Context: Mechanically controlled variable capacitors allow the plate spacing to be adjusted, for example by rotating or sliding a set of movable plates into alignment with a set of stationary plates. Low cost variable capacitors squeeze together alternating layers of aluminum and plastic with a screw. Electrical control of capacitance is achievable with varactors (or varicaps), which are reverse-biased semiconductor diodes whose depletion region width varies with applied voltage. They are used in phase-locked loops, amongst other applications.

Question: What do mechanically controlled variable capacitors enable to be modified?
Answer: the plate spacing

Question: How do cheap variable capacitors vary the separation distance between layers of a plastic and aluminum?
Answer: with a screw

Question: Of what type of diodes are varactors or varicaps comprised?
Answer: reverse-biased semiconductor diodes

Question: What varies as a function of voltage in varactors?
Answer: depletion region

Question: What is an application of varactors?
Answer: phase-locked loops

Question: What do mechanically controlled variable capacitors disable to be modified?
Answer: Unanswerable

Question: How do cheap variable capacitors vary the separation distance against layers of a plastic and aluminum?
Answer: Unanswerable

Question:  Of what type of diodes are varactors or varicaps uncomprised?
Answer: Unanswerable

Question: What doesn't vary as a function of voltage in varactors?
Answer: Unanswerable

Question:  What is not an application of varactors?
Answer: Unanswerable