We describe the results of experimental studies of Liquid Encapsulated Czochralski (LEC) semi-insulating GaAs detectors with different contact areas. The detector active area spreading at different applied bias is proven by three various experiments. In the first, the scan of the detector electrode and adjacent area was performed using the 660nm pulsed laser beam, with a spot diameter of 50 microm. The other experiments use the spectra measured by tested detectors, when irradiated by 241Am alpha source (5.48 MeV). The experiments show relation between bias voltage applied and the range of active detector area from the edge of the detector contact.