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Optoelectronic junction devices, semiconductor diodes in which carriers are generated by photons (photo-excitation). These devices are called optoelectronic devices., Examples, (i) Photodiodes used for detecting optical signal (photodetectors)., (ii) Light emitting diodes (LED) which convert electrical energy into light., (iii) Photovoltaic devices which convert optical radiation into electricity (solar cells).
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Photodiode, A Photodiode is a special purpose p-n junction diode fabricated with a transparent window to allow light to fall on the diode. , It is operated under reverse bias., When the photodiode is illuminated with light (photons) with energy (hν) greater than the energy gap (Eg ) of the semiconductor, then electron-hole pairs are generated due to the absorption of photons. , The diode is fabricated such that the generation of e-h pairs takes place in or near the depletion region of the diode. Due to electric field of the junction, electrons and holes are separated before they recombine. The direction of the electric field is such that electrons reach n-side and holes reach p-side. Electrons are collected on n-side and holes are collected on p-side giving rise to an emf. When an external load is connected, current flows.
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The magnitude of the photocurrent depends on the intensity of incident light (photocurrent is proportional to incident light intensity). There is change in the current with change in the light intensity, if a reverse bias is applied. Thus photodiode can be used as a photodetector to detect optical signals.
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(ii) Light emitting diode, It is a heavily doped p-n junction which under forward bias emits spontaneous radiation., The diode is encapsulated with a transparent cover so that emitted light can come out., When the diode is forward biased, electrons are sent from n → p (where they are minority carriers) and holes are sent from p → n (where they are minority carriers). , At the junction boundary the concentration of minority carriers increases compared to the equilibrium concentration (i.e., when there is no bias). Thus at the junction boundary on either side of the junction, excess minority carriers are there which recombine with majority carriers near the junction. , On recombination, the energy is released in the form of photons. Photons with energy equal to or slightly less than the band gap are emitted. , When the forward current of the diode is small, the intensity of light emitted is small. As the forward current increases, intensity of light increases and reaches a maximum. Further increase in the forward current results in decrease of light intensity. LEDs are biased such that the light emitting efficiency is maximum.
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The V-I characteristics of a LED is similar to that of a Si junction diode., But the threshold voltages are much higher and slightly different for each colour. The reverse breakdown voltages of LEDs are very low, typically around 5V., LEDs that can emit red, yellow, orange, green and blue light are commercially available. The semiconductor used for fabrication of visible LEDs must at least have a band gap of 1.8 eV , (spectral range of visible light is from about 0.4 μm to 0.7 μm, i.e., from about 3 eV to 1.8 eV). , The compound semiconductor Gallium Arsenide – Phosphide (GaAs1–x Px ) is used for making LEDs of different colours. , GaAs0.6 P0.4 (Eg ~ 1.9 eV) is used for red LED. , GaAs (E g ~ 1.4 eV) is used for making infrared LED., Uses : used in remote controls, burglar alarm systems, optical communication, LEDs which can replace incandescent lamps.
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LEDs have the following advantages over conventional incandescent low power lamps:, (i) Low operational voltage and less power., (ii) Fast action and no warm-up time required., (iii) The bandwidth of emitted light is 100 Å to 500 Å or in other words it, is nearly (but not exactly) monochromatic., (iv) Long life and ruggedness., (v) Fast on-off switching capability.
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(iii) Solar cell, A solar cell is basically a p-n junction which, generates emf when solar radiation falls on the p-n junction. , It works on the same principle (photovoltaic effect) as the photodiode, (i.e.diode is illuminated with light (photons) with energy (hν) greater than the energy gap (Eg ) of the semiconductor, then electron-hole pairs are generated due to the absorption of photons) except that no external bias is applied and , the junction area is kept much larger for solar radiation to be incident because we are interested in more power.
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A p-Si wafer of about 300 μm is taken over which a thin layer (~0.3 μm) of n-Si is grown on one-side by diffusion process. The other side of p-Si is coated with a metal (back contact). On the, top of n-Si layer, metal finger electrode (or metallic grid) is deposited. This acts as a front contact. The metallic grid occupies only a very small fraction, of the cell area (<15%) so that light can be incident on the cell from the top.
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The generation of emf by a solar cell, when light falls on, it is due to the following three basic processes: generation, separation and collection—, (i) generation of e-h pairs due to light (with hν > E g ) close to the junction;, (ii) separation of electrons and holes due to electric field of the depletion region. Electrons are swept to n-side and holes to p-side;, (iii) the electrons reaching the n-side are collected by the front contact and holes reaching p-side are collected by the back contact. Thus p-side becomes positive and n-side becomes negative giving rise to photovoltage. When an external load is connected as shown in the Fig. photocurrent I L flows through the load.
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Semiconductors with band gap close to 1.5 eV are ideal materials for solar cell fabrication. , Solar cells are made with semiconductors like Si (Eg = 1.1 eV), GaAs (Eg = 1.43 eV), CdTe (E g = 1.45 eV), CuInSe2 (E g = 1.04 eV), etc. , The important criteria for the selection of a material for solar cell fabrication are (i) band gap (~1.0 to 1.8 eV), (ii) high optical absorption (~104 cm–1), (iii) electrical conductivity, (iv) availability of the raw material, and (v) cost. , sunlight is not always required for a solar cell. Any light with photon energies greater than the band gap will do., uses, They are used to power electronic devices in satellites and space vehicles and also as power supply to some calculators.
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Photodiodes, detect the optical signal operated under reverse bias , Uses , 1in the photo detection for the optical signal, 2 modulation for optical signal , 3 switching the light on and off 4 optical communication equipment , 5 in reading of punch cards and the tape computer, 6 Logic circuit that requires stability and high speed., Photo voltaic devices (solar cells), converts optical radiation into electricity, no external bias is used uses, 1charging the storage batteries , 2used in artificial satellite, 3 for generating electricity in solar cooker and Solar pump solar calculator wrist watches and light metre, 5 power traffic signs, 6 remote radio telephone , 7 to produce power in remote areas, LED , light emitting diode converts electrical energy into light , it is forward bias heavily doped., Uses , 1 remote control , 2 burglar alarm system, 3 optical communication , 4 in calculator digital watches , 5 in the computers optical mouse of computer, 6 picture phone video display , 7traffic light illuminate
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Photodiodes, Photo voltaic devices (solar cells), LED
