Lead ion selective membrane (Pb-ISM) coated AlGaN/GaN high electron mobility transistors (HEMT) was used to demonstrate a whole new methodology for ion-selective FET sensors, which can create ultra-high sensitivity (-36 mV/log [Pb2+]) surpassing the limit of ideal sensitivity (-29.58 mV/log [Pb2+]) in a typical Nernst equation for lead ion. The largely improved sensitivity has tremendously reduced the detection limit (10-10 M) for several orders of magnitude of lead ion concentration compared to typical ion-selective electrode (ISE) (10-7 M). The high sensitivity was obtained by creating a strong filed between the gate electrode and the HEMT channel. Systematical investigation was done by measuring different design of the sensor and gate bias, indicating ultra-high sensitivity and ultra-low detection limit obtained only in sufficiently strong field. Theoretical study in the sensitivity consistently agrees with the experimental finding and predicts the maximum and minimum sensitivity. The detection limit of our sensor is comparable to that of Inductively-Coupled-Plasma Mass Spectrum (ICP-MS), which also has detection limit near 10-10 M.