The phenomenon in which a substance exists in more than one form is called ......

$n$-type semiconductor is formed when:

$A$ solid which is very hard,has a very high melting point in the solid state,is an insulator,and becomes a conductor of electricity in its molten state is a/an ..... solid.

Write a note on $n$-type and $p$-type semiconductors.

Which of the following is a non-crystalline solid?

Quartz is a crystalline variety of