Semiconductor materials are those which have electrical conductivity between conductors (such as metals) and insulators (such as non-metals). They possess a unique property that allows them to conduct electricity under certain conditions but not as efficiently as conductors.

The given options are Germanium, Selenium, Radium, and Silicon. Among these, Radium is not a semiconductor.

Let us understand why Radium is not a semiconductor:

1. Radium is a metal: Radium is a highly radioactive metal that belongs to the alkaline earth metal group in the periodic table. It is a dense and silvery-white metal with a high melting point. Unlike semiconductors, metals have high electrical conductivity due to the presence of a large number of free electrons that can easily move through the material.

2. Conductivity of Radium: Radium is a good conductor of electricity. It possesses metallic bonding, where valence electrons are highly mobile and can move freely throughout the crystal lattice. This characteristic makes it a conductor rather than a semiconductor.

3. Radioactive nature: Radium is highly radioactive and emits alpha particles, beta particles, and gamma rays. Its radioactive nature makes it unsuitable for use as a semiconductor in electronic devices.

4. Semiconductor properties: Semiconductors like Germanium and Silicon have a specific energy band structure that allows them to exhibit unique electrical properties. They possess a band gap, which is the energy difference between the valence band and the conduction band. This band gap determines their electrical conductivity. In the case of Radium, it does not possess a significant band gap, and therefore, it cannot exhibit the characteristic electrical properties of a semiconductor.

In conclusion, Radium is not a semiconductor because it is a metal with high electrical conductivity and does not possess the necessary band gap required for semiconductor behavior.