Reliability Of Four-Terminal Polycrystalline-Silicon Thin-Film Transistors With A Carrier Injection Terminal

Hot-carrier(HC)induced degradation is a critical reliability issue of n-channel low temperature polycrystalline-silicon(LTPS)thin-film transistors(TFTs)in TFT-based circuits.In this work,a kind of four-terminal LTPS TFT,which has an additional p+-doped lateral carrier injection terminal connecting to the floating channel,is introduced.First,degradation of n-type LTPS TFTs is investigated under bipolar gate pulses with a small drain bias.It is found that such degradation behaviors are similar to those caused by the bipolar gate pulses with source and drain grounded.Dynamic HC degradation is the dominant mechanism.In addition,experiments prove the dynamic HC degradation strongly depends on drain bias both in conventional and four-terminal TFTs.More positive drain bias will significantly enhance degradation.However,four-terminal TFTs degrade much less than conventional TFTs under the same stress condition.The degradation behaviors can be well explained based on the non-equilibrium PN junction degradation model.Then,n-channel four-terminal LTPS TFT is systematically compared to conventional TFT and lightly doped drain(LDD)TFT,focusing on TFTs performance and their instabilities under both DC and AC HC conditions.Note that three kinds of n-channel LTPS TFTs are fabricated on the same glass plate using the same CMOS LTPS mass production technology.Compared to LDD TFTs,four-terminal TFTs can also reduce kink current and suppress DC HC degradation,but without any tradeoffs in TFT performance and process complexity.Moreover,four-terminal TFTs can greatly suppress the dynamic HC degradation,far more effective than LDD TFTs.Thus,four-terminal TFTs are more desirable than LDD TFTs in CMOS LTPS TFT technology for their high device performance,low manufacture cost and excellent immunity against both DC and AC HC degradation.In the end,the geometry effect of carrier injection terminal on dynamic HC degradation is analyzed from the aspects of its width and location,which is helpful to optimize device structure and reliability.The experimental results show that the wider the carrier injection terminal in the middle of channel is,the more effective the four-terminal TFT can suppress the dynamic HC degradation.In addition,it is found that the four-terminal TFT can provide the best immunity to dynamic HC degradation,when the carrier injection terminal is near the drain depletion border at falling time of gate pulse.Based on the non-equilibrium PN junction degradation model related to drain depletion boundary,the geometry effect of carrier injection terminal on the device degradation can be well understood.