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increase in its conductivity. The electric properies of a substance depend on its crystal structure and its electronic configuration. A large number of semiconducting substances have been produced systematically by preparing suitable compounds. In this chapter, however, we will study only two most important semiconducting elements, Si and Ge; which have crystal structure of diamond." Kakani. Saxena, Chhajer & Locha, op.cit, pp. 536-- “Impurities added 10 pure germaniuni or silicon will cause a change in the lattice structure, which might add a free electron or create a hole. The importance of hole is that it may serve as a carrier of electricity and a free electron from a neighbouring atom may drop into it.” "The study of transistors is based on semiconductors. Semiconductors which sind use in transistors are silicon and germanium, out of which germanium is commonly used. “When an electric field is applied between both the ends of a semiconductor crystal, the free carriers such as free clectrons and holes attain drift velocity. The electrons drift towards the positive electrode and holes move towards the negative electrode." Satish K. Gupta, op.cit pp. 3,4 **I11 600 B.C. the Greek philosopher Thales observed that when anber* was rubbed with a woolen cloth, it acquired the property of attracting light objects like feathers of birds, small bits of paper. pieces of dry leaves, etc. In 100 A.D., Dr. William Gilbert was led to the conclusion that there are many other substances like amber, which acquire the property of attracting such light objects after they had been rubbed with some other suitable substances. For example, if on a dry day, we comb dry hair with a rubber comb, the coinb acquires the property of attracting the small bits of paper and dry grass. Similarly, a glass rod acquires the attractive property on being rubbed with silk and an ebonite rod on being rubbed with flannel or cikin. The amber, the rubber comb, the glass rod, the ebonite rod, etc. are said to be charged with electricity.
45. (a)
* It is hardened up of a tree similar to a pine tree. In Greek, the nic:ining of amher is electrum and
probably the words like electric charge, electric force, electric potential, clectricity and finally ck ctron ove their roin wclectruin.
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