Why does my current sweep saturate at much lower levels (10-50 uA) than expected?

You most likely have connected your device under test (DUT) to the Sense terminals of the 4200-SCS. Make sure that you use the Force terminal of each SMU (or the Force terminal of the PreAmp, if installed) to connect to the DUT. Using Sense terminals alone may damage the internal circuitry of the 4200-SCS. The Sense terminals can be used in conjunction with the Force terminals for the remote-sense (also known as Kelvin) measurements. Please see the 4200 Reference Manual for details on making SMU connections.