High Input impedance OPamp

Both, CMOS and JFET input op amp may have input impedance in the range of 1Tohm but for the low voltage and RRIO CMOS would be the only choice.  Please take a look at OPA328, OPA392, OPA391, OPA320, OPA325, OPA333 for 5V supply or LMP7701 (Vs up to 12V) and OPA928 (Vs up to 36V), etc.

There are three basic type of transistors that might be used to implement input stage of any op amp circuit:  one of the two field effect transistor type: CMOS (MOSFET) and JFET (junction field effect transistor) or BJT (bipolar junction transistor). For more info, please click on the following link:

 https://www.electronicshub.org/transistors-classification-and-types/

All op amps on the list above are CMOS - this is typical for RRIO op amps. 

Glad to help.  Good luck with your project!