Magnetic sensors with polysilicon TFTs

This paper deals with magnetic position sensors compatible with large area electronics using polycrystalline silicon deposited by a low-pressure chemical vapour deposition (LPCVD) technique. The principle of this large area position sensor is a matrix of thin film field effect transistors (TFT) with two additional Hall probes. The performances of the TFT based cells are linked to the crystalline quality of the active polysilicon layer, which depends on the deposition conditions and on the technological process. Layers are made from two precursor gases, silane or disilane and two processes. We have compared the sensitivity (absolute or relative) of devices and measured their power consumption. Sensors made from disilane have a sensitivity of 18 mV/T, and the ones made with a monolayer process a sensitivity of 28 m V/T. We propose a simple model, which describes the bias dependency of the sensitivity. The offset voltage is also studied in order to determine the role of geometry and of the layer morphology.