9. True or False (9 marks): answer each of the following statements by respondin
ID: 3280511 • Letter: 9
Question
9. True or False (9 marks): answer each of the following statements by responding "True" (or circling T) if it is true or "False" (or circling F) if it is false. Ifyou answer on this exam paper then make sure that you hand in this exam paper with your SID and name on it, as well as your exam booklets. TF a) Some of the unique electrical properties of solid-state semiconductor materials, like silicon, are due to the very regular crystal lattice structure of the material. TF b) At room temperature, electrons in the valence band will freely move about the material and have more energy than those in the conduction band. TF c) In solid silicon, the thermal energy available at room temperature is enough to cause some electrons to move to the conduction band TF d) Insulator materials have a small bandgap and a partially filled conduction band at room temperature. TF e) In electronics, the term "hole" defines a location in a solid-state material that can be filled by an impurity atom. T F f) At high temperatures, an intrinsic semiconductor material will have more electrons than holes. TF g) In extrinsic silicon, the Fermi energy will be closer to the conduction band when there are more electrons in the conduction band than holes in the valence band. TF h) Adding impurities to a semiconductor material will not affect (at room temperature) intrinsic carrier concentration. TF i) A hole is defined as the absence of an electron in the valence band.Explanation / Answer
a. T The electrical properties are related to the avaibility of free electron and silicon [ four valance electron] crystal structure crystal lattice has a tetrahedral pattern with each atom sharing one valence electron with each of its four neighbors to form the covalent bonds. If an electron gains enough thermal energy (1.1 eV for Si ), it may break the covalent bond and becomes a free electron of negative charge, while leaving a vacancy or a hole of positive charge. In an electric field, a free electron may move to a new location to fill a hole there, i.e., both such electrons and holes contribute to electrical conduction. Electrical conductivity can be resulted from crystal structure deffects and doping .In all these crystal structure is important.
b. F at room temparature the electrons in the valance band cannot move freely in the crystal as they are involved in a covalent bonding. The elctrons which are there in the conduction band has more energy and they are free to move in the material
c. T At room temperature, relatively few electrons gain enough energy to become free electrons, the over all conductivity of such materials is low, thereby their name semiconductors. At room temparature what happens is that a small number of electrons in the valance band gain enough thermal energy and reaches the valance band there by leading to the process of conduction. The gap between valance band and conduction band are smallthe fermi energy is kept between the valance band and conduction band. the valance band is almost filled and there few electrons in the conduction band.
d. F For the insulator the fermi level is at the top of the valance band which is completely filled. in the conduction band there are no electrons sothat the procees of conduction is not possible. There exist a large band gap between the valance band and the conduction bad.
e. F As said earlier If an electron gains enough thermal energy (1.1 eV for Si or 0.7 eV for Ge), it may break the covalent bond and becomes a free electron of negative charge, while leaving a vacancy or a hole of positive charge. So hole means vacancy of an electron not an impurity atom.
f. F The number electrons and holes remain equall at all temparature.
g. F In an intrinsic semiconductor the fermi level lies exactly at the middle of the bandgap
h. F The carrier concentrationwill change due to addition of impurities.
i , T Already explained.
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