In the definition of a new device, we have to specify the donor density, i.e. the spatial distribution of the donors in the device. This is done in order to solve correctly the Poisson equation. In fact, this last equation needs both the donor and acceptor distribution, so it is necessary to specify them. If the user does not specify any constant value or distribution of the donors, **GNU Archimedes** will consider that the donor distribution is constant on all the device and it is equal to the intrinsic density as default. Furthermore, if the user specify the value of the donor distribution only on a part of the device, the restant part will be considered equal to the intrinsic density.

Let us see, now, how to specify the donor distribution on a device.
In **GNU Archimedes** a sub-domain (but also the entire device) on which we want to specify an donor value for the spatial distribution is just a simple rectangle. This means that we can specify the value of the donor density on a rectangle (the entire device or a part of it), specifying only five numbers i.e.

- The x-coordinate value of the left-bottom vertex of the rectangle. Let us denote it by
- The y-coordinate value of the left-bottom vertex of the rectangle. Let us denote it by
- The x-coordinate value of the right-upper vertex of the rectangle. Let us denote it by
- The y-coordinate value of the right-upper vertex of the rectangle. Let us denote it by
- The value of the donor density on the rectangular sub-domain. Let us denote it by

An example will clarify everything.

# donor spatial distribution on the rectangle [0.0,1.0e-6]x[0.0,0.1e-6] # donor density on this rectangle is equal to 1.e20 DONORDENSITY 0. 0. 1.0e-6 0.1e-6 1.e20