Mechanism Of Bias Stress Induced Current Instability Of OTFTs

Significant efforts and progress have been made towards diverse applications oforganic thin film transistors (OTFTs) in recent20years, covering the fields of flat-paneldisplays, radio-frequency identification tags, chemical and biological sensing, etc. Forthe ambipolar OFETs, they could be deployed in organic circuits, such as inverters,oscillators, and logic circuits without advanced patterning techniques are desirable.More recently, organic light-emitting field-effect transistors (OLEFETs) based onambipolar OFETs are attracting increasing attention.However, even though a significant improvement has made, the OTFTs also meetsome challenges. For all these practical applications, operation stability of OTFTs iscritically important, and thus the typically strong bias stress effect in OTFTs must beeffectively suppressed. The bias stress effect refers to the drain current (ID) instability ofOTFTs with prolonged operation time. In this thesis, bias stress induced instability ofOTFTs is studied (unipolar and ambipolar device) and the mechanism is talked. Contactresistance can be measured by determining the resistance of transistors with differentchannel lengths (transfer line method). The Probe Station and4200SCS are used tomeasure and analyse the performance. The Atomic Force microscope (AFM) and X-raydiffraction (XRD) can be utilized to study the film growth and morphology.For the unibolar OTFTs, we report a study on the contact resistance instabilityinduced by the bias stress in staggered pentacene thin film transistors. The contactresistance is increased with the stress time, and two device parameters are found tocontribute to this contact resistance instability: one is the threshold voltage increase dueto the charge trapping in the charge accumulation layer; the other is the effective contactlength increase due to the charge trapping in the pentacene bulk in the contact region.This work suggests that the time-dependent charge trapping is responsible for the biasstress effect in organic thin film transistors.For the ambipolar OTFTs, We find that regulating the thickness and deposition temperature of p and n-type semiconductor material is an effective method to obtain astandard device with balanced characteristic. The morphology of the P/N semiconductorinterface is critically important to the charge transport of the carrier type determined bythe top semiconductor material, influencing the characteristics such like the mobility,on-off radio obviously. Even the regular changes of bias stress effect depend on thethickness of semiconductor layer were observed and studied. The charge trapping andscattering influenced by the interface morphology are investigated and the model can bebuilt to explain the working mechanism.The thesis is helpful for the understanding of bias stress effect in OTFT.Additionally, it is meanful for improving the current stability and performance ofdevices.