We have developed a method of PECVD depositing a-SiN.sub.x:H films which are useful in a TFT device as gate dielectric and passivation layers, when a series of TFT devices are arrayed over a substrate having a surface area larger than about 1 m.sup.2, which may be in the range of about 4.1 m.sup.2, and even as large as 9 m.sup.2. The a-SiN.sub.x:H films provide a uniformity of film thickness and uniformity of film properties, including chemical composition, which are necessary over such large substrate surface areas. The films produced by the method are useful for both liquid crystal active matrix displays and for organic light emitting diode control.