When using the SMU to measure only, why are the polarities of my current measurements negative?

When using the SMU to measure only, why are the polarities of my current measurements negative?

Why are the polarities of my current measurements the opposite of what I expect?

The instrument always assumes that it is measuring a current from its internal source. In this case, a POSITIVE current will flow OUT of its terminals and a NEGATIVE current will flow IN to them. As a result of this convention, a Source Measure Unit (SMU) operating in the "measure current only" mode will measure polarities opposite from what is expected with a typical ammeter. A positive current flowing from your circuit into the SMU will be measured as a negative current and vice versa.