NIOS Class 12 Physics Chapter 28 Semiconductors and Semiconducting Devices

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NIOS Class 12 Physics Chapter 28 Semiconductors and Semiconducting Devices

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Semiconductors and Semiconducting Devices

Chapter: 28

Module-VIII: Semiconductor Devices And Communication

INTEXT QUESTIONS 28.1 

1. At 300 K, pure silicon has intrinsic carrier concentration of 1.5 × 10¹⁶ m⁻³. What is the concentration of holes and electrons?

Ans Given that silicon is intrinsic (pure), the number of free electrons n equals the number of holes p and both equal the intrinsic carrier concentration n_i. 

Hence:

n = n_i = 1.5 × 10¹⁶ m^-3

p = n_i = 1.5 × 10¹⁶ m^-3

Therefore, both the electron concentration and the hole concentration are 1.5 × 10¹⁶ m^-3 at 300 K. Remember that intrinsic semiconductors always maintain charge neutrality because every electron promoted to the conduction band leaves behind exactly one hole in the valence band. 

2. The n-type semiconductor is obtained by doping with:

(i) Trivalent impurity. 

(ii) Pentavalent impurity.

(iii) Tetravalent impurity.

(iv) Trivalent as well as tetravalent.

Ans An n-type semiconductor must gain extra conduction electrons. This happens when an impurity atom possessing five valence electrons (one more than silicon or germanium) substitutes a host atom. Four of the five electrons participate in covalent bonding; the fifth remains loosely bound and contributes a free electron to the crystal. Such impurities belong to Group V of the periodic table and are therefore called pentavalent donors. Common examples are phosphorus (P), arsenic (As) and antimony (Sb). Options (i), (iii) and (iv) do not provide the necessary extra electron: trivalent dopants create holes (p-type); tetravalent atoms simply replace Si without changing carrier count; mixing trivalent and tetravalent simultaneously would not systematically create an n-type. 

Accordingly, the correct choice is (ii) pentavalent impurity.

3. An intrinsic semiconductor can be converted into an extrinsic semiconductor by addition of ____________ This process is called ____________.

Ans: Impurity and doping.

4. Electrons in n-type semiconductor and holes in p-type semiconductor are the __________ carriers.

Ans: Majority.

Electrons dominate conduction in n-type material, while holes dominate in p-type; the less numerous species are called minority carriers.

5. An extrinsic semiconductor has _____________ resistivity as compared to an intrinsic semiconductor.

Ans Lower.

INTEXT QUESTIONS 28.2 

1. Fill in the blanks:

(a) When a p-n junction is formed, the _____________ diffuse across the junction.

Ans: Majority carriers.

(b) The region containing uncompensated acceptor and donor ions is called ___________ region.

Ans: Depletion.

(c) The barrier potential in silicon is ______________ V and in germanium, it is ___________ V.

Ans: 0.7 V (Si) and 0.3 V (Ge).

(d) In a p-n junction with no applied electric field, the electrons diffuse from n-region to p-type region as there is ___________ concentration of ____________ in n-region as compared to p-region.

Ans: Higher, electrons.

2. Choose the correct option:

(a) The potential barrier at the p-n junction is due to the charges on either side of the junction. These charges are:

(i) Majority carriers. 

(ii) Minority carriers.

(iii) Fixed donor and acceptor ions.

(iv) None of above.

Ans: (iii) fixed donor and acceptor ions.

(b) In a p-n junction without any external voltage, the junction current at equilibrium is:

(i) Due to diffusion of minority carriers only. 

(ii) Due to diffusion of majority carriers only.

(iii) Zero, as no charges are crossing the junction. 

(iv) Zero, as equal and opposite charges are crossing the junction.

Ans: (iv) zero, as equal and opposite charges are crossing the junction.

(c) In a semiconductor diode, the barrier potential repels:

(i) Minority carriers in both the regions.

(ii) Majority carriers in both the regions.

(iii) Both the majority and the minority carriers. 

(iv) None of the above.

Ans: (ii) Majority carriers in both the regions.

3. Why is depletion region named so? What is depletion region made of?

Ans The term depletion region reflects the fact that this narrow zone on either side of the p-n junction has been depleted of all mobile charge carriers (free electrons in n-type and holes in p-type). As majority carriers diffuse and recombine, they leave their parent donor or acceptor atoms ionised but immobile. Consequently, the region contains only fixed, uncovered ions: positively charged donor ions on the n-side and negatively charged acceptor ions on the p-side. These opposite charges establish an internal electric field that opposes further carrier diffusion, creating a built-in potential barrier. The absence of mobile carriers gives the region high resistance and justifies the name “depletion.”