Contents I. Introduction
Active matrix liquid crystal displays (AMLCDs) incorporating hydrogenated amorphous silicon (a-Si : H) thin film transistors (TFTs) currently dominate the market for laptop and desktop flat panel displays greater than a few inches in size. There are two main reasons for this dominance. The first is the fact that a-Si : H, and also its accompanying gate insulator material, amorphous silicon nitride (a-SiN), can be deposited uniformly over very large areas by RF plasma-enhanced chemical vapor deposition (PECVD). This allows the active matrix TFTs to be spread across the whole of a display. The latest generation of state-of-the-art deposition systems are designed to use substrates in excess of 1 in area. Second, carrier transport in a-Si : H at room temperature is dominated by the high density of localized band tail states which act as traps. The carrier can move between these trap states either directly via a tunneling mechanism, known as hopping [1], or indirectly via extended state conduction above the mobility edge [2]. This is a relatively inefficient process compared with crystalline silicon, where the density of trap states is low, and so a TFT incorporating an a-Si : H channel layer has a very low OFF state current, and therefore charge can be stored for a long time on an electrode on one side of a liquid crystal causing the liquid crystal to twist and change its optical transmission characteristics.
