Hi,
I've designed the voltage reference for 1.25V with 20ppm/deg C over -40 to 125deg C.I chose the BG schematic for PTAT current generation to balance for the -ve temp Coifficient of Vbe.The results are satisfactory.

But,if I cannot use bipolar transistors and have to generate the same results of 1.2V and 20ppm/deg C for supply variation from 2.25 to 5.5V.(Using only MOS)
I need to bias MOS in weak inversion to generate a temperature dependent current to compensate for Vth variation with temperature.

Now, can you suggest me a proper topology to implement this.?or some material related.
Suggest me how to simulate or verify parameters of MOS in weak inversion?
Is it possible to attain a BG voltage of 1.25V with out bipolars.

I've come across papers using only MOS for a reference voltage less than 1V.