Science (English Medium) Class 12 - CBSE Important Questions for Physics

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?

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.

A semiconductor device is connected in series with a battery, an ammeter and a resistor. A current flows in the circuit. If. the polarity of the battery is reversed, the current in the circuit almost becomes zero. The device is a/an ______.

With the help of a circuit diagram, explain how a full wave rectifier gives output rectified voltage corresponding to both halves of the input ac voltage.

• Assertion (A): The resistance of an intrinsic semiconductor decreases with an increase in its temperature.
• Reason (R): The number of conduction electrons as well as hole increase in an intrinsic semiconductor with the rise in its temperature.

• Assertion (A): Putting the p-type semiconductor slab directly in physical contact with the n-type semiconductor slab cannot form the pn junction.
• Reason (R): The roughness at contact will be much more than inter atomic crystal spacing and continuous flow of charge carriers is not possible.