From BEAM Robotics Wiki
An electret is a stable dielectric material with a quasi-permanent electric charge that due to the high resistance of the material does not decay for hundreds of years. The term (coined this term in 1885 by Oliver Heaviside) is a combination of the two words electrostatic and magnet.
A typical electret microphone preamp circuit uses an FET in a common source configuration. The two-terminal electret capsule contains an FET which must be externally powered by supply voltage V+. The resistor sets the gain and output impedance. The audio signal appears at the output, after a DC-blocking capacitor.
 External References
- Powering microphones by: Tomi Engdahl