Four Al0.52In0.48P p(+)-i-n(+) mesa photodiodes with 6 mu m thick i layers and two different diameters (217 mu m +/- 15 mu m and 409 mu m +/- 28 mu m) were studied at room temperature (24 degrees C). Electrical characterisation measurements are reported along with measurements showing the performance of the devices as x-ray detectors. The devices exhibited leakage currents <3 pA (corresponding to leakage current densities <2 nA cm(-2)) at 100 V reverse bias (electric field strength of 167 kV cm(-1)). The photodiodes were coupled to a custom-made low-noise charge-sensitive preamplifier, the noise characteristics of the resultant spectrometers were investigated as functions of shaping times. The best energy resolutions (full width at half maximum of the 5.9 keV photopeak from an Fe-55 radioisotope x-ray source) achieved with the 217 mu m +/- 15 mu m and 409 mu m +/- 28 mu m diameter photodiodes were 0.89 keV and 1.05 keV, respectively. The dielectric dissipation factor of Al0.52In0.48P was estimated to be (2.2 +/- 1.1) x 10(-3) at room temperature.

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