The design and management of a wafer fabrication facility is a very hard task because the manufacturing process needs a large amount of re-entrant operation sequences. Besides, for the new 300mm wafer generation, ergonomic and productivity reasons impose the use of an automated material handling system to transfer wafer lots between operations. The point is now to model and to optimize performance parameters and control logic (i.e. routing and scheduling rules) in order to improve system's productivity. In such a context, this paper studies the wafer transportation problem in a semiconductor fabrication facility. First of all, we shortly describe the production process as a way to understand its complexity. Secondly, we propose an analytical model of this problem.