Which parameters will decide the threshold of a mos?

i'm designing a circuit using siemens's bsp299 as a switch. my problem is that i can't see how threshold varies with respect to parameters other than temperature. can anyone help me out here?

the curve attached is from bsp299's datasheet and the dot lines with a percentage number really confuse me. do you know the meaning of those percentages?

Threshold varies with doping density, which means it will vary with process. Since in the IC processing, you get a doping profile.,which means not every transistor is doped the same way. Hence Vt can vary from one device to another. So the curve indicates that the given device can have Vts within a bound.

wholx,

The dotted lines represent the 2 sigma limits of the parameter; e.g., at 25 Deg C, 98% of the devices will have a Vgs below 4.0V, and 2% of the devices will have a Vgs below 2.1V.

Gfs indirectly affects the threshold voltage in a practical circuit. For example, if the drain current in will be .1A, then you are interested in the Vgs required to conduct .1A. Since Vgs is specified at .001A, you need to determine the increase in Vgs (DeltaVgs) that is required:
Delta Vgs = (.1 - .001)/Gfs
So the net Vgs = Vgs(specified) + Delta Vgs. Since Gfs varies with both temperature (it decreases as temperature increases) and drain current (it increases with drain current), you must use the value of Vgs at the coldest operating temperature, and the lowest drain current to calculate worst case Vgs in an actual circuit.

I hope this helps,
Regards,
Kral