The growth of pentacene films on oxide surfaces plays a major role in determining the device performance. A combination of synchrotron x-ray diffraction and atomic force microscopy was used to probe this interface. In-plane diffraction from films down to one monolayer thick was observed, which allowed to probe the early stages of film growth. Depositions at various substrate temperatures and deposition rates were found to yield films with crystallite sizes from hundreds of nanometers to tens of microns. The performance of these films in thin film transistors was investigated. The scaling of the transistor characteristics down to nanometer size channel lengths is discussed. Finally, applications in sensors are demonstrated.