Name the p-n junction diode which emit spontaneous radiation when frrward biased. How do we choose the semiconductor, to be used in those diodes, if the emitted rediation is to be in the visible region?
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Light emitting diode (LED) emit spontaneous radiation when forward biased. For visible LED's phosphor doped Ga As and GaP are the suitable semiconductors.
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Assertion : Light emitting diode (LED) emits spontaneous radiation. Reason : LED are forward biased p-n junctions.
Explain with help of circuit diagram, the action of a forward biased p-n junction diode which emits spontaneous radiation. State the least band gap energy of this diode to have emission in visible region.
Read the following text and answer the following questions on the basis of the same: 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 to p (where they are minority carriers) and holes are sent from p to n(where they are minority carriers). At the junction boundary, the concentration of minority carriers increases as 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. LED's are biased such that the light emitting efficiency is maximum. 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 LED's are very low, typically around 5 V. So care should be taken that high reverse voltages do not appear across them. LED's that can emit red, yellow, orange, green and blue light are commercially available. During recombination at the junction, emitted photons have:
