CLASS 12 – PHYSICS-- Chapter 7: ELECTRONIC

CLASS 12 – PHYSICS
Chapter 7: ELECTRONIC
Short answer question:

Q1. How does the motion of an electron in an n-type material differ from the motion of holes in a p-type material?
Ans. In the n-type material the electrons are majority charge carriers and holes are minority charge carrier, where in p-type material the holes are majority charge carriers and electrons are minority charge carriers. In n-type material the current produces by the flow of electrons called electronic current, while in p-type material the current produces by the flow of holes called convention current. When the battery is connected to the n-type and p-type materials, the conventional current and electronic current flow in opposite direction i.e. the motion of electronic in n-type material is opposite to the motion of holes in p-type material.

Q2. What is the net charge on an n–type or a p-type material?
Ans. The net charge on n-type or a p-type material is zero, because both these substances are electrical neutral in absence of electric field.

Q3. The anode of a diode is 0.2V positive with respect to the cathode. Is forward biased?
Ans. As the anode (p-type) of the diode is at high potential with respect to cathode (n-type) so it is forward biased. In this case potential of anode is 0.2V which is greater than the potential of cathode, therefore p-n junction is forward biased and the flow of electron takes place from cathode to anode.

Q4. Why charge carrier are not present in the depletion region of a p-n junction?
Ans. In p-n junction, the region contains free electrons as majority charge carrier and p-region contain holes takes place across the junction. As a result of diffusion the recombination of electrons and holes takes place within the depletion region. Since the charges in the depletion region are used in recombination due to which a charge-less region is formed around the junction in which charge carrier are not present.

Q5. What is the effect of forward and reverse biased the width of the depletion region?
Ans. When the p-n junction is forward biased the width of the depletion decreased, but in the reverse biased the width of the depletion region increases.

Q6. Why ordinary silicon diode does not emit light?
Ans. When the electron recombines with a hole 0.7ev energy is released in the form of photon
Energy of photon = 0.7ev but lev = 1.6 x10 ᴧ -19 J
Energy of photon = 0.7 x 1.6 x 10 ᴧ-19J
Energy of photon = hf but f = c/λ
Energy of photon E = hc/λ
Λ = hc/E
Λ = 1775nm
Hence the photon emitted from the silicon diode has a wavelength much greater than the wavelength of visible light which is from 750-400nm which is not visible. Therefore silicon diode do not emits light.

Q7. Why a photodiode is operated in a reverse biased state?
Ans. In the reverse biased state, the reverse current is negligible in the absence of light. But when p-n junction of the photo diode is exposed to light, the photons in the light give energy to electrons and holes due to which the current begins to flow in the reverse direction. When the intensity of light increases, the number of photons increases which increases the number of electrons and holes and thus increases the reverse current.

Q8. Why is the base current in a transistor very small?
Ans. The base of the transistor is very thin and has very small doping level as compared to emitter and collector. Thus it has only a small number of electrons and holes to recombine. Therefore the majority of the electrons coming from emitter does not combine with some holes and thus escape out of the base. Hence the majority of electrons are attracted towards the collector.

Q9. What is the blasing requirement of the junctions of a transistor for its normal operation?
Ans. The requirement for the normal operation of a transistor is that the emitter base junction should be forward biased and base-collector junction should be reverse biased.

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