CBSE Class 12 Physics 2017 Outside Delhi Set 3 Paper

Question : 8

Total: 8

(a) In the following diagram ' S ' is a semiconductor. Would you increase or decrease the value of R to keep the reading of the ammeter A constant when S is heated? Give reason for your answer.

(b) Draw the circuit diagram of a photodiode and explain its working. Draw its I−V characteristics.

Solution:

(a)

When you heat the semiconductor, the resistance in the circuit will decrease. On making current fixed, we have to increase the resistance in the circuit to keep the reading of ammeter A constant.
(b)

Photo diode is light sensitive electronic device that converts light into voltage signal or current signal. It carries two terminals - anode and cathode showing light rays reflection in form of photo detector. It works normally on principle of photo electric effect.
Working: In this, the photons hit the diode resulting in electron hole pairs where intensity of photon absorption is based on photon energy where less photon energy resulting in high absorption.

In the p−n junction as shown, in case of absorption in depletion region, the hole pairs are removed from the p−n junction due to inside electric field in depletion region that makes the holes to move towards anode and electrons toward cathode forming photocurrent. Here, total photocurrents and dark currents flowing with/without light is sum of current through photodiode.
V-I Characteristics

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