Chapter 14: Semiconductor Electronics: Materials, Devices and Simple Circuits Short Questions | physics Class 12 CBSE

Chapter 14: Semiconductor Electronics: Materials, Devices and Simple Circuits Short Questions | physics Class 12 CBSE | ByteNotes.in

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Question 1

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What are the two main types of charge carriers in a semiconductor and what charges do they carry?

The two charge carriers are free electrons and holes; electrons carry a negative charge of magnitude q.
A hole is a vacancy in the covalent bond structure and behaves as an apparent free particle with an effective positive charge +q.
Under an applied electric field, electrons move towards the positive terminal while holes move towards the negative terminal, both contributing to the total current.

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Question 2

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Distinguish between the conductivity ranges of metals, semiconductors, and insulators.

Metals have very low resistivity (10^-2 to 10^-8 Ω m) corresponding to conductivity of 10^2 to 10^8 S m^-1.
Semiconductors have intermediate resistivity (10^-5 to 10^6 Ω m) and conductivity between 10^5 and 10^-6 S m^-1.
Insulators have very high resistivity (10^11 to 10^19 Ω m) and very low conductivity (10^-11 to 10^-19 S m^-1).

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Question 3

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What is meant by the energy band gap and how does its value determine whether a material is a conductor, semiconductor, or insulator?

The energy band gap Eg is the energy difference between the top of the valence band (EV) and the bottom of the conduction band (EC).
For metals, Eg ≈ 0 (bands overlap or conduction band is partially filled), making conduction easy.
For insulators Eg > 3 eV; for semiconductors Eg is between 0.2 eV and 3 eV, allowing limited thermal excitation of electrons into the conduction band.

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Question 4

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Why does the resistivity of a semiconductor decrease with increase in temperature?

At higher temperatures, more thermal energy is available to electrons in the valence band.
Some of these electrons gain enough energy to cross the energy band gap and move into the conduction band, increasing the number of free electrons and holes.
The increase in carrier concentration (ne and nh) leads to higher conductivity and hence lower resistivity.

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Question 5

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What is doping and why must the dopant atom size be similar to the host semiconductor atom?

Doping is the deliberate addition of a small amount of suitable impurity (a few ppm) into a pure semiconductor to increase its electrical conductivity.
The dopant must have an atomic size nearly equal to that of Si or Ge so that it can substitute into the crystal lattice without causing significant lattice distortion.
If the sizes differ greatly, the dopant cannot occupy the original atomic sites and would disturb the crystal structure, preventing effective doping.

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Question 6

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Define donor impurity and give two examples of donor atoms used with silicon.

A donor impurity is a pentavalent atom that, when added to Si or Ge, donates one extra free electron for conduction while four of its electrons form covalent bonds with neighbouring atoms.
Examples of donor atoms used with silicon are Arsenic (As), Antimony (Sb), and Phosphorous (P).
The ionisation energy required to free the fifth electron is very small (~0.05 eV for Si), so these electrons are available for conduction even at room temperature.

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Question 7

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State the relationship between electron and hole concentrations in an extrinsic semiconductor and explain its significance.

In any semiconductor (intrinsic or extrinsic), the product of electron concentration and hole concentration equals the square of intrinsic carrier concentration: nenh = ni^2.
This means that when doping increases one type of carrier (majority carriers), the concentration of the other type (minority carriers) decreases proportionally.
This inverse relationship is crucial because it shows that adding majority carriers indirectly suppresses minority carrier concentration through enhanced recombination.

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Question 8

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What is the depletion region in a p-n junction and how is it formed?

The depletion region is a narrow zone (~0.1 μm) on either side of the p-n junction that is devoid of free charge carriers.
It forms when electrons diffuse from n-side to p-side and holes diffuse from p-side to n-side due to concentration gradients, leaving behind immobile ionised cores.
The positive ions on the n-side and negative ions on the p-side form the space-charge region that creates an internal electric field opposing further diffusion.

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Question 9

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What is barrier potential and how does forward biasing affect it?

Barrier potential (V0) is the built-in electric potential that develops across the p-n junction at equilibrium due to the space-charge region, opposing further diffusion of majority carriers.
In forward bias, the applied voltage opposes the barrier potential, reducing the effective barrier height to (V0 - V) and narrowing the depletion region.
As the barrier decreases, more majority carriers gain sufficient energy to cross the junction, resulting in a significantly increased current (in mA).

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Question 10

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Define threshold voltage of a diode and give its values for Ge and Si diodes.

Threshold voltage (or cut-in voltage) is the minimum forward bias voltage at which the diode current begins to increase significantly (exponentially).
Below this voltage, the current is negligibly small because only very few carriers have enough energy to cross the reduced barrier.
The threshold voltage is approximately 0.2 V for germanium diodes and approximately 0.7 V for silicon diodes.

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Question 11

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What happens to diffusion current and drift current when a p-n junction is reverse biased?

In reverse bias, the applied voltage adds to the barrier potential, increasing the effective barrier to (V0 + V) and widening the depletion region.
This increase in barrier suppresses the flow of majority carriers, so the diffusion current decreases enormously compared to forward bias.
However, minority carriers on each side are swept across the junction by the electric field, producing a small drift current of the order of a few μA, which remains nearly constant with increasing reverse voltage.

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Question 12

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What is reverse saturation current and why is it nearly independent of applied voltage?

Reverse saturation current is the small current (~μA) that flows through a p-n junction diode under reverse bias due to the drift of minority carriers across the junction.
It is nearly independent of applied voltage because even a small reverse voltage is sufficient to sweep all available minority carriers from one side to the other.
The current magnitude is limited not by the applied voltage but by the concentration of minority carriers, which depends only on temperature.

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Question 13

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Define dynamic resistance of a diode and state its formula.

Dynamic resistance (rd) of a diode is defined as the ratio of a small change in the voltage across the diode to the corresponding small change in current through it.
Its formula is rd = ΔV/ΔI, where ΔV and ΔI are small incremental changes in voltage and current respectively.
The forward bias dynamic resistance is very low (in ohms) while the reverse bias dynamic resistance is very high (in megaohms), reflecting the asymmetric conduction property of the diode.

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Chapter 14: Semiconductor Electronics: Materials, Devices and Simple Circuits | Short Questions

What are the two main types of charge carriers in a semiconductor and what charges do they carry?

Distinguish between the conductivity ranges of metals, semiconductors, and insulators.

What is meant by the energy band gap and how does its value determine whether a material is a conductor, semiconductor, or insulator?

Why does the resistivity of a semiconductor decrease with increase in temperature?

What is doping and why must the dopant atom size be similar to the host semiconductor atom?

Define donor impurity and give two examples of donor atoms used with silicon.

State the relationship between electron and hole concentrations in an extrinsic semiconductor and explain its significance.

What is the depletion region in a p-n junction and how is it formed?

What is barrier potential and how does forward biasing affect it?

Define threshold voltage of a diode and give its values for Ge and Si diodes.

What happens to diffusion current and drift current when a p-n junction is reverse biased?

What is reverse saturation current and why is it nearly independent of applied voltage?

Define dynamic resistance of a diode and state its formula.

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