A semiconductor is neither a complete conductor nor a complete insulator. Before we go into the details of these two, we need to understand another term. It is called conductivity. The reason being a semiconductor and insulator are both defined in terms of their conductivity. Hence to proceed further without a proper understanding of conductivity would not be of much help. So, what is conductivity? The degree or extent to which a material allows heat or current to conduct through it is called conductivity.
Now let us get into understanding semiconductors and insulators. A material whose conductivity lies between that of a metal like copper or gold and an insulator like glass is called a semiconductor. The semiconductors’ resistance decreases with increase in temperature. With metals, it is the opposite where temperature rise causes increased resistance. The conducting property of semiconductors can be improved by selectively adding impurities. This is called "doping". By introducing them into the crystal structure of the original material the conductivity can be improved. In the same crystal, when two differently or rather oppositely doped regions exist, there is created a semiconductor junction. In modern electronics, diodes, transistors etc: show different behavior depending on this junction. Some of the semiconductors are silicon, germanium.
Insulators are resources that oppose the flow of electric current. The same is true for heat also. Wood is an excellent insulator of heat while metal is a good conductor. Insulators are embedded in household appliances as protection devices. The internal electric charges in an insulator will not flow as freely as in a conductor. The resistivity of insulators is very high and consequently very low conductivity. The atoms of insulating material do not have free-flowing electrons. Their electrons are tightly bound causing them to be bad conductors. It is important to note that there is no perfect insulator. When a sufficiently large voltage is applied across an insulator, even the tightly bound electrons are pulled and they start to conduct. The voltage at which the electrons are pulled away and they get to conduct is called the breakdown voltage of an insulator. Some materials have very high resistivity. Example: Glass, paper are excellent insulators. Polymers and plastics made by way of thermosetting or thermo plasticizing are also good insulators. We see these in the protection on electric wires and in switches of appliances. Insulators are used to support and separate the conductors so that there is no physical harm to the user.