Semiconductor Physics: Medium-Level Quiz
Semiconductors form the backbone of modern electronics, from diodes and transistors to solar cells and integrated circuits. This quiz tests your understanding of energy bands, charge carriers, doping, p-n junctions, and device behaviour at a medium difficulty level.
- 20 carefully designed multiple-choice questions
- Concept application and multi-step reasoning
- Clear explanations for every answer
Review the fundamentals of intrinsic and extrinsic semiconductors, drift and diffusion, and junction physics before you begin. Good luck!
Q1. In an intrinsic semiconductor at room temperature, the number of electrons in the conduction band is:
In an intrinsic (undoped) semiconductor, every electron excited to the conduction band leaves behind one hole, so ##n = p##.
Q2. The energy gap of a semiconductor is typically in the range:
Semiconductors have a small forbidden gap, roughly ##0.1## to ##3\ \text{eV}##, which allows thermal excitation of carriers at ordinary temperatures.
Q3. Adding a pentavalent impurity such as phosphorus to silicon produces:
Pentavalent donors contribute extra electrons, making the material n-type with electrons as majority carriers.
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.
Q5. The Fermi level in an n-type semiconductor at thermal equilibrium lies:
Donor doping shifts the Fermi level upward, near the conduction band edge.
Q6. At the junction of a p-n diode, the region depleted of mobile carriers is called the:
The diffusion of carriers leaves behind fixed ionized dopants, forming the depletion (space-charge) region.
Q7. The built-in potential barrier of a silicon p-n junction is approximately:
For silicon diodes the barrier is about ##0.7\ \text{V}##; germanium is roughly ##0.3\ \text{V}##.
Q8. A p-n junction diode under forward bias:
Forward bias lowers the potential barrier and narrows the depletion region, allowing current flow.
Q9. The current in a reverse-biased p-n junction is mainly due to:
Reverse bias current is a small drift current of thermally generated minority carriers.
Q10. The relation between the current gain ##\alpha## and ##\beta## of a transistor is:
Since ##\alpha = \dfrac{\beta}{1+\beta}##, rearranging gives ##\beta = \dfrac{\alpha}{1-\alpha}##.
Q11. In a common-emitter transistor amplifier, the phase difference between input and output voltage is:
The common-emitter configuration inverts the signal, giving a ##180^\circ## phase shift.
Q12. The conductivity of a semiconductor increases with temperature because:
Thermal energy excites more carriers across the gap, and this dominates over the decrease in mobility.
Q13. A Zener diode is normally operated in:
Zener diodes are designed to work in reverse breakdown, where the voltage stays nearly constant.
Q14. The drift velocity of charge carriers in a semiconductor is directly proportional to:
Drift velocity ##v_d = \mu E##, so it is proportional to the electric field ##E##.
Q15. The diffusion current in a semiconductor is caused by:
Diffusion current flows due to a gradient in carrier concentration, from high to low density.
Q16. A full-wave rectifier using a centre-tapped transformer produces output ripple frequency equal to:
Both half-cycles are rectified, so the ripple frequency is ##2f##.
Q17. The forbidden energy gap of an insulator compared with a semiconductor is:
Insulators have a large band gap (typically several eV), preventing easy carrier excitation.
Q18. In an unbiased p-n junction, the net current is zero because:
At equilibrium, the drift and diffusion currents of each carrier type cancel exactly.
Q19. The mobility of electrons in silicon compared with that of holes is generally:
Electrons have higher mobility than holes in silicon, which is why n-channel devices are often faster.
Q20. A light-emitting diode emits light when:
Recombination of electrons and holes releases energy as photons, producing light.