Grade 10 electricity – Semiconductor Theory Quiz
1. What is a semiconductor?
Semiconductors (like silicon) have conductivity between conductors and insulators and can change conductivity with temperature, doping or light.
2. What is the purpose of doping a semiconductor?
Doping adds small amounts of impurities (donors or acceptors) that increase free charge carriers and so change conductivity.
3. Which element is commonly used as the base material for making semiconductor devices in Kenya?
Silicon is the main base material for most semiconductors and solar cells due to its suitable band gap and abundance.
4. What is an n-type semiconductor?
N-type semiconductors are doped with donor atoms (e.g., phosphorus) that supply extra electrons as majority carriers.
5. In a p-type semiconductor, which are the majority charge carriers?
P-type semiconductors are doped with acceptor atoms (e.g., boron) which create holes that act as the majority carriers.
6. What is meant by an intrinsic semiconductor?
Intrinsic semiconductors are undoped and have equal numbers of electrons and holes generated thermally.
7. What happens to the conductivity of a semiconductor when temperature increases?
Higher temperature provides energy for more electrons to move from the valence band to the conduction band, increasing conductivity.
8. What are the conduction band and valence band in a semiconductor?
The conduction band is where free electrons can conduct electricity; the valence band contains electrons bound to atoms; the band gap separates them.
9. What is the band gap of a semiconductor?
The band gap is the energy needed for an electron to move from the valence to the conduction band; it determines the semiconductor's electrical and optical properties.
10. What creates the depletion region in a p-n junction?
When electrons and holes diffuse across the p-n boundary they recombine, leaving immobile charged donor and acceptor ions that form the depletion region.
11. What is the direction of the built-in electric field in a p-n junction depletion region?
Positive donor ions on the n-side and negative acceptor ions on the p-side create an electric field pointing from n to p that opposes further diffusion.
12. How does a diode behave when forward biased (p-side positive relative to n-side)?
Forward bias reduces the depletion barrier, allowing charge carriers to cross the junction and current to flow once the barrier (≈0.7 V for silicon) is exceeded.
13. What happens to a diode under reverse bias (n-side positive relative to p-side)?
Under reverse bias the depletion region widens and only a tiny leakage current flows; large reverse voltage can cause breakdown and large current.
14. Approximately what is the forward voltage drop of a silicon diode when conducting?
A silicon diode typically requires about 0.6–0.8 V across it in forward bias for significant current; 0.7 V is a commonly used approximation.
15. What is the main function of a light-emitting diode (LED)?
An LED emits photons when electrons recombine with holes across the band gap; they are widely used for lighting and indicators.
16. What is a photodiode used for?
A photodiode generates current when light falls on it by creating electron-hole pairs; used in light sensors and solar cells.
17. Which statement describes majority and minority carriers in an n-type semiconductor?
In n-type material doping adds extra electrons, making electrons the majority and holes the minority carriers present in much smaller numbers.
18. What is the role of a transistor in electronic circuits commonly taught at age 15?
Transistors control current flow between terminals and can amplify weak signals or switch currents on and off in circuits.
19. In a typical NPN transistor, which junctions are forward or reverse biased for normal active operation?
Active mode requires the emitter-base junction forward biased to inject carriers and the base-collector junction reverse biased to collect them for amplification.
20. Why does increasing doping concentration increase conductivity in a semiconductor?
Dopant atoms donate or accept electrons, increasing the number of free carriers (electrons or holes) and so raising conductivity.
21. Which device uses a p-n junction to allow current to flow mainly in one direction and is used in phone chargers and power supplies in Kenya?
A diode's p-n junction conducts in forward bias and blocks in reverse bias, making it ideal for rectifying AC to DC in chargers and supplies.
22. What is Zener breakdown used for in electronics?
Zener diodes are designed to break down at a specific reverse voltage and maintain that voltage, providing a simple voltage regulator.
23. How does light affect the conductivity of a photoconductive semiconductor?
Photons with enough energy create electron-hole pairs, increasing charge carriers and therefore conductivity; this is used in light sensors.
24. Which of the following best describes diffusion current in semiconductors?
Diffusion current results from concentration gradients of carriers; for example electrons move from high to low electron concentration regions.