Q14 of 26 Page 1

(i) Explain briefly the process of emission of light by a Light Emitting Diode (LED).
(ii) Which semiconductors are preferred to make LEDs and why?
(iii) Give two advantages of using LEDs over conventional incandescent lamps.

(i) A light emitting diode is a p-n junction diode which emits photons when it is forward biased. When we forward bias a diode, electrons move from n-type towards p-type, combining to complete atoms and giving of a packet of energy, a photon.


(ii) Semiconductors having energy band gaps such that the wavelength of photon emitted lies in the visible spectrum are chosen as materials for constructing an LED. Usually this wavelength is around 1.4-1.5eV. Apart from this, the cost of synthesizing the material, their availability and optical absorption are also considered. Some of the materials which are used are GaAs, CdTe etc.


(iii) The two advantages of LED’s are: they operate on low voltages and thus are power efficient. They have longer lifetime and are more robust than filament bulbs.


More from this chapter

All 26 →
12

(a) A parallel plate capacitor (C1) having charge Q is connected, to an identical uncharged capacitor C2 in series. What would be the charge accumulated on the capacitor C2?
(b) Three identical capacitors each of capacitance 3mF are connected, in turn, in series and in parallel combination to the common source of V volt. Find out the ratio of the energies stored in two configurations.

 13

(a) Define the term intensity of radiation in photon picture.
(b) Plot a graph showing the variation of photo current vs collector potential for three different intensities I1>I2>I3, two of which (I1 and I2) have the same frequency n and the third has frequency n1>n.
(c) Explain the nature of the curves on the basis of Einsteins equation.

 15

A capacitor of unknown capacitance, a resistor of 100 V and an inductor of self-inductance L5(4/p2) henry are connected in series to an ac source of 200V and 50 Hz. Calculate the value of the capacitance and impedance of the circuit when the current is in phase with the voltage. Calculate the power dissipated in the circuit.

 16

(i) For a glass prism (