  Hybrid pixel detectors (HPDs) have been shown to be highly effective for
diffraction-based and time-resolved studies in transmission electron
microscopy, but their performance is limited by the fact that high-energy
electrons scatter over long distances in their thick Si sensors. An advantage
of HPDs compared to monolithic active pixel sensors (MAPS) is that their sensor
does not need to be fabricated from Si. We have compared the performance of the
Medipix3 HPD with a Si sensor and with a GaAs:Cr sensor using primary electrons
in the energy range of 60 - 300keV. We describe the measurement and calculation
of the detectors' modulation transfer function (MTF) and detective quantum
efficiency (DQE), which show that the performance of the GaAs:Cr device is
markedly superior to that of the Si device for high-energy electrons.
