This chapter deals with the semiconductor devices that are the foundation for the HVDC industry: the power thyristor that was introduced in the HVDC line commutated technology in the 1970s, and the insulated gate bipolar transistor (IGBT) that was introduced in HVDC in the 1990s in the voltage source converter (VSC) technology. It also covers diodes that are a necessary part of VSC HVDC applications. Although the chapter covers these devices, it also discusses semiconductor materials. It is important to note that for power applications, the current dependent on-state losses but also switching losses are quite high, i.e. in the megawatt range. This is important for the device application. The high switching loss is a result of the simultaneous presence, for a short duration, of both high voltage and high current. Furthermore, the rates of rise and fall of current and voltage are also critical. Other semiconductor devices which might become relevant to HVDC in the future are covered in this chapter, although such devices have not been applied in HVDC as of the time of writing (2022).