الفهرس 
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Abstract
Materials are generally classified according to their electronic structures and conductivity into three main categories: insulators, semiconductors, and conductors [22]. The main parameter that determines a difference in conductivity between these categories is the energy band gab which depends on their electronic structures. The bang gap is a forbidden energy region located between the valance band (VB) which filled by electrons and the next higher energy band namely the conduction band (CB), which is partially filled or empty from electrons [23].The larger the band gap energy, the more difficult it is for electrons to transfer from VB to CB resulting in lower possibility for electrons to conduct, as shown in Figure (2). Conductors have no band gab, so electrons can freely move between valance band and conduction band; make it a good material to conduct the current. on other hand, insulators have a large band gab, so it is hard to electrons to move between VB and CB that make it can‟t conduct current. But the semiconductor materials enjoy with intermediate band gab, which makes it have features of both insulators and conductors. Because of its band gab, it is hard for electron to move between the VB and the CB until it is excited by external influence, making the electron move from the VB to the CB with creating a hole that causes conduction of current [22-23].