The behaviour of the charge collection efficiency of particle detectors fabricated from Semi-Insulating (SI) Liquid Encapsulated Czochralski (LEC) GaAs as a function of applied bias is investigated for alpha particles and it is shown that it cannot be explained by simple models that assume a uniform electric field across the detector. To investigate this the electric field within the SI GaAs has been studied. We have attempted to correlate the measured charge collection efficiency with the observed field distributions. Some correlation is found but other effects may also be present.