A capacitative voltage doubler uses transistors and a 'flying' capacitor to double or invert an input voltage.
A clock signal and an inverter drive the switches. First, the input switches are closed, charging C1 to VCC. Then the input switches are opened, and the output switches are closed, putting the positive terminal of C1 at VCC × 2, and charging the output capacitor C2.
Voltage doublers are only capable of supplying a small current, but they are particularly useful in situations where a voltage outside the range of the supply terminals is required.