Why are heat sinks required for power electronics devices?

Heat sinks dissipate heat generated by power losses in semiconductor devices, keeping junction temperature below maximum rating (typically 125-175C). Without adequate cooling, excessive temperature causes device failure. Heat sink thermal resistance (degrees C/W) determines temperature rise for given power loss: Tj = Ta + Ploss x (Rth_jc + Rth_cs + Rth_sa). Selection considers power dissipation, ambient temperature, and available space. Thermal interface materials (thermal grease/pads) reduce contact resistance between device and heat sink.