Why A Schottky diode uses a metal-to-semiconductor junction instead of a P-N junction, so there is no stored minority charge to sweep out. That gives it very fast switching, low junction capacitance, and a low forward drop of roughly 0.3 V, which makes it ideal for mixing and detecting signals at VHF, UHF and microwave frequencies where an ordinary silicon junction diode would be too slow.
Watch out The negative-resistance oscillator element is a tunnel diode, voltage-variable capacitance for AFC comes from a varactor, and a constant voltage reference is a Zener diode.
Schottky = metal junction, no charge storage = fast. Think 'Schottky for switching and SHF-speed mixers'.