Semiconductors flash cards

1. 1.On the basis of conductivity, into which three categories are solids classified?

   Conductors ($\rho \approx 10^{-2}$ to $10^{-8}\,\Omega\,\text{m}$), semiconductors (intermediate), and insulators ($\rho \approx 10^{11}$ to $10^{19}\,\Omega\,\text{m}$).

   Hint: Think in terms of resistivity ranges.

2. 2.What is the origin of energy bands in solids?

   When many atoms come close in a crystal, the discrete energy levels of individual atoms split and merge into closely spaced levels forming continuous energy bands.

   Hint: Interaction of a large number of atoms.

3. 3.Define the valence band and the conduction band.

   The valence band is the highest energy band completely or partly filled with valence electrons. The conduction band is the next higher band where electrons are free to conduct.

   Hint: One holds bound electrons, the other free ones.

4. 4.What is the energy gap (band gap) $E_g$?

   The gap between the top of the valence band and the bottom of the conduction band; no allowed electron energy states exist within it.

   Hint: A forbidden region of energy.

5. 5.How does band structure distinguish a conductor?

   In a conductor the conduction and valence bands overlap (or the conduction band is partially filled), so $E_g \approx 0$ and electrons move freely.

   Hint: Bands overlap — no gap.

6. 6.How does band structure describe an insulator?

   An insulator has a large energy gap ($E_g > 3\,\text{eV}$), so electrons cannot be excited from the valence to the conduction band at ordinary temperatures.

   Hint: Big forbidden gap.

7. 7.What is the band-gap condition for a semiconductor?

   A small energy gap ($E_g < 3\,\text{eV}$, e.g. Si $\approx 1.1\,\text{eV}$, Ge $\approx 0.7\,\text{eV}$) so some electrons can jump to the conduction band by thermal energy.

   Hint: Small gap — thermally bridgeable.

8. 8.What are the two most common elemental semiconductors?

   Silicon ($\text{Si}$) and Germanium ($\text{Ge}$), both tetravalent group-14 elements.

   Hint: Group 14, four valence electrons.

9. 9.Give examples of compound (non-elemental) semiconductors.

   Inorganic: $\text{GaAs}$, $\text{CdS}$, $\text{InP}$; organic: anthracene, doped phthalocyanines.

   Hint: III-V and II-VI compounds.

10. 10.What is an intrinsic semiconductor?

    A pure semiconductor with no added impurity, where the number of free electrons equals the number of holes ($n_e = n_h = n_i$).

    Hint: Pure crystal, equal carriers.

11. 11.What is a hole in a semiconductor?

    A vacancy created in the valence band when an electron leaves a covalent bond; it behaves as a positive charge carrier.

    Hint: Absence of an electron.

12. 12.State the mass-action law relating carrier concentrations.

    $n_e\,n_h = n_i^2$, where $n_i$ is the intrinsic carrier concentration; it holds for both intrinsic and extrinsic semiconductors.

    Hint: Product of electron and hole densities is constant.

13. 13.How does the conductivity of an intrinsic semiconductor change with temperature?

    It increases with temperature because more electron-hole pairs are thermally generated (semiconductors have a negative temperature coefficient of resistance).

    Hint: Opposite behaviour to metals.

14. 14.At absolute zero, how does an intrinsic semiconductor behave?

    Its conduction band is empty and valence band full, so it behaves as a perfect insulator.

    Hint: No thermal energy to excite electrons.

15. 15.What is doping?

    The deliberate addition of a small, controlled amount of impurity atoms to a pure semiconductor to increase its conductivity.

    Hint: Adding impurity to change carrier count.

16. 16.What is an extrinsic semiconductor?

    A doped semiconductor whose conductivity is controlled by impurity atoms; carriers are no longer equal in number.

    Hint: Doped — impure by design.

17. 17.What type of impurity produces an n-type semiconductor?

    A pentavalent (donor) impurity such as arsenic, antimony, or phosphorus, which donates an extra free electron.

    Hint: Five valence electrons — one spare.

18. 18.In an n-type semiconductor, name the majority and minority carriers.

    Majority carriers are electrons; minority carriers are holes ($n_e \gg n_h$).

    Hint: Donors give electrons.

19. 19.What type of impurity produces a p-type semiconductor?

    A trivalent (acceptor) impurity such as boron, aluminium, or indium, which creates a hole by accepting an electron.

    Hint: Three valence electrons — one short.

20. 20.In a p-type semiconductor, name the majority and minority carriers.

    Majority carriers are holes; minority carriers are electrons ($n_h \gg n_e$).

    Hint: Acceptors create holes.

21. 21.Is an n-type or p-type semiconductor electrically charged?

    Neither — both remain electrically neutral overall, since dopant atoms are neutral; doping only changes the relative number of carriers.

    Hint: Neutral atoms added.

22. 22.Where do donor and acceptor energy levels lie in the band diagram?

    Donor levels lie just below the conduction band; acceptor levels lie just above the valence band, so carriers are easily released thermally.

    Hint: Close to the nearest band.

23. 23.How is a p-n junction formed?

    By diffusing a suitable dopant into a semiconductor wafer so that one region becomes p-type and the adjoining region becomes n-type within a single crystal.

    Hint: p and n regions in one crystal.

24. 24.What two processes occur at a p-n junction during its formation?

    Diffusion (holes and electrons cross due to concentration gradient) and drift (carriers move back due to the electric field); equilibrium is reached when they balance.

    Hint: Concentration gradient vs. field.

25. 25.What is the depletion region (depletion layer)?

    A narrow region around the junction depleted of free charge carriers, containing only immobile ionised donor and acceptor atoms.

    Hint: No mobile carriers left there.

26. 26.What is the barrier potential (potential barrier)?

    The potential difference developed across the depletion layer that opposes further diffusion of majority carriers (about $0.3\,\text{V}$ for Ge, $0.7\,\text{V}$ for Si).

    Hint: Built-in potential across the junction.

27. 27.In which direction does the junction field point in the depletion region?

    From the n-side (positive ions) to the p-side (negative ions), opposing the diffusion of majority carriers.

    Hint: From positive to negative immobile ions.

28. 28.What is forward biasing of a p-n junction?

    Connecting the p-side to the positive terminal and n-side to the negative terminal of the battery, which reduces the barrier and depletion width.

    Hint: p to +, n to −.

29. 29.What is reverse biasing of a p-n junction?

    Connecting the p-side to the negative terminal and n-side to the positive terminal, which increases the barrier height and depletion width.

    Hint: p to −, n to +.

30. 30.What happens to the depletion width and barrier under forward bias?

    Both decrease; the applied voltage opposes the built-in field, allowing majority carriers to cross the junction.

    Hint: Barrier lowered.

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