We perform measurements which show that focused, picosecond pulses of x-rays can be used to generate single event transients (SET) in a GaAs heterostructure field effect transistor (HFET) and a GaN high electron mobility transistor. X-ray pulses with photon energies of 8, 10 and 12 keV from the Advanced Photon Source at Argonne National Laboratory were used to excite transients. SETs are observed when x-ray pulses are incident upon metal layers above sensitive areas on the transistors. We use focused ion beam (FIB) cross-sectioning and scanning transmission electron microscopy energy dispersive x-ray spectroscopy (STEM-EDXS) to determine the compositional structure of the devices. We present a first order analysis of energy deposition in the devices and correlate it to the transient response to make preliminary interpretations of the results. We compare the x-ray transients from the GaAs HFET with transients generated by 750 nm and 870 nm femtosecond laser pulses. We also present results on the total dose susceptibility of the GaN HEMTs.