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Optical Devices – How they work

Electronic optical devices have varied applications in telecommunication, military services, medical field and in automated control systems. Solid state devices like sensors, IR and laser emitters are widely used in these applications. Optoelectronic devices use the energy from the sunlight to activate or inhibit electronic circuits. Check out how they work…

There are two types of optoelectronic devices. These are Photoconductive devices and Photovoltaic devices. Photoconductive devices detect variations in light intensity to activate or inhibit electronic circuits. LDR, Photodiodes and Phototransistors fall in this category. Photovoltaic devices on the other hand generate voltage and current by accepting energy from the sunlight. Depending on the intensity of light, a current will be generated in the p-n junction of these devices. Solar cells and Photovoltaic cells fall in this category.

Photo conductive devices

The electrical conductivity of the semi conductive material used in Photoconductive devices varies according to the intensity of light. Typically these devices have high resistance in dark and very low resistance in bright light.

Light Dependent Resistor – LDR

LDR finds many applications in alarm systems, object counters etc. Cadmium sulphide is used in LDR as the semi conductor material. CdS cells have a light sensitive area that contains small amount of silver, antimony or indium impurities. When light stimulates the semiconductor, its conductivity increases followed by an increase in the flow of electron-hole pairs. Besides Cadmium sulphide, Lead sulphide, Cadmium selenide etc are also used to make LDR. In absolute darkness, the resistance of LDR is as high as 10 MΩ while in bright light it reduces to few ohms. The LDR has a thin film of semiconductor enclosed in a transparent case. LDR has high dissipation capacity and excellent sensitivity in the visible spectrum of light. It can handle many watts and even AC will pass through it without damaging the device.

LDR

Photodiodes

These are high impedance devices used in reverse biased mode to increase the performance. Photodiodes are high speed devices which generate current (in µA) when light fall on it. The photodiode has a large p-n junction and the photons impinging the junction, cause rupturing of covalent bonds in the semiconductor to produce electron-hole pairs. When the intensity of illumination increases, additional electron-hole pairs are produced and the flow of current increases.

photodiode

Phototransistors

Phototransistors are similar to photodiodes but they amplify the current generated by the reverse biased p-n junction. Phototransistors are light sensitive duo diode devices with two junctions separated by a wide base region. It has an n-p-n junction. The n-p junction remains slightly forward biased and the p-n junction is slightly reverse biased. When light illuminates the n-p junction, electron-hole pairs are generated. The electrons then diffuse out of p region while the holes remain in the p region. This forms a positive charge. This increases the forward bias of n-p junction and current flow increases. Phototransistors are usually connected in common base mode with the base connection left unconnected. Only the collector and emitter are connected in the circuit. There is a transparent window with a focusing lens to pass light into the semiconductor junction. Darlington phototransistors are also available with greater sensitivity.

phototransistor

Infrared Diodes

These are Photodiodes emit light in the infrared region which is invisible to eye. The p-n junction of IR diode is made up of Gallium arsenide. There is a recombination junction between the p and n type semiconductor. When a potential difference is applied between the anode and cathode of IR diode, electrons move from the n region and combines with the holes of the p region. Recombination of electrons and holes takes place in the recombination region and photo emission takes place in the form of infrared. The rays have a wavelength around 900 nanometers.

infrared diode

Photovoltaic devices- Solar Cell

These are photoelectric transducers generating electric current by accepting light energy. The photocell has many p-n junctions connected in series. One of these junctions is very narrow to allow the passage of light energy. When sunlight incident on the p-n junction, electron-hole pairs are formed. These charge carriers generate current proportional to the incident light energy. The sunlight incident on the semiconductor junction of solar cell collides with the valance electrons. This causes formation of charge carriers which cross the p-n junction in an opposing manner and create a voltage across the p-n junction. Approximately 0.6 volt will be generated from each solar cell.

solar cell

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