Diode
The varicap or varactor diode is used in many appliances. It uses the region between the p-side and n-side of the diode where electrons and holes balance each other.  This is called the depletion zone.  By changing the amount of reverse voltage, the size of the depletion zone changes.  There is some capacitance in this area, and it changes based on the size of the depletion zone.  This is called variable capacitance, or varicap for short.  It is used in PLLs (Phase-locked loops) which are used to control the high-speed frequency a chip runs at.