A code has been set up to simulate the response of GaAs microstrip detectors to m.i.p.s. From preliminary results it is evident that charge sharing is possible in principle also in SIU-GaAs detectors where the charge transport is not perfect, provided that the strip parameters and noise performances are optimized. More extensive results on various geometry and parameters will be presented soon. Limitations of the present code are in the calculation of details of the electric field near the strip and at the interface with passivating layers. This will be improved using a more suitable package for the field calculation. Further developments will also aim at including the possibility of drift in a magnetic field and the effects of diffusion. Because of the peculiar band structure of GaAs these processes are strongly dependent on the local intensity of the electric field even when the drift velocity is saturated. A detailed knowledge of the electric field is needed to improve the simulation.
A possible explanation of the measured electric field profile has been proposed, based on a transition of the EL2 defects which makes them electrically and optically inactive. This model can explain consistently a variety of features of SIU-GaAs, like the slow transients and a saturation in generation current.