TCAD Analysis Of The Four-Terminal Poly-Si TFTs On Suppression Of The Hot-Carrier Degradation

Low temperature poly-Si thin-film transistors(TFTs)are widely used for flat panel displays,for integration of pixel matrix and driver circuits on the same panel.However,poly-Si TFTs suffer from various electric stresses in practical applications.The degradation of TFTs ultimately affects the display quality and lifetime of the display products.For poly-Si TFTs,the hot carrier(HC)is the main degradation mechanism.A novel four-terminal TFT with a counter-doped carrier injection terminal connected to the channel of the conventional TFT is obtained,thereby achieving a significant suppression on HC degradation at the level of devices.In this paper,with the Technology Computer Aided Design(TCAD)simulation tool,the suppression effect of the position and width of the body terminal on both direct current(DC)and alternating current(AC)HC degradation of TFTs as well as the underlying mechanisms are systematically investigated.Thus,the four-terminal TFTs can be further optimized towards superior reliability under DC/AC HC condition.1.Simulation under DC HC Condition:Simulation shows that the four-terminal TFTs can effectively suppress the kink effect and relevant HC degradation.And a wider body terminal or that closer to the drain can suppress kink effect more effectively.The suppression mechanism of kink effect can be given as below:a wider body terminal or that closer to the drain collects more holes generated from impact ionization in the drain depletion region and suppresses the parasitic bipolar junction transistor effect,subsequently reduces the kink current and relevant HC degradation more effectively.According to the simulation,the suppression of kink current is irrelevant to the decrease of E-field near the drain region2.Simulation under AC HC Condition:In this paper,the simulation is carried out from the injection current,the process of the junction turns on,the variation of hole concentration in the channel and E-field near the drain region.Simulation shows that the four-terminal TFTs can effectively suppress the dynamic HC degradation.And a wider body terminal or that closer to the drain can suppress the dynamic HC degradation more effectively.The suppression mechanism of dynamic HC degradation can be given as below:during the falling time of Vg pulse,a wider body terminal can inject more holes and the holes injected by that closer to the drain diffuse to drain depletion region first,partially removes the non-equilibrium state,significantly relieves the Emax in the drain depletion region,thus suppresses the dynamic HC degradation better.In addition,the suppression effect of two kinds of TFTs with different leakage currents is also studied.Simulation shows that the conclusions of this paper are applicable to both TFTs.