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...

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.