This paper reports on the design, fabrication and detailed characteristics of multi-gate vibrating-body field effect transistors (VB-FETs). Double-gate and four-gate VB-FETs with resonance frequencies of 2 MHz and 71 MHz, respectively, are successfully demonstrated. The VB-FETs exhibit built-in amplification (more than +30 dB improvement of signal detection compared with capacitive detection), low motional resistances (in the orders kOhms down to tens of Ohms) and frequency tuning by applied voltages. For the first time, we experimentally demonstrate an active MEM resonator concept, with built-in amplification, which has a negative resistance of -30 Ohms, enabling the possibility to build an oscillator without any sustaining amplifier, thus reducing the power consumption and oscillator size. Also for the first time, we demonstrate a VB-FET mixer-filter based on a single-device operating at 9.84 MHz and a VB-FET oscillator at 2.6 MHz.