If you are using the IVCharacteristics study object, you may also use it to calculate analysis objects, as discussed in the manual, https://docs.quantumatk.com/manual/Types/IVCharacteristics/IVCharacteristics.html and https://docs.quantumatk.com/tutorials/ivcharacteristics/ivcharacteristics.html.
You also extract a particular device configuration for a given gate and bias voltage, and then do analysis from file in a usual way.
iv_characteristics=nlread('device-ivcharacteristics.hdf5', IVCharacteristics)[-1]
device_configuration=iv_characteristics.configuration(1.0*Volt, 0.0*Volt)
nlsave('device-0.0-bias.hdf5', device_configuration)
where gate_source_voltage=1 .0*Volt and drain_source_voltage=0.0*Volt