After being attached on a carrier chip and receiving gold micro-welds, measurements of these devices  at low temperatures using an optical cryostat (between 12 to 300K) can be made. IxV curves can either be performed in ambient light (receiving external radiation) or in the dark (devices sit inside a temperature controlled copper cup) using a lockin or a sub-femtoammeter (model 6430) from Keithley (now Tektronix), which makes it possible to determine the photocurrent (in the light) or the dark current (in the dark) of photodetectors with high sensitivity.