Microstrip detectors made of GaAs will be used in a part of the forward tracker detector in ATLAS at the LHC because of their larger radiation hardness when compared to silicon devices. Test beam results are reported elsewere in these proceedings. In order to optimise the parameters of the devices, such as total thickness, strip pitch and aspect ratio for a required spatial resolution, we are developing a program which is able to calculate the current signal from the various geometrical configurations and crystal properties. A more detailed study will be published later; in the following we describe the method and stress the importance of a detailed knowledge of the electric field inside the detector.
The second part of this paper deals with a model for the electric field.
Semi-Insulating-Undoped (SIU) GaAs has a resistivity of the order of
which is achieved through a compensation by deep levels.
These levels are mainly of the intrinsic type called EL2, which is due to the
non perfect stoichiometric ratio between Ga and As. In detectors made of
this material
the region which is active for detection has
a thickness which increases with bias. At a bias of 200 V about 180 
are active [1, 2, 3].
Various models [4, 5, 6]
have been proposed to calculate the electric field, and two
measurements have been made [7],[8]. The measurements
show that the electric field is approximately uniform at high voltages,
which means that the sensitive region has a very low fixed charge density.
This observation is very difficult to explain in homogeneous materials, where
the fixed charge density is constant over the depth.
A possible explanation of such a field shape is that some of the EL2 defects,
under the influence of a strong electric field,
make a transition to a normally metastable state featuring no energy levels
in the forbidden gap. The existence of such a level has been reported
[9], but neither evidence nor exclusion of a
voltage-induced transformation has yet been reported.