  GaAs devices have relatively high atomic numbers (Z=31, 33) and thus extend
the X-ray absorption edge beyond that of Si (Z=14) devices. In this study,
radiation detectors were processed on GaAs substrates with 110 $\mu\textrm{m}$
- 130 $\mu\textrm{m}$ thick epitaxial absorption volume. Thick undoped and
heavily doped p$^+$ epitaxial layers were grown using a custom-made horizontal
Chloride Vapor Phase Epitaxy (CVPE) reactor, the growth rate of which was about
10 $\mu\textrm{m}$/h. The GaAs p$^+$/i/n$^+$ detectors were characterized by
Capacitance Voltage ($CV$), Current Voltage ($IV$), Transient Current Technique
(TCT) and Deep Level Transient Spectroscopy (DLTS) measurements. The full
depletion voltage ($V_{\textrm{fd}}$) of the detectors with 110 $\mu\textrm{m}$
epi-layer thickness is in the range of 8 V - 15 V and the leakage current
density is about 10 nA/cm$^2$. The signal transit time determined by TCT is
about 5 ns when the bias voltage is well above the value that produces the peak
saturation drift velocity of electrons in GaAs at a given thickness. Numerical
simulations with an appropriate defect model agree with the experimental
results.
