Level 1 — RecognitionMaterials & Atomic Structure

Materials & Atomic Structure

⏱ 20 minutes40 marksprintable — key stays hidden on paper

Level 1 Examination: Recognition

Time limit: 20 minutes Total marks: 40

Section A — Multiple Choice (1 mark each)

Select the single best answer.

Q1. According to the Bohr model, the maximum number of electrons in the third shell (n = 3) is:

(a) 2 (b) 8 (c) 18 (d) 32

Q2. How many valence electrons does a silicon atom have?

(a) 2 (b) 4 (c) 6 (d) 8

Q3. In a pure silicon crystal, each atom forms covalent bonds with how many neighbouring atoms?

(a) 2 (b) 3 (c) 4 (d) 6

Q4. An intrinsic semiconductor is one that is:

(a) heavily doped with phosphorus
(b) pure, with no added impurities
(c) doped with boron
(d) a metal alloy

Q5. When a covalent bond in silicon breaks due to thermal energy, it creates:

(a) a free electron only
(b) a hole only
(c) an electron–hole pair
(d) a positive ion only

Q6. As temperature increases, the electrical conductivity of an intrinsic semiconductor:

(a) decreases
(b) stays constant
(c) increases
(d) drops to zero

Q7. Carrier mobility is best described as:

(a) the number of carriers per unit volume
(b) the drift velocity of a carrier per unit electric field
(c) the charge on a carrier
(d) the energy gap of the material

Q8. The maximum number of electrons a single atomic shell can hold is given by:

(a) $2n$ (b) $n^2$ (c) $2n^2$ (d) $n^3$

Q9. Which semiconductor is preferred for high-temperature, high-power, and high-frequency applications such as LEDs and RF devices?

(a) Germanium (b) Compound semiconductors like GaN/GaAs (c) Copper (d) Carbon

Q10. The bandgap of silicon at room temperature is approximately:

(a) 0.67 eV (b) 1.12 eV (c) 3.4 eV (d) 5.5 eV

Section B — Matching (1 mark each, 6 marks)

Q11. Match each material/term in Column X to its correct property in Column Y.

Column X	Column Y
(i) Germanium	(A) Wide bandgap, used in blue LEDs
(ii) GaAs	(B) Small bandgap (~0.67 eV), leaky at high temp
(iii) GaN	(C) High electron mobility, used in RF/microwave
(iv) Silicon	(D) Abundant, forms stable native oxide (SiO₂)
(v) SiC	(E) High thermal conductivity, high-power devices
(vi) Hole	(F) Absence of an electron, behaves as +charge

Section C — True/False with Justification (3 marks each: 1 for T/F, 2 for justification)

Q12. "Silicon dominates over germanium mainly because silicon is cheaper and forms a stable insulating oxide layer." — True or False? Justify.

Q13. "In an extrinsic n-type semiconductor, holes are the majority carriers." — True or False? Justify.

Q14. "Doping intrinsic silicon with a Group V element (e.g., phosphorus) increases the number of free electrons." — True or False? Justify.

Q15. "At absolute zero temperature (0 K), pure silicon behaves like a perfect insulator." — True or False? Justify.

Q16. "Carrier mobility is independent of temperature in a semiconductor." — True or False? Justify.

END OF PAPER

Answer keyMark scheme & solutions

Section A — MCQ (1 mark each)

Q1 — (c) 18. Shell capacity is $2n^2$ ; for $n=3$ , $2(3)^2 = 18$ . (1 mark)

Q2 — (b) 4. Silicon is in Group IV, so it has 4 valence electrons. (1 mark)

Q3 — (c) 4. Each Si atom shares its 4 valence electrons with 4 neighbours to complete an octet. (1 mark)

Q4 — (b) pure, with no added impurities. Intrinsic = pure material; conductivity due only to thermally generated pairs. (1 mark)

Q5 — (c) an electron–hole pair. A freed electron leaves behind a vacancy (hole); they are always created together. (1 mark)

Q6 — (c) increases. Higher temperature breaks more bonds → more carriers → higher conductivity (opposite to metals). (1 mark)

Q7 — (b) drift velocity per unit electric field. $\mu = v_d / E$ , units m²/(V·s). (1 mark)

Q8 — (c) $2n^2$ . Standard shell-filling rule. (1 mark)

Q9 — (b) Compound semiconductors like GaN/GaAs. Wide/direct bandgaps suit high-power, high-freq, optoelectronic use. (1 mark)

Q10 — (b) 1.12 eV. Standard room-temperature bandgap of silicon. (1 mark)

Section B — Matching (1 mark each)

Q11:

(i) Germanium → B (small bandgap ~0.67 eV, leaky at high temp)
(ii) GaAs → C (high electron mobility, RF/microwave)
(iii) GaN → A (wide bandgap, blue LEDs)
(iv) Silicon → D (abundant, stable SiO₂)
(v) SiC → E (high thermal conductivity, high-power)
(vi) Hole → F (absence of electron, +charge behaviour)

(6 marks total, 1 each)

Section C — True/False with Justification

Q12 — TRUE. (T/F = 1) Justification: Silicon is more abundant/cheaper than germanium, and crucially forms SiO₂, a stable, high-quality native oxide used as an insulator/mask in fabrication — germanium's oxide is water-soluble and unstable. (2 marks)

Q13 — FALSE. (T/F = 1) Justification: In n-type material, donor doping supplies extra electrons, so electrons are the majority carriers and holes are the minority. (2 marks)

Q14 — TRUE. (T/F = 1) Justification: A Group V atom has 5 valence electrons; 4 bond with silicon and the 5th is loosely bound and easily freed, adding conduction electrons (donor). (2 marks)

Q15 — TRUE. (T/F = 1) Justification: At 0 K there is no thermal energy to break covalent bonds, so no free carriers exist → no conduction → behaves as an insulator. (2 marks)

Q16 — FALSE. (T/F = 1) Justification: Mobility decreases with rising temperature because increased lattice vibrations (phonon scattering) impede carrier drift. (2 marks)

[
  {"claim": "Third shell capacity 2n^2 = 18 for n=3", "code": "n=3; result = (2*n**2 == 18)"},
  {"claim": "Silicon Group IV has 4 valence electrons matching 4 covalent neighbours", "code": "valence=4; neighbours=4; result = (valence == neighbours == 4)"},
  {"claim": "Second shell (n=2) capacity is 8 electrons", "code": "n=2; result = (2*n**2 == 8)"},
  {"claim": "Silicon bandgap 1.12 eV exceeds germanium 0.67 eV", "code": "si=1.12; ge=0.67; result = (si > ge)"}
]