High-temperature characteristics of a symmetrically-graded delta-doped InAlAs/InxGa1-xAs/GaAs (x=0.5 rarr 0.65 rarr 0.5) metamorphic high electron mobility transistor (MHEMT) have been investigated. The thermal threshold coefficients, defined as deltaVth/deltaT, are superiorly low to be 0.9 mV/K from 300 K to 420 K and -0.75 mV/K from 420 K to 500 K, respectively. The present MHEMT device, with stabilized thermal threshold variations and superior high-temperature linearity characteristics, is promising for high-temperature integrated circuit (IC) applications