Why A Schottky barrier diode is formed where a metal contacts a lightly doped semiconductor rather than by joining P-type and N-type silicon. That metal-semiconductor barrier conducts by majority carriers only, so there is no minority carrier storage time, giving very fast switching and a low forward drop of roughly 0.3 V. That makes Schottky devices favorites for high-frequency detectors, mixers and low-loss rectifiers.
Watch out A PIN junction is a different device entirely, with an undoped intrinsic layer between P and N regions, used as an RF switch or attenuator; the thermionic emission diode is a vacuum tube.
Schottky = metal meets semiconductor: no P-N sandwich, ~0.3 V drop, very fast.