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Gallium arsenide surface barrier diodes have been fabricated from high purity level materials. These devices have an Au surface barrier having a depletion layer thickness of from 70Â¿mto 1 mm and an area of from 3mm2-27mm2. These devices have been operated as Â¿-particles, / Ã-ray, and Â¿-ray spectrometers and detectors. The best energy resolutions taken with a GaAs detector made from liquid phase epitaxial GaAs wafer were 20keV (fwhm) and 8 keV for 5.486 MeV Â¿-particles from 241Am and 115 keV conversion electrons from 57Co at room temperature, respectively. For room temperature applications, a simple charge sensitive preamplifier was constructed using two FETs and one OP amp. The combination of a encapsulated GaAs detector and two small semiconductor thermoelements (electrical cooling device) was studied to make a simple X-or Â¿-ray spectrometer. A special tiny GaAs detector was also fabricated as in vivo Ã-ray counting detector in biomedical applications. The energy per electron-hole pair (Â¿) in GaAs was measured at 4.35 + 0.02 eV for Â¿-particles with a linear variation with bandgap energy (Eg) of 2.53 over a temperature range of 1950Â°K to 330Â°K, and 4.57 eV for conversion electrons (115keV) at 300Â°K, respectively. The Â¿ vs Eg relationship was also investigated for Ge, Si, GaAs and CdTe using experimental values and led to Â¿ = 2. 596 Eg + 0.714 (eV) with correlation coefficient of 0.999 at 300Â°K. The related problems for intrinsic material constant (Â¿) are discussed for several semiconductor materials.