Hi,
For Mosfet model you can refer to the paper
"Cunha, Ana I.A; Schneider, M.C; Galup-Montoro, C. An MOS-transistor Model for Analog Circuit Design. IEEE journal of Solid State Circuits, Vol 33, Oct 1998, No. 10."
As for circuits with nA currents you can use the CMOS Circuit Design book by Baker 2nd edition. you just need to size the Mosfets accordingly ( typicaly large L are required) you may have to use series connection of Mosfets to achieve the required length. You could search for series parallel OTA paper.
I had designed an OTA with 20nA bias current at 180nm Technology.
Hope this helps!
-Zopeon