How to build a 100uA current source?

Without knowing too much about the load I can suggest an option that may be able to trigger some ideas for you.  If you can find a low power Rail-Rail op-amp that can operate from the 2.5V source then you can build a decent 100uA Source.  Use the 2.5V supply to create a low voltage reference via either an LDO linear reference or a zener based one.  I suggest using one around a value of 1V.  Connect the reference to the non-inverting input of the op-amp.  On the output of the op-amp you can drive the base of a BJT.  On the emitter of the BJT there is a resistor connected to ground.  The voltage node at the emitter and resistor (R) gets routed back to the negative terminal of the op-amp.  The 100uA source is created by taking Vref/R (ex: Ref = 1.25V, R = 12500 ohms, yields Iload = 100uA).  So you can tie your load between the 2.5V source and the collector of the BJT.  This works because the op-amp will always try to drive the BJT such that the voltage into both the inverting and non-interting inputs have the same voltage. 

The above suggestion doesnt give you a ground reference for your load which may or maynot be good for your application.  If you need an Isource that provides you a ground connection then I can offer up a suggestion which uses a PNP transitor and a P-Channel MOSFET. 

Thanks, for your suggestion. Coincidently, I found some interesting solutions in the latest EDN :-)

Use REF200

Current output current shunt monitors also can make good current sources. A differential measurement over a shunt resistor produces a corresponding current output that can be adjusted by adjusting the voltage drop over the shunt resistor. TI's INA138 would be a good example although this has a minimum 2.7V supply.

Developing current sources or voltage references I recommend taking a look into this book:

"Current Sources and Voltage References: A Design Reference for Electronics Engineers" - Linden T. Harrison

http://www.amazon.com/Current-Sources-Voltage-References-Electronics/dp/075067752X/ref=sr_1_1?s=books&ie=UTF8&qid=1291279747&sr=1-1

One of my favourite at all