Condenser mics can be either tube or FET (or op amp, which may have an FET input stage). The capsule of the mic (which is a capacitor=condenser) needs to be coupled to the rest of the mic's circuitry (head amp) which amplifies and balances the signal before it goes out of the mic to be further processed. This condenser produces a very low current, which needs to go into a very high impedance device so enough voltage is produced to be used further on in the circuit with a reasonable signal to noise ratio.
FETs (field effect transistors) and tubes are two of these very high impedance devices used, and depending on which, the mic is called an FET or a tube mic. Both can be large diaphragm condensers.