increasing doping concentration increases vt
(i) if you increase the doping concentration of sub, it means that the majority carriers concentration is increased, the Vgs for depletion and inversion is increased,that's to say, the threshold voltage is increased.
(ii) if you increase the width of the oxide layer, the cap of the oxide layer will be decreased, the change of charge is not so sensitive with the change of Vgs, so, the Vth will be increased.
the threshold voltage is related with
1,the doping concentration of the sub,
2,the work function between the gate and the sub
3,the impurities in the oxide layer
4,tox of the oxide layer