how to fabricate SOI MOSFET macro model ?

The parameters of SOI NMOSFET I extracted can't simulate kink effect. The real device is body tied. But it still has kink effect. so I want to fabricate a macro model. I used a MOSFET and a BJT transistor. The parasitical capacitor will be taken into account through the parameters of MOSFET. But how to simulate the floating body? By default the voltage of base electrode is zero if no bias added. Then the kink effect caused by Vt enhancement still can't be simulated. How should I do ? Thanks!