The characterisation of semiconductor devices has an important role in Microelectronics, especially in the case of power semiconductor devices, in which the electrothermal characterisation is an essential aspect to study their behaviour. In several Power Electronics applications, non-controlled electrothermal phenomena are the responsible for the device destruction. The internal self-heating experienced by Power Devices generates an internal temperature rise, which may negatively affect their internal physical behaviour. In this framework, this book covers the development of an experimental rig for temperature and free-carrier concentration measurement in power devices, based on the internal IR-laser deflection (IIR-LD) and free-carrier absorption (FCA) techniques. IIR-LD allows a complete characterisation of power devices, extracting its temperature and free-carrier concentration gradients. By contrast, FCA provides the carrier concentration at the drift region of bipolar power devices. With both techniques, paramount information related to power devices physical behaviour are derived.