Thin film transistors (TFTs) fabricated on polycrystalline silicon (polysilicon) have gained much attention in flat panel displays such as active matrix LCDs and in static random access memory units. In the future it can be expected that the degree of circuit integration will continue to increase as device characteristics improve further, so that an entire system will be formed on a single panel. In addition, in flat panel displays polysilicon thin film transistor technology enables the integration of row and column drive circuitry, and also additional functionality such as image reversal, aspect ratio control and level shifting among others. In addition to display elements and circuitry (both analog and digital), memories, solar cells, touch sensors and other sensors may all be integrated on the panel. For example, electrically erasable and programmable read only memories (EEPROMS) have been fabricated using a polysilicon TFT process.
In comparison with thick film devices TFT devices made with a thin film have the advantages of lower grain boundary trap density, higher mobility, and higher on-state current. It is desirable to make the thin film transistor as thin as possible in order to provide a high supply current in the on-state.
However, thin film devices experience a higher lateral electric field, in particular at the junction between the channel and drain which arises from the reduced junction depth. This increase in lateral electric field is known to be a major cause of impact ionisation in the channel/drain region which results in the accumulation of holes. These holes are known to be the cause of a pronounced "kink" effect in the IV characteristics of a TFT device which in turn degrades the output characteristics of the device, and in particular reduces output resistance at high drain voltages and its gain, see for example M. Valdinoci. L. Colalongo, G. Baccarani. G. Fortunato, A. Pecora and I. Policicchio, "Investigations on the Kink Effect in Poly-TFTs", Proceediings of the ESSDERC, pp. 1055-1058, 1996 and A. G. Lewis, T. Y. Huang, R. H. Bruce, M. Koyangi, A. Chiang and I. W. Wu, "Polysilicon Thin Film Transistor for Analogue Circuit Applications", IEDM Tech. Digest, pp. 264-267, 1988. In addition the kink effect also causes avalanche induced short channel effects.
Furthermore the high lateral electric field causes anomalous leakage current in the off state that counteracts the benefit provided by a thin film in which the lower trapped charge content would otherwise tend to reduce leakage current. This anomalous leakage current is a serious problem in poly-Si TFTs.