voltage before recombination takes place. Thus, holes move towards the n-side and electrons towards the p-side. When the external circuit is made, the electrons flow through the external circuit and constitute the photocurrent.

When the incident light is zero, there exists a reverse current which is negligible. This reverse current in the absence of any incident light is called dark current and is due to the thermally generated minority carriers.

Applications

 Alarm system

• Count items on a conveyer belt

• Photoconductors

• Compact disc players, smoke detectors

• Medical applications such as detectors for computed tomography etc.

 

3. Solar cell

A solar cell, also known as photovoltaic cell, converts light energy directly into electricity or electric potential difference by photovoltaic effect. It is basically a p-n junction which generates emf when solar radiation falls on the p-n junction. A solar cell is of two types: p-type and n-type.

Both types use a combination of p-type and n-type Silicon which together forms the p-n junction of the solar cell. The difference is that p-type solar cells use p-type Silicon as the base with an ultra-thin layer of n-type Silicon as shown in Figure 9.24, while n-type solar cell uses the opposite combination. The other side of the p-Silicon is coated with metal which forms the back electrical contact. On top of the n-type Silicon, metal grid is deposited which acts as the front electrical contact. The top of the solar cell is coated with anti-reflection coating and toughened glass.


In a solar cell, electron–hole pairs are generated due to the absorption of light near the junction. Then the charge carriers are separated due to the electric field of the depletion region. Electrons move towards n–type Silicon and holes move towards p-type Silicon layer. The electrons reaching the n-side are collected by the front contact and holes reaching p-side are collected by the back electrical contact. Thus a potential difference is developed across solar cell.When an external load is connected to the solar cell, photocurrent flows through the load.

Many solar cells are connected together either in series or in parallel combination to form solar panel or module. Many solar panels are connected with each other to form solar arrays. For high power applications, solar panels and solar arrays are used.

Applications:

• Solar cells are widely used in calculators, watches, toys, portable power supplies, etc.

• Solar cells are used in satellites and space applications

• Solar panels are used to generate electricity.

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12th Physics : UNIT 10a : Semiconductor Electronics : Optoelectronic devices using Diodes |

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