urgent : Tox V/S threshold

[konqueror]

when i was simulating a dual gate mosfet using medici 4 , I found out that as i was decreasing the oxide thickness the threshold voltage was increasing rather than going down which should be the case as Tox decreases the Cox increases and Vth is inversely proportional to Cox so Vth must decrease.
Can some one please explain what is the problem? please help as early as possible

the parameters for the mosfet are
Tox=5n
Thin film thickness=50n
Channel doping = 1E16
source/drain doping = 5E19
Gate metal work function=4.4

thanks in advance