Semiconductor Physics: Medium-Level Mastery Quiz
Semiconductors sit at the heart of modern electronics, bridging the gap between conductors and insulators. Their behaviour is governed by energy bands, charge carriers, and the controlled introduction of impurities through doping.
This quiz tests your understanding of:
- Intrinsic versus extrinsic semiconductors
- P-type and N-type doping and majority carriers
- Energy band gaps and Fermi levels
- PN junction behaviour, depletion regions, and biasing
- Diode characteristics and rectification
Work carefully through each question. Use the explanations to reinforce the underlying physics, and remember that medium difficulty means applying concepts, not just recalling definitions.
Q1. In an intrinsic semiconductor at room temperature, the number of electrons in the conduction band is:
In an intrinsic (pure) semiconductor, thermal excitation creates electron-hole pairs, so the electron concentration equals the hole concentration: ##n = p = n_i##.
Q2. The energy band gap of a semiconductor is typically in the range:
Semiconductors have band gaps of roughly ##1\ \text{eV}## to ##3\ \text{eV}## (e.g., ##Si \approx 1.1## eV, ##Ge \approx 0.67## eV). Conductors have negligible gaps and insulators have gaps above about 5 eV.
Q3. When a pentavalent impurity such as phosphorus is added to silicon, the resulting material is:
Pentavalent (donor) impurities contribute a free electron, making electrons the majority carriers and producing an N-type semiconductor.
Q4. In a P-type semiconductor, the majority charge carriers are:
Trivalent (acceptor) impurities create holes, so holes are the majority carriers in P-type material, while electrons are the minority carriers.
Q5. The Fermi level in an intrinsic semiconductor lies:
For an intrinsic semiconductor, the Fermi level lies near the centre of the forbidden gap, slightly shifted by the effective masses of electrons and holes.
Q6. As temperature increases in an intrinsic semiconductor, its conductivity:
Rising temperature generates more electron-hole pairs, increasing carrier concentration and hence conductivity, even though mobility slightly decreases.
Q7. The depletion region of a PN junction is characterised by:
Diffusion of carriers across the junction leaves behind fixed ionised dopants, creating a region depleted of mobile carriers and a built-in electric field.
Q8. When a PN junction is forward biased, the width of the depletion region:
Forward bias reduces the potential barrier, allowing the depletion region to narrow and enabling a larger diffusion current to flow.
Q9. The barrier potential of a silicon PN junction is approximately:
A silicon diode has a forward barrier potential of about ##0.7\ \text{V}##, while a germanium diode is about ##0.3\ \text{V}##.
Q10. In reverse bias, the current through an ideal PN junction diode is:
Reverse bias widens the depletion region and blocks majority carrier flow; only a small drift current from minority carriers flows.
Q11. A half-wave rectifier using a single diode converts:
A single diode conducts during one half-cycle, producing a pulsating DC output from an AC input.
Q12. The relation between electron concentration ##n##, hole concentration ##p##, and intrinsic concentration ##n_i## in a semiconductor at equilibrium is:
The mass-action law states that at thermal equilibrium the product of electron and hole concentrations equals the square of the intrinsic concentration: ##np = n_i^2##.
Q13. Which of the following is a direct band gap semiconductor commonly used in LEDs?
Gallium arsenide (GaAs) has a direct band gap, allowing efficient radiative recombination, which makes it suitable for LEDs and lasers.
Q14. The mobility of charge carriers in a semiconductor is defined as:
Mobility ##\mu = v_d / E## is the drift velocity acquired per unit applied electric field, with units of ##\text{m}^2/(\text{V}\cdot\text{s})##.
Q15. In an N-type semiconductor, the Fermi level lies:
Donor doping raises the Fermi level toward the conduction band, since electrons are the majority carriers.
Q16. The current in a PN junction diode under forward bias is mainly due to:
Forward bias lowers the barrier, so majority carriers diffuse across the junction, producing the dominant diffusion current.
Q17. A Zener diode is designed to operate reliably in:
Zener diodes are heavily doped to operate safely in reverse breakdown, providing a stable reference voltage.
Q18. If the temperature of a doped semiconductor increases significantly, the Fermi level:
At high temperatures, intrinsic carrier generation dominates, and the Fermi level shifts toward the middle of the band gap (the intrinsic level).
Q19. The conductivity of a semiconductor is given by:
Conductivity sums the contributions of both carrier types: ##\sigma = e(n\mu_e + p\mu_h)##, where ##\mu_e## and ##\mu_h## are electron and hole mobilities.
Q20. In a PN junction, the direction of the built-in electric field in the depletion region is:
The built-in field points from the positively charged donor ions (N-side) toward the negatively charged acceptor ions (P-side), opposing further diffusion.