The thermal stability and reliability of AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrates with a 2-μm thick copper interconnection (Cu-INTC) were evaluated. The use of metallic copper as a conducting metal has the advantages of higher thermal conductivity, low cost and low sheet resistance. For comparison, traditional gold metal interconnection (Au-INTC) devices were fabricated under the same process conditions. Thermal infrared (IR) microscopy measurements show that the Cu-INTC devices could function at a lower channel temperature (T CHANNEL) than traditional Au-INTC devices with the same drain current density, because of the low resistivity of the metal. The typical peak transconductance (g m), output power (P OUT), power gain (Gp) and power-added-efficiency (PAE) during operation at 100 °C were 87.53 mS/mm, 22.85 dBm, 11.1 dB and 25.9% for a Cu-INTC power device with gate width of 1 mm and these measured results were better than those of Au-INTC devices. They indicate that the copper metal provides great potential for high-power AlGaN/GaN HEMT applications.