In this work we report the fabrication, characterization and validation of a cortisol biosensor, together with a unified predictive calibrated model. We demonstrated the possibility of using a classical submicron semiconductor FET as the transducer for a cortisol biosensor, extending its gate with a graphene on platinum electrode decorated with cortisol specific aptamers. The sensor outperforms the so far any reported cortisol sensors, in terms of performance and integration capability: (i) we report sensor validation over 4 orders of concertation (1 nM - 10 mu M, matching human sweat concentration range), (ii) with excellent voltage (14.7 mV/dec.) and current (80% relative change with respect baseline) sensitivity, (iii) low drift, smaller than 10 mV/h, (iv) low power consumption (sub-nW DC power), (v) record low detection limit (LOD) for cortisol of 0.2nM, and (vi) selectivity over other hormones such as testosterone. Moreover, we have developed and validated the first unified compact analog predictive calibrated model for cortisol FET sensors based on experimental data, valid from weak to strong inversion, and able to capture the output current dependence on hormone concentrations. In addition, this model is accurate in the prediction of I-D, g(m) and transconductance efficiency, I-D/g(m), enabling simulation and optimization of analog design readout, together with power and signal-to-noise ratio trade-offs.