Which of the following elements is NOT suitable for the fabrication of a light emitting diode structure?

Germanium

The correct answer is option 3. Germanium.

Out of the listed elements, Germanium (Ge) is NOT suitable for the fabrication of a light-emitting diode (LED) structure.

Here's why:

Gallium phosphide (GaP), Indium gallium nitride (InGaN), and Gallium arsenide (GaAs) are all semiconductors with direct band gaps. This means that electrons in these materials can readily recombine with holes (vacant electron states) and emit light efficiently. These properties make them suitable for use in LEDs.

Germanium (Ge), while also a semiconductor, has an indirect band gap. In such materials, electron-hole recombination is less efficient, and most of the energy is lost as heat instead of being released as light. This makes germanium unsuitable for efficient light emission in LEDs.

Therefore, due to its indirect band gap, germanium is not a viable option for constructing LED structures.