Science (English Medium) Class 12 - CBSE Important Questions

In a pure semiconductor crystal of Si, if antimony is added then what type of extrinsic semiconductor is obtained. Draw the energy band diagram of this extrinsic semiconductor so formed.

Name the device which converts the change in intensity of illumination to change in electric current flowing through it. Plot I-V characteristics of this device for different intensities. State any two applications of this device.

Explain with a proper diagram how an ac signal can be converted into dc (pulsating) signal with output frequency as double than the input frequency using pn junction diode. Give its input and output waveforms.

ASSERTION (A): The electrical conductivity of a semiconductor increases on doping.

REASON (R): Doping always increases the number of electrons in the semiconductor.

The figure shows a piece of pure semiconductor S in series with a variable resistor R and a source of constant voltage V. Should the value of R be increased or decreased to keep the reading of the ammeter constant, when semiconductor S is heated? Justify your answer

The graph of potential barrier versus width of depletion region for an unbiased diode is shown in graph A. In comparison to A, graphs B and C are obtained after biasing the diode in different ways. Identify the type of biasing in B and C and justify your answer

‘A’	‘B’	‘C’

Read the following paragraph and answer the questions.

1. Why are LEDs made of compound semiconductor and not of elemental semiconductors?
2. What should be the order of bandgap of an LED, if it is required to emit light in the visible range?
3. A student connects the blue coloured LED as shown in the figure. The LED did not glow when switch S is closed. Explain why?
   
   OR
   iii. Draw V-I characteristic of a p-n junction diode in
   (i) forward bias and (ii) reverse bias

With the help of a circuit diagram, explain briefly how a p-n junction diode works as a half-wave rectifier.

Draw V-I characteristics of a p-n Junction diode.

Differentiate between the threshold voltage and the breakdown voltage for a diode.

Two crystals C₁ and C₂, made of pure silicon, are doped with arsenic and aluminium respectively.

Identify the extrinsic semiconductors so formed.

Two crystals C₁ and C₂, made of pure silicon, are doped with arsenic and aluminium respectively.

Why is doping of intrinsic semiconductors necessary?

Give two differences between a half-wave rectifier and a full-wave rectifier.

Briefly explain how emf is generated in a solar cell.

Draw solar cells of I-V characteristics.

Draw the circuit diagram of an illuminated photodiode and its I-V characteristics.

How can a photodiode be used to measure light intensity?

Draw the energy band diagrams for conductors, semiconductors and insulators. Which band determines the electrical conductivity of a solid? How is the electrical conductivity of a semiconductor affected with rise in its temperature? Explain.

Name the extrinsic semiconductors formed when pure germanium is doped with a trivalent impurity. Draw the energy band diagram of extrinsic semiconductors so formed.

Name the extrinsic semiconductors formed when pure germanium is doped with a Pentavalent impurity. Draw the energy band diagram of extrinsic semiconductors so formed.