Plastic relaxation in metamorphic high electron mobility transistor (MHEMT) structures was investigated by x-ray reciprocal mapping and high-resolution transmission electron microscopy (HRTEM). X-ray data indicates that In(Ga)AlAs M buffers with a linearly graded buffer and an inverse step are completely strain compensated at the buffer-active area interface. HRTEM shows reduction of dislocation density from 10(9) to 10(6) cm(-2) through the M buffer. Optimized MHEMT structures were found to exhibit low rms roughness of around 2 nm and excellent electrical transport properties. MHEMT devices with 0.15 mum gates were fabricated with a transconductance of 710 mS/mm, maximum current of 500 mA/mm, and gate-drain breakdown of 6.6 V. A maximum f(t) value of 118 GHz and a maximum rf gain of 18 dB at 10 GHz were measured at a drain current of 200 mA/mm.