A multi-energy (2–60 keV) calibration of 200 μ m and 400 μm diameter spectroscopic GaAs X-ray photodiodes

Thin (2 μm active layer) spectroscopic p⁺-i-n⁺ GaAs X-ray photodiodes of circular mesa geometry (200 μm and 400 μm diameter; one representative diode of each diameter) have been characterised for their energy response using high-purity X-ray fluorescence calibration samples excited by an X-ray tube, giving energies between 2.1 keV (Au Mα₁) and 21.18 keV (Pd Kα₁), and an ²⁴¹Am radioisotope γ-ray source (26.3 keV, 59.5 keV). The photodiodes were operated uncooled at +33°C. The 200 μm diameter device's energy resolution (FWHM) was found to be constant (0.79 keV) and primarily limited by electronics noise at energies between 2.1 keV and 21.18 keV, but it broadened to 0.85 keV at 26.3 keV, and to 1 keV at 59.5 keV. The 400 μm diameter device's energy resolution (FWHM) was constant (1.1 keV) for photon energies between 4.95 keV and 9.89 keV, but increased to 1.15 keV at 16.62 keV, 1.25 keV at 21.18 keV, 1.3 keV at 26.3 keV and 1.66 keV at 59.5 keV. The broadening of energy resolution (FWHM) observed in both cases is greater than can be attributed solely to increasing Fano noise and is hypothesised to be at least in part due to energy dependent charge trapping. However, for both types of device, the peak charge output from the devices was found to be linearly (R² ⩾ 0.9999) dependent on incident X-ray energy.