Semiconductor Materials: Properties, Carriers & Doping: 30 flashcards
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1. What is a semiconductor?
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A material whose electrical conductivity lies between that of a conductor and an insulator, roughly $10^{-6}$ to $10^3$ S/cm.
2. Name the three factors that can tune semiconductor conductivity.
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Doping, temperature change, and illumination.
3. Which Group IV elements are the most common elemental semiconductors?
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Silicon (Si), Germanium (Ge), and Tin (Sn).
4. What is silicon's electronic configuration?
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$1s^2\,2s^2\,2p^6\,3s^2\,3p^2$
5. How many valence electrons does a silicon atom have?
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Four valence electrons.
6. How many covalent bonds does each silicon atom form in a crystal?
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Four covalent bonds, completing an octet with neighbouring atoms.
7. Why do semiconductors behave as insulators at absolute zero?
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All valence electrons are locked in covalent bonds, so no free carriers exist to conduct current.
8. What is a hole in a semiconductor?
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The absence of an electron at a bond site; it behaves as if it has positive charge.
9. Is a hole a real positively charged particle?
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No. A hole is not a real particle and carries no actual positive charge; it only behaves like a positive carrier.
10. In which direction does a hole appear to move relative to the electron that filled it?
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Opposite in direction to the bound electron motion that creates it.
11. What two carriers contribute to current in an intrinsic semiconductor?
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Free electrons and holes, in equal numbers.
12. What is the typical band gap range for semiconductors?
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$0.1$ to $3$ eV.
13. What is the typical resistivity range of semiconductors?
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$10^{-4}$ to $10^8\ \Omega\cdot\text{cm}$.
14. What is the temperature coefficient of resistivity for semiconductors?
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Negative — resistivity decreases as temperature rises, opposite to metals.
15. Do semiconductors obey Ohm's law?
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No. Semiconductors are nonlinear conductors and do not obey Ohm's law.
16. In which crystal structures do most semiconductors crystallize?
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Zinc Blende or Wurtzite structure, with covalent bonding.
17. What is doping?
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Adding impurities (about one atom in $10^7$ host atoms) to boost conductivity.
18. What is an extrinsic semiconductor?
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A doped semiconductor in which electron and hole concentrations are unequal.
19. Which dopants act as donors, and what type of semiconductor do they produce?
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Pentavalent dopants (As, Sb, Bi, P) act as donors and produce n-type semiconductors.
20. In an n-type semiconductor, which carriers are majority and which are minority?
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Electrons are majority carriers; holes are minority carriers.
21. Where does the donor level $E_d$ lie in the band diagram?
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Just below the conduction band $E_c$.
22. Which dopants act as acceptors, and what type of semiconductor do they produce?
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Trivalent dopants (In, B, Al, Ga) act as acceptors and produce p-type semiconductors.
23. In a p-type semiconductor, which carriers are majority and which are minority?
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Holes are majority carriers; electrons are minority carriers.
24. Where does the acceptor level $E_a$ lie in the band diagram?
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Just above the valence band $E_v$.
25. What are III-V semiconductors?
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Compounds combining one Group III element with one Group V element.
26. What are II-VI semiconductors?
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Compounds combining one Group II element with one Group VI element.
27. Does doping change the overall charge of a semiconductor crystal?
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No. Doping changes the dominant carrier type but leaves the crystal electrically neutral overall.
28. What happens to semiconductor conductivity when temperature rises?
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Conductivity increases because more covalent bonds break, freeing more carriers.
29. What happens to semiconductor conductivity under illumination?
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Conductivity increases when light of suitable wavelength is absorbed.
30. Why are intrinsic semiconductors not enough for practical devices?
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Pure silicon has too few free carriers, so its conductivity is too low and not controllable.
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