Metal-semiconductor-metal detectors have been fabricated based on AlGaN grown on Si by molecular beam epitaxy. Field distribution and collection efficiency were studied with the ion beam induced charge collection method. The results were explained by numerical two-dimensional calculations of the electric field distribution. The calculated field map and charge buildup at the electrodes are used to explain the bias and position dependence of the ion beam induced charge collection. The similarities and differences with the case of optical detection are discussed. (C) 2002 American Institute of Physics.