Abstract: An improved semiconductor radiation detector which involves engineering the internal electrical field through an external infrared light source. A planar semiconductor radiation detector is applied with a bias voltage, and an optical light beam with a selected photon energy is used to illuminate the detector and engineer the internal electric field. Different light beam intensities or photon energies produce different distributions of the internal electric field. The width of the electric field can be fine-tuned by changing the optical beam intensity and wavelength, so that the radiation detector performance can be optimized. The detector is portable, small in size, and operates at room temperature.