Final answer:
PN junctions are considered extrinsic semiconductors due to intentional impurities during fabrication, with n-type and p-type materials forming a p-n junction that creates a built-in electric field at the depletion zone.
Step-by-step explanation:
PN junctions are considered extrinsic semiconductors because they contain impurities intentionally added during fabrication. These impurities create doping that significantly alters the electrical properties of the semiconductor material. In an n-type semiconductor, the majority carriers are free electrons contributed by donor impurities, while in a p-type semiconductor, the majority carriers are holes contributed by acceptor impurities. When these two types of semiconductors are joined, a p-n junction is formed.
At the junction, electrons from the n-type material diffuse into the p-type material to fill the holes, which creates a depletion zone with a built-in electric field. This electric field is critical for the functioning of devices like diodes and transistors by controlling the flow of charge carriers. Hence, a uniform electric field generated near the p-n junction is due to the separation of charge as a result of this diffusion process.