Before the transistor radio was invented there existed something called a crystal radio. In this radio there was a crystal that removed the noise from the radio signal. This crystal was very important to be able to use the radio. Although not known at the time, this crystal was constructed of a material called semiconductor, which is the key to the transistor's function.
In the crystal radio the crystal was often made of galena (lead sulfide), iron pyrite (fool's gold), zincite (zinc oxide) and sometimes the elements germanium or silicon. These substances are all examples of semiconductors.