20uA 2 terminal current source?
The simplest current source I know is as follows:



simulate this circuit &ndash; Schematic created using CircuitLab
Not sure it will work but you can play with it. D1 and D2 are being used to establish a voltage reference (probably around 1V in this case). R1 is being used to limit the current flowing through D1 and D2. That current should be limited to a small fraction of the total desired current, otherwise it won't really behave like a current source at all.
The base of Q1 should be held at a fairly constant voltage regardless of V1, since D1 and D2 are a voltage reference.
Since Q1 is set up as a voltage follower, the voltage at R2 will be Vb - Vbe, again, independent of V1. This means the voltage at R2 will be constant. Which means the emitter current will be constant. Which means the collector current will be pretty much constant also.
I have used this circuit before but not at such low current levels. I have used it to drive LED's from variable voltage sources, and it works great for that purpose. So you can try it and see if it works for you.