In a transistor an insulator is placed between the plates to increase its capacitivity. Why can't a conductor be used? I am talking about a parallel plate capacitor in which two plates are parallel to each other. One has a positive charge while the other has a negative charge. The insulator (dielectric) is in between. I know why the insulator is used but why can't it be replaced by a conductor.

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A parallel plate capacitor has two plates which are oppositely charged. The opposite charge between the two plates allows it to function as a capacitor and to be used in various applications. A capacitor's capacitance is inversely proportional to the distance between the plates. The closer they are, the higher is the resulting capacitance. This makes the dielectric between the plates an essential component. The sheet of dielectric or insulator that is inserted between the plates allows them to stay apart while making the distance of separation very small.

If a capacitor were used instead of an insulator the charge on the plates would flow from one to the other and both of them would have a neutral charge. The device would no longer remain a capacitor. This is the reason why a conductor cannot be inserted between the plates of the parallel plate capacitor.

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