Strictly-speaking, intrinsic semiconductors can only be used as a material for perfect , ballistic leads attached to a resistor in the central region where the actual scattering takes place, and attached to bulk reservoirs of electrons in the left and right infinity. I note that the reservoirs are then not explicitly included in the simulations in spirit of the Landauer-Büttiker formalism of the electron transport that is equivalent to nonequilibrium Green's-function formalism (used in QuantumATK) within the framework of the independent particle approximation adopted for ground-state density functional or semiempirical calculations of the electron transport.
In this way, only the geometrical part of contact resistance (=conductance of bulk, perfect semiconductor leads) is taken into account, and metal contact resistance that arises from the details of the actual metal-semiconductor interface is neglected, i.e., the contacts are assumed to be transparent.
If you want to model more realistic electrodes, you may attach the leads made of the intrinsic semiconductor material to some metal electrodes. In some cases, having metallic-like electrodes can be achieved by heavy doping the semiconductor leads.
It would be instructive to read some textbooks on the electron transport to understand these matters, e.g., a commonly-used book by S. Datta
https://www.cambridge.org/core/books/electronic-transport-in-mesoscopic-systems/1E55DEF5978AA7B843FF70337C220D8B.