In this paper, we present a mixed-signal integrate and fire neuron designed in a 22-nm FDSOI technology. In this novel design, we deploy the back-gate terminal of FDSOI technology for a tunable design. For the first time, we show analytically and with pre-and post-layout simulations a neuron with tunable spiking frequency using the back-gate voltage of FDSOI technology. The neuron circuit is designed in the subthreshold region and dissipates an ultra-low energy per spike of the order of Femto Joules per spike. With the layout area of only 30 µm 2 , this is the smallest neuron circuit reported to date.