Use of Matched Transistors

Most integrated circuit amplifiers have inherently matched input devices because all the circuit elements are developed on a single die in the same process. For special amplifiers, input devices "may" be trimmed prior to final packaging or external trim pin(s) may be included in the final packaging.

For high input impedance, see Bi-FET input op-amps like...
http://www.frc.utexas.edu/page11/files/AD745.pdf

and CMOS op-amps like...
http://focus.ti.com/lit/ds/symlink/opa2334.pdf

Many more out there if you take the time to search for them. This really looks like a homework question, so I'm done on this topic.

Back a few years ago, 34 to be exact, I worked for a major smoke detector manufacturer. We used the CA4007, a commercial CMOS device with uncommitted transistors, for a front-end. Chamber currents in that smoke detector were on the shy side of 20 pA (0.000,000,000,020 amperes). We needed to measure without disturbing. Check out the U.S. patent - 4,138,664. Yes, matched FET's are very useful in electrometer applications.

QL