Title ELECTRONICS.pdf ✅

2020 TEACHERS OF PHYSICS www.teachersofphysics.com 7/17/2020 ELECTRONICS
www.teachersofphysics.com Copyright © 2020 1. Distinguish between semiconductor and conductors (2mks)  Semiconductors are materials whose electrical conductivity lies between that of good conductors and insulators.  Conductors are those materials which allow current to flow through them easily/materials with free electrons. 2. Give one example of a semiconductor and one for a conductor (2mks)  Semiconductors-silicon,germanium  Conductors-copper,aluminium 3. State difference between semiconductors and metallic conductors. (1mk)  The electrical conductivity of semiconductors lie between that of conductors and insulators while metallic conductors allow current to flow through them easily since they have mobile electrons. 4. State the effect on conductivity of increasing the temperature of a semi-conductor  Increase in temperature increase the electrical conductivity of semiconductors. When the temperature is raised the electrons gain more energy and are able to move from the valence band across the forbidden gap to the conduction band 5. Name two ways of enhancing the conductivity of a semiconductor (2mks)  Increasing the temperature  Doping 6. Explain the effects of an increase in temperature in the conductivities of: a) A metallic conductor (2mks)  Increase in temperature increase the vibration of the atoms this interferes with the electron flow due to resistance which result from increased collisions. Hence the conductivity is decreased. b) A semi-conductor (2mks)  Increase in temperature increases the conductivity of semiconductors. Increasing the temperature increases the chances of electrons moving from the valence band to the conduction band. Electrical resistance therefore reduces as the current flow is due to the flow of electrons and holes. DOPING 7. Explain what you understand by the term doping as used in semiconductors(2mk)  Doping is the process of adding an impurity to a pure semiconductor to improve its electrical conductivity 8. State how the conductivity of an intrinsic semi-conductor can be improved.(1mk)  By increasing the temperature 9. Explain the meaning of ‘donor’ and ‘acceptor’ atoms in relation to semi Conductors.  Donor-This is a group five element having five electrons in there outermost energy levels. Four of the electrons participate in bonding and donates the remaining for electrical conductivity. When Silicon is doped using Boron its three electrons on the outermost energy level participate in bonding with the neighboring atoms while silicon will have an extra electron. This creates a missing electron which is treated as a hole. Such an impurity is called an acceptor impurity since it creates a hole which can accept an electron.  Acceptor-This is a group three element like Boron
www.teachersofphysics.com Copyright © 2020 10. Distinguish between intrinsic and extrinsic conductors (1mk)  Intrinsic semiconductors are the pure form of semiconductor materials While extrinsic semiconductors are impure semiconductor formed by adding an impurity to a pure semiconductor. 11. Distinguish between a p-type and an n–type extrinsic semiconductors (2mk)  A p-type semiconductor material is formed by the addition of group III elements or we can say trivalent impurity to a pure (intrinsic) semiconductor. On the contrary, the n-type semiconductor material is formed by the addition of group V elements i.e., a pentavalent impurity to a pure or intrinsic semiconductor. 12. Using examples explain the difference between n-type and p-type semiconductor. (1mk)  N-type semiconductors are conductors that are formed by doping (process of adding impurity atoms to a pure or intrinsic semiconductor) a pure semiconductor such as silicon with donor or pentavalent impurity atoms like phosphorous and arsenic in order to increase the number of charge carriers(free electrons). The majority charge carriers in n-type semiconductors are the free electrons. While P-type semiconductors are made by adding trivalent (acceptor atoms) such as boron and aluminium to a pure semiconductor like silicon in order to increase the number of charge carriers (holes). The majority charge carriers in p-type semiconductors are the holes (positive charge) 13. What is meant by donor impurity in a semiconductor? (1 mk)  This is a dopant having 5 electrons in its valence shell which holds the ability to donate an extra electron present in its valence shell to the neighboring atom. 14. Sketch the crystal structure of Germanium containing a donor impurity atom. 15. State the majority carriers for a p-type semi conductor.  Holes 16. Extrinsic semiconductors are made through a process called doping. Explain how doping produces an n-type semi-conductor (1mk)  When a pure semiconductor is doped using a group five element like phosphorous having five electrons in their outermost energy levels, four of the electrons participate in bonding with the neighboring atoms while the remaining electron is used for electrical conductivity in the P Ge Ge Ge Ge e- Donor atom
www.teachersofphysics.com Copyright © 2020 semiconductor. Hence electrons will be the majority charge carriers while holes will be the minority charge carriers this gives rise to an n-type semiconductor. 17. Briefly describe how an n-type semiconductor is made from a pure silicon crystal.  When a silicon is doped using a group five element like phosphorous having five electrons in their outermost energy levels, four of the electrons participate in bonding with the neighboring atoms while the remaining electron is used for electrical conductivity in the semiconductor. Hence electrons will be the majority charge carriers while holes will be the minority charge carriers. 18. Silicon was doped with boron to form an extrinsic semi-conductor what are the majority charge carriers.  Holes 19. Explain how doping produces a p-type semi-conductor from pure semi-conductor material. (2mk  When a pure semiconductor like silicon is doped using a group three element like boron, all the three electrons on the outermost energy level of boron atom participate in bonding with the neighboring atoms while silicon will have an extra electron. A vacancy will therefore exist due to the missing electron which is responsible for the electrical conductivity of the doped semiconductor. Hence holes are the majority charge carriers while electrons are the minority charge carriers. 20. Explain why the conductivity of a metallic conductor decreases with increase in temperature while that of an intrinsic semiconductor increases with increase in temperature. (2mks)  In metals, conductivity is due to movement of free electrons. When temperature increases, the vibration of metal ions increases this result in an increase in resistance of metal and hence, decrease in conductivity while When the temperature of the semiconductor is raised the electrons gain more energy and are able to move from the valence band across the forbidden gap to the conduction band this increases the electrical conductivity of the semiconductor. P Si Si Si Si e-