Choose precision current source mosfet and opamp
The load circuit is probably a PRT or BJT for measuring temperature. Thus you don't want to drive the long leads to the sensor and back to the inverting input of the op-amp, it will oscillate. By using a MOSFET, you decouple the feed back making it stabile. The op-amp is easy. The hard part is finding a MOSFET with very low leakage over temperature. A low Vth is typically 0.45V @ 250uA. Remember, we want the MOSFET in the saturated region (Vds &gt; Vgs). So what's happening at 10nA? We don't know, no specs are given at that low of a current. The BJT will have a very low Beta at that current level and thus will be very inaccurate. The sense resistor has iC+iB, so the load current will be iB less than iC. The required accuracy for a DT-670 silicon temperature sensor is 10uA +/- 0.1% (My problem). Also 10Meg ohms requires, clean boards with no contamination.