CUET UG Physics Booster Test 2- Fundamentals and Materials -

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CUET UG Physics Booster Test 2- Fundamentals and Materials

⏱️ 30 Minutes

❓ 20 Questions

📌 Answers are locked once submitted — results and explanations appear at the end.

CUET UG Physics Booster Test 2- Fundamentals and Materials

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QUESTION 1 OF 20

In a vacuum tube operating with moving electrons

Answer Review

1 In a vacuum tube operating with moving electrons

A the moving electrons may lose their energy on collision with air molecules if vacuum is missing

B the moving electrons easily reverse direction for alternating currents without restrictions

C the moving electrons gain kinetic energy from the grid only

D the moving electrons do not require any voltage difference to flow between electrodes

💡 Short Explanation

Vacuum minimizes electron collisions. Air molecules hinder electron motion. Energy loss reduces device efficiency.

✅ Correct Answer Explanation

Vacuum tubes require an evacuated space because moving electrons can collide with air molecules if air is present. These collisions cause electrons to lose kinetic energy and disrupt their controlled motion between cathode and anode. The vacuum ensures efficient electron transport and proper device operation.

❓ Why Other Options Are Incorrect

Option B → Electron flow remains controlled and does not freely reverse direction.
Option C → Electrons gain energy primarily from the electric field between electrodes, not only from the grid.
Option D → A potential difference is essential for electron movement.

🎯 Strategy

Used

Elimination

Application:

Eliminate statements that contradict the basic operating principle of vacuum tubes.

Final Logic:

Without vacuum, collisions with air molecules reduce electron energy.

🧠 Memory Trick

"Vacuum Prevents Collisions."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Vacuum Tube Operation

2 Incorrect statement about the early realization of semiconductors:

A It was recognized that junctions offer control over the direction of flow

B The number of charge carriers can be controlled by light or heat

C Small applied voltages can change the flow of charge carriers within the material

D Mobile electrons had to be obtained from an external heated cathode to function

💡 Short Explanation

Semiconductors generate carriers internally. Light and heat affect conductivity. Heated cathodes are features of vacuum tubes.

✅ Correct Answer Explanation

A major realization about semiconductors was that charge carriers already exist within the material and their concentration can be modified by light, heat, or applied voltages. Therefore, semiconductors do not require an external heated cathode to supply electrons. Option D incorrectly describes vacuum-tube technology rather than semiconductor devices.

❓ Why Other Options Are Incorrect

Option A → Junctions control current direction.
Option B → Heat and light affect carrier concentration.
Option C → Small voltages can regulate carrier flow.

🎯 Strategy

Used

Odd One Out

Application:

Identify the statement associated with vacuum tubes rather than semiconductors.

Final Logic:

External heated cathodes are unnecessary in semiconductors.

🧠 Memory Trick

"Semi Has Its Own Carriers."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Semiconductor Fundamentals

3 The reason vacuum tube devices are historically referred to as "valves" is because

A they operate consistently at high voltages (~100 V)

B they contain a grid that physically opens and closes

C electrons can flow only in one direction, from cathode to anode

D they require five electrodes to function as a pentode

💡 Short Explanation

Vacuum tubes permit one-way electron flow. They behave like electrical valves. Current direction is controlled.

✅ Correct Answer Explanation

Vacuum tubes were called "valves" because they allow electron flow primarily in one direction—from the heated cathode to the positively charged anode. This one-way control resembles the operation of a valve controlling fluid flow.

❓ Why Other Options Are Incorrect

Option A → High voltage is not the reason for the name.
Option B → The grid does not physically open or close.
Option D → Pentodes are only one category of vacuum tube.

🎯 Strategy

Used

Contextual/Tonal Matching

Application:

Relate the term "valve" to its functional meaning.

Final Logic:

One-way electron flow gives rise to the term "valve."

🧠 Memory Trick

"Valve = One-Way Flow."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Vacuum Valves

4 Galena detector historical statements:
A. Used much before the full implications of semiconductors were formally understood
B. Comprised a naturally occurring crystal of Lead sulphide (PbS)
C. Relied on an internal heated cathode for electron emission
D. Acted as a detector of radio waves with an attached metal point contact

A (A), (B), (D)

B (A), (C), (D)

C (B), (C)

D (A), (B), (C), (D)

💡 Short Explanation

Galena was an early semiconductor detector. It used PbS crystal. No heated cathode was required.

✅ Correct Answer Explanation

Galena detectors were among the earliest semiconductor devices. They used naturally occurring lead sulphide (PbS) crystals and a metal point contact to detect radio waves. These devices were used before modern semiconductor theory was fully developed. Statement C is incorrect because galena detectors did not use thermionic emission or heated cathodes.

❓ Why Other Options Are Incorrect

Option B → Includes incorrect statement C.
Option C → Includes incorrect statement C and omits valid statements.
Option D → Includes incorrect statement C.

🎯 Strategy

Used

Option Grouping

Application:

Check each statement independently and identify the correct combination.

Final Logic:

Only A, B, and D correctly describe galena detectors.

🧠 Memory Trick

"Galena = PbS + Point Contact."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Early Semiconductor Devices

5 Physical scale characteristics (Vacuum Tubes vs Solid State) respectively:

A Small, Bulky

B Bulky, Small

C Bulky, Bulky

D Small, Small

💡 Short Explanation

Vacuum tubes require large evacuated spaces. Semiconductor devices are compact. Miniaturization became possible through solid-state electronics.

✅ Correct Answer Explanation

Vacuum tubes are bulky because they require cathodes, anodes, grids, and vacuum chambers. Semiconductor devices operate entirely within solids and therefore occupy much less space.

❓ Why Other Options Are Incorrect

Option A → Reverses actual characteristics.
Option C → Semiconductor devices are not bulky.
Option D → Vacuum tubes are not small.

🎯 Strategy

Used

Odd One Out

Application:

Compare physical dimensions of the two technologies.

Final Logic:

Vacuum tubes are bulky, whereas semiconductor devices are compact.

🧠 Memory Trick

"Tube Big, Chip Small."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Advantages of Semiconductors

6 CorreA. Vacuum tubes generally consume high power
B. Semiconductor devices strictly require external heating
C. Semiconductor devices operate at low voltages and consume low power
D. CRT monitors work on solid state principlect statements about operational requirements:

A (A), (C)

B (B), (D)

C (A), (B)

D (C), (D)

💡 Short Explanation

Vacuum tubes consume more power. Semiconductors use low power. CRT monitors are vacuum-tube based.

✅ Correct Answer Explanation

Vacuum tubes require continuous heating and therefore consume substantial power. Semiconductor devices operate at relatively low voltages and low power levels. Statement B is incorrect because semiconductors do not require external heating. Statement D is incorrect because CRT monitors operate using vacuum-tube principles.

❓ Why Other Options Are Incorrect

Option B → Both statements are incorrect.
Option C → Includes incorrect statement B.
Option D → Includes incorrect statement D.

🎯 Strategy

Used

Option Grouping

Application:

Evaluate each operational characteristic individually.

Final Logic:

Only statements A and C are correct.

🧠 Memory Trick

"Tube = High Power, Semi = Low Power."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Vacuum Tubes vs Semiconductors

7 Match List I (Technology) with List II (Characteristics)

List I	List II
I. CRT monitors	P. Long life and high reliability
II. LCD monitors	Q. Works on the principle of vacuum tubes
III. Vacuum tubes	R. Supported by solid state electronics
IV. Semiconductor devices	S. Limited life and low reliability

A I–Q, II–R, III–S, IV–P

B I–R, II–Q, III–P, IV–S

C I–S, II–P, III–Q, IV–R

D I–P, II–S, III–R, IV–Q

💡 Short Explanation

CRT monitors operate using vacuum tube technology. LCD monitors are based on solid-state electronics. Vacuum tubes have limited life and lower reliability. Semiconductor devices offer long life and high reliability.

✅ Correct Answer Explanation

CRT (Cathode Ray Tube) monitors function on the principle of vacuum tubes. Therefore, CRT monitors → Q (Works on the principle of vacuum tubes). LCD monitors use semiconductor-based electronic circuits and hence correspond to R (Supported by solid state electronics). Vacuum tubes suffer from heating, filament failure, and larger power consumption, resulting in limited life and low reliability, so Vacuum tubes → S. Semiconductor devices are compact, durable, and efficient, giving them long life and high reliability, so Semiconductor devices → P. Thus, the correct matching is: I–Q, II–R, III–S, IV–P

❓ Why Other Options Are Incorrect

Option B → I–R, II–Q, III–P, IV–S

CRT and LCD monitor principles are interchanged, and the reliability characteristics are reversed.

Option C → I–S, II–P, III–Q, IV–R

CRT monitors are not characterized by limited life, and vacuum tubes do not operate on solid-state principles.

Option D → I–P, II–S, III–R, IV–Q

Semiconductor devices do not work on vacuum tube principles, and LCD monitors are not associated with low reliability.

🎯 Strategy

Used

Concept-Based Matching

Application:

Identify whether the device belongs to vacuum-tube technology or solid-state technology, then connect it to its reliability characteristics.

Final Logic:

CRT → Vacuum tube principle
LCD → Solid-state electronics
Vacuum tubes → Limited life and reliability
Semiconductor devices → Long life and reliability

🧠 Memory Trick

Semiconductors → Longer life, more reliable

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Advantages of Semiconductor Devices over Vacuum Tubes

8 If V_(vac)is a typical vacuum tube voltage and V_(semi)is a typical semiconductor operating voltage, their expected numerical relation based on standard operational limits will be:

A −(Vvac + Vsemi) > 0

B (Vvac − Vsemi) > 0

C (Vsemi − Vvac) > 0

D (Vvac + Vsemi) < 0

💡 Short Explanation

Vacuum tubes require higher voltages. Semiconductor devices operate at lower voltages. Therefore Vvac > Vsemi.

✅ Correct Answer Explanation

Vacuum tubes generally operate around hundreds of volts, whereas semiconductor devices function effectively at much lower voltages. Therefore: V_(vac)>V_(semi) which means: (V_(vac)-V_(semi))>0

❓ Why Other Options Are Incorrect

Option A → Sum of positive voltages cannot satisfy this inequality.
Option C → Reverses the actual relationship.
Option D → Sum of positive voltages cannot be negative.

🎯 Strategy

Used

Substitution

Application:

Compare typical operating voltages of the two technologies.

Final Logic:

Vacuum tubes operate at higher voltages than semiconductors.

🧠 Memory Trick

"Tube Voltage > Chip Voltage."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Semiconductor Advantages

9 If an insulator has a conductivity of 10^(-15)S m⁻¹, what will be its theoretical resistivity?

A 10^(11)Ω m

B 10^(-15)Ω m

C 10^(15)Ω m

D 10^(19)Ω m

💡 Short Explanation

Resistivity is reciprocal of conductivity. ρ=1/σ Reciprocal of 10^(-15)is 10^(15).

✅ Correct Answer Explanation

The relationship between conductivity and resistivity is: ρ=1/σ Given: σ=10^(-15) S m^(-1) Therefore, ρ=1/10^(-15)=10^(15) Ωm

❓ Why Other Options Are Incorrect

Option A→ Incorrect magnitude.
Option B → Same as conductivity, not reciprocal.
Option D → Larger than calculated value.

🎯 Strategy

Used

Dimensional/Unit Analysis

Application:

Apply the reciprocal relation between conductivity and resistivity.

Final Logic:

ρ=1/σgives 10^(15) Ωm.

🧠 Memory Trick

"Resistivity = Reciprocal of Conductivity."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Conductivity and Resistivity

10 Incorrect statement about resistivity limits:

A Metals: 10^(-8)to 10^(-2)Ω m

B Semiconductors: 10^(-5)to 10^6Ω m

C Insulators: 10^(11)to 10^(19)Ω m

D Insulators have lower resistivity limits than elemental semiconductors

💡 Short Explanation

Insulators possess the highest resistivity. Semiconductors have intermediate resistivity. Metals have the lowest resistivity.

✅ Correct Answer Explanation

Insulators exhibit extremely high resistivity values, typically ranging from 10^(11)to 10^(19)Ω m. Semiconductors have much lower resistivity values. Therefore, it is incorrect to state that insulators have lower resistivity than semiconductors.

❓ Why Other Options Are Incorrect

Option A → Represents metallic low resistivity range.
Option B → Represents semiconductor resistivity range.
Option C → Represents insulator resistivity range.

🎯 Strategy

Used

Extreme Word Filter

Application:

Compare the relative resistivity levels of metals, semiconductors, and insulators.

Final Logic:

Insulators always have much higher resistivity than semiconductors.

🧠 Memory Trick

"Metal < Semiconductor < Insulator."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Resistivity Classification of Materials

11 Conduction mechanisms in metals statements:
A. The conduction band can be partially filled, allowing easy flow.
B. The valence band can be partially empty.
C. Electrons from the valence band easily move into the conduction band if there is overlap.
D. The energy gap strictly must be greater than 3 eV.

A (A), (B), (C)

B (C), (D)

C (A), (D)

D (B), (C), (D)

💡 Short Explanation

Metals have free electrons. Bands may overlap. Large band gaps are absent.

✅ Correct Answer Explanation

In metals, electrical conduction is possible because the conduction band may be partially filled, the valence band may be partially empty, or both bands may overlap. This allows electrons to move easily under an applied electric field. Statement D is incorrect because a band gap greater than 3 eV is characteristic of insulators, not metals.

❓ Why Other Options Are Incorrect

Option B → Includes statement D, which is incorrect.
Option C → Includes statement D, which is incorrect.
Option D → Includes statement D, which is incorrect.

🎯 Strategy

Used

Option Grouping

Application:

Evaluate each statement based on metallic band theory.

Final Logic:

Only A, B, and C correctly describe metallic conduction.

🧠 Memory Trick

"Metal = Overlap + Free Flow."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Metallic Energy Bands

12 In the energy state of insulators under normal thermal excitation

A holes easily diffuse across the barrier unprompted

B the energy gap is sufficiently small for electrons to cross dynamically

C the conduction band is completely filled

D the energy gap is so large that electrons cannot be excited from the valence band to the conduction band

💡 Short Explanation

Insulators possess large band gaps. Thermal energy is insufficient. Conduction band remains empty.

✅ Correct Answer Explanation

Insulators have a forbidden energy gap typically greater than 3 eV. At room temperature, thermal energy is too small to excite valence electrons into the conduction band. As a result, electrical conduction is practically absent.

❓ Why Other Options Are Incorrect

Option A → Hole diffusion does not occur freely due to lack of mobile carriers.
Option B → Small energy gaps are characteristic of semiconductors.
Option C → Conduction band is nearly empty, not filled.

🎯 Strategy

Used

Elimination

Application:

Remove options inconsistent with insulator band theory.

Final Logic:

Large band gaps prevent thermal excitation.

🧠 Memory Trick

"Large Gap = No Jump."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Insulating Materials

13 The fundamental reason Silicon acts as a semiconductor while diamond (Carbon) acts as an insulator is that

A Silicon has 4 valence electrons, whereas Carbon has 6

B The ionization energy required to free an electron is significantly smaller for Si (~1.1 eV) than for C (~5.4 eV)

C Silicon intrinsically possesses a larger energy band gap than Carbon

D Carbon atoms completely lack valence bands

💡 Short Explanation

Silicon has a smaller band gap. Carbon (diamond) has a large band gap. Thermal excitation is easier in silicon.

✅ Correct Answer Explanation

Silicon has an energy band gap of approximately 1.1 eV, whereas diamond has a much larger gap of about 5.4 eV. Because less energy is needed to excite electrons in silicon, it behaves as a semiconductor. Diamond's large gap prevents excitation at room temperature, making it an insulator.

❓ Why Other Options Are Incorrect

Option A → Both Silicon and Carbon have four valence electrons.
Option C → Carbon has the larger band gap.
Option D → Carbon possesses valence bands like other solids.

🎯 Strategy

Used

Elimination

Application:

Compare energy-gap values of Silicon and Carbon.

Final Logic:

Smaller band gap makes Silicon a semiconductor.

🧠 Memory Trick

"Si = 1.1 eV, C = 5.4 eV."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Elemental Semiconductors

14 Correct statements about inorganic semiconductors:
A. They include compounds like GaAs and InP
B. The general concepts of elemental semiconductors apply to most of them
C. They are exclusively comprised of long hydrocarbon chains
D. They consist of materials such as polyaniline

A (A), (B)

B (C), (D)

C (A), (C)

D (B), (D)

💡 Short Explanation

GaAs and InP are inorganic semiconductors. Band theory applies to them. Hydrocarbon chains belong to organic semiconductors.

✅ Correct Answer Explanation

Compound inorganic semiconductors include materials such as GaAs, CdS, InP, and CdSe. The principles governing elemental semiconductors largely apply to them as well. Statements C and D refer to organic and polymer-based semiconductors rather than inorganic compounds.

❓ Why Other Options Are Incorrect

Option B → Both statements are incorrect.
Option C → Statement C is incorrect.
Option D → Statement D is incorrect.

🎯 Strategy

Used

Option Grouping

Application:

Classify examples into inorganic and organic categories.

Final Logic:

Only A and B correctly describe inorganic semiconductors.

🧠 Memory Trick

"GaAs & InP = Inorganic Pair."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Compound Semiconductors

15 Match List I (Material) with List II (Categorization)

List I	List II
I. Doped phthalocyanines	P. Inorganic compound
II. Germanium	Q. Organic semiconductor
III. GaAs	R. Organic polymer
IV. Polythiophene	S. Elemental semiconductor

A I–Q, II–S, III–P, IV–R

B I–P, II–Q, III–R, IV–S

C I–R, II–P, III–S, IV–Q

D I–S, II–R, III–Q, IV–P

💡 Short Explanation

Doped phthalocyanines are organic semiconductors. Germanium is an elemental semiconductor. GaAs (Gallium Arsenide) is an inorganic compound semiconductor. Polythiophene is an organic polymer.

✅ Correct Answer Explanation

Semiconducting materials can be classified as elemental, compound, organic, or polymer-based. Doped phthalocyanines belong to the category of organic semiconductors, giving I–Q. Germanium is a pure element and is therefore an elemental semiconductor, giving II–S. Gallium Arsenide (GaAs) consists of gallium and arsenic and is an inorganic compound semiconductor, corresponding to III–P. Polythiophene is a conducting organic polymer widely used in organic electronics, corresponding to IV–R. Thus, the correct matching is: I–Q, II–S, III–P, IV–R

❓ Why Other Options Are Incorrect

Option B → I–P, II–Q, III–R, IV–S

Organic semiconductors, elemental semiconductors, and polymers are incorrectly interchanged.

Option C → I–R, II–P, III–S, IV–Q

Doped phthalocyanines are not polymers, and GaAs is not an elemental semiconductor.

Option D → I–S, II–R, III–Q, IV–P

Germanium is not a polymer, and GaAs is not an organic semiconductor.

🎯 Strategy

Used

Classification-Based Matching

Application:

Classify each material as:
Organic semiconductor
Elemental semiconductor
Compound semiconductor
Organic polymer

Final Logic:

Doped phthalocyanines → Organic semiconductor
Germanium → Elemental semiconductor
GaAs → Inorganic compound
Polythiophene → Organic polymer

🧠 Memory Trick

"Ge = Element, GaAs = Compound, Polythiophene = Polymer, Phthalocyanine = Organic."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Types of Semiconductor Materials

16 Incorrect statement about polymeric electronics:

A It relies on organic polymers like polypyrrole

B It signals the birth of a futuristic technology of molecular-electronics

C Polymeric materials behave strictly as inorganic elemental metals

D Polyaniline is an example of a semiconducting polymer

💡 Short Explanation

Polymeric electronics uses organic polymers. It supports molecular electronics. Polymers are not inorganic metals.

✅ Correct Answer Explanation

Polymeric electronics is based on organic conducting polymers such as polypyrrole, polyaniline, and polythiophene. These materials are associated with future molecular-electronics technologies. Statement C is incorrect because polymeric materials are organic and do not behave as inorganic elemental metals.

❓ Why Other Options Are Incorrect

Option A → Correct description.
Option B → Correct technological significance.
Option D → Correct example.

🎯 Strategy

Used

Odd One Out

Application:

Identify the statement inconsistent with polymeric electronics.

Final Logic:

Organic polymers are not inorganic metals.

🧠 Memory Trick

"Polymer = Organic Future."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Polymeric Electronics

17 Energy level properties (Isolated atom vs Atom in solid):

A Discrete, Continuous

B Continuous, Discrete

C Continuous, Continuous

D Discrete, Discrete

💡 Short Explanation

Isolated atoms have discrete levels. Solids form energy bands. Energy becomes nearly continuous.

✅ Correct Answer Explanation

For an isolated atom, electron energies are quantized into discrete levels. In solids, interactions between a large number of atoms split these levels into numerous closely spaced states that form continuous energy bands.

❓ Why Other Options Are Incorrect

Option B → Reverses the actual relationship.
Option C → Isolated atoms do not have continuous bands.
Option D → Solids exhibit energy bands, not discrete levels.

🎯 Strategy

Used

Contextual/Tonal Matching

Application:

Compare isolated atomic structure with solid-state structure.

Final Logic:

Atoms → Discrete Levels; Solids → Continuous Bands.

🧠 Memory Trick

"Atom = Levels, Solid = Bands."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Energy Band Formation

18 Statements regarding electron orbit overlap in solids:
A. Occurs because atoms come very close to each other
B. Makes the nature of electron motion identical to that in an isolated atom
C. Leads to continuous energy variation forming energy bands
D. Allows no two electrons to see exactly the same pattern of surrounding charges

A (A), (C), (D)

B (A), (B), (C)

C (B), (D)

D (C), (D)

💡 Short Explanation

Neighboring atoms overlap. Energy bands are formed. Electron environments become unique.

✅ Correct Answer Explanation

In solids, atoms are packed closely together, causing outer electron orbits to overlap. This overlap produces a continuous range of energy states known as energy bands. Since each electron experiences a slightly different surrounding charge environment, exact energy levels differ slightly. Statement B is incorrect because electron behavior in solids differs from isolated atoms.

❓ Why Other Options Are Incorrect

Option B → Includes incorrect statement B.
Option C → Omits correct statements A and C.
Option D → Omits correct statement A.

🎯 Strategy

Used

Option Grouping

Application:

Evaluate the physical consequences of orbit overlap.

Final Logic:

A, C, and D correctly describe band formation.

🧠 Memory Trick

"Overlap → Bands."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Energy Band Theory

19 If a Ge crystal has N atoms, the maximum possible number of electrons in the outer orbit states will be:

A 4N

B 8N

C 2N

D N

💡 Short Explanation

Ge has four valence electrons. Outer orbit states split into 8N energy states. Maximum occupancy equals 8N electrons.

✅ Correct Answer Explanation

In a crystal containing N germanium atoms, the outermost energy levels split into a large number of closely spaced states. Since each energy state can accommodate two electrons and there are 4N valence electrons, the total available outer-orbit states become 8N. Therefore, the maximum possible occupancy is 8N electrons.

❓ Why Other Options Are Incorrect

Option A → Represents only the number of valence electrons.
Option C → Too small compared with available states.
Option D → Does not correspond to band theory.

🎯 Strategy

Used

Substitution

Application:

Apply the NCERT relation for N atoms and available outer-orbit states.

Final Logic:

N atoms produce 8N available outer energy states.

🧠 Memory Trick

"N Atoms → 8N States."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Energy Band Formation in Crystals

20 If Ge has a lattice spacing of 5.66 Å, and C (diamond) has 3.56 Å, what is the spatial difference between their spacings in meters?

A 2.10 × 10⁻¹⁰ m

B 2.10 × 10⁻⁸ m

C 2.10 × 10⁻¹² m

D 2.10 × 10⁻⁶ m

💡 Short Explanation

Difference = 5.66 − 3.56 Å Difference = 2.10 Å 1 Å = 10⁻¹⁰ m

✅ Correct Answer Explanation

Δa=5.66-3.56=2.10 A Since 1 A=10^(-10) m Therefore, 2.10 A=2.10×10^(-10) m Hence Option A is correct.

❓ Why Other Options Are Incorrect

Option B → Incorrect exponent.
Option C → Two powers of ten too small.
Option D → Micrometer-scale value, not atomic-scale.

🎯 Strategy

Used

Dimensional/Unit Analysis

Application:

Convert angstroms directly into meters after subtraction.

Final Logic:

2.10 Å equals 2.10 × 10⁻¹⁰ m.

🧠 Memory Trick

"Å Means Minus Ten."

📚 Class XII – Physics Part II – Chapter: Semiconductor Electronics: Materials, Devices and Simple Circuits – Topic: Crystal Lattice Spacing

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