Answer: semiconductor materials.
This question mainly examines the basic properties and material characteristics of light-emitting diodes.
Option A: Ceramic is an insulator. It does not have conductivity, so it cannot be used as the main material of light-emitting diodes. So option A is wrong.
Option B: The core component of light-emitting diodes is semiconductor materials. The conductivity of semiconductor materials is between conductors and insulators, and it has unique electrical properties, making it an ideal material for making electronic devices such as light-emitting diodes. So option B is correct.
Option C: Although semiconductor materials have a certain conductivity, the conductivity of diodes is not "very good", but has unidirectional conductivity. That is, the diode only allows current to flow from its positive pole to the negative pole, but not in the opposite direction. So option C is wrong.
Option D: The main function of the diode is rectification (that is, converting AC to DC) or as a switching element, rather than amplifying electrical signals. Amplifying electrical signals is the main function of devices such as transistors. So option D is wrong.
In summary, the correct answer is B.