According to the article, why are metals not used on transistors and integrated circuits?

The usage of metals are generally avoided in semiconductor technology, since it acts as a deep-level trap and recombination center, i.e. charge carriers of opposite sign do recombine at Au defects in Si and get lost to the current. Metal is also effectively diffusing from a deposited thin film into the underlying substrates even at moderate temperatures. This makes metal rather poisonous to semiconductors. More information can be found in textbooks on semiconductors or on Wikipedia “Carrier recombination”. However, metal layers are often applied to immobilize organic molecules and biomolecules on solid supports via thiol bonds. If semiconductors are used as substrates for bioelectronics applications, for instance, special precautions have to be taken to block the Au diffusion.

b) brahmans   is nomadic society

i believe it was spain