Why Si and Ge are used in semiconductors ?

Si and Ge are the two materials that have received the broadest range of interest in the development of semiconductor devices. The reasons for this are as follows:
(1) They can be manufactured to a very high purity level. In fact recent advances have reduced impurity levels in pure material to one part in 10 billion (1:10 000,000,000)
(2) The addition of one part of impurity (of the proper types) per million in a wafer of silicon material can change that material from a relativity poor conductor to a good conductor of electricity.
(3) Their characteristics can be altered significantly through the application of heat and light an important consideration in the development of heat and light sensitive devices.