Low Voltage Organic Field Effect Transistor Of High Transconductance Efficiency And Sensor Applications

The organic field-effect transistor?OFET?is able to be fabricated by low temperature solution/printing based processes,and owns excellent intrinsic mechanical flexibility.These attributes enable the OFET to be manufactured on low thermal budget plastic substrates to construct truly flexible electronics.Moreover,OFETs are also easier to be functionalized for sensing applications.Therefore,it would be well complementary to the silicon FET?Si-FET?to form an ideal technology platform for developing various large area/flexible/ubiquitous systems.In such a hybrid architecture,the OFET transducer part can be customized to convert different biological,chemical and physical signals to electrical outputs with amplification.The electrical outputs from the OFET transducer will then be transmitted to the subsequent high performance Si-FET chips for more complex data processing,storage and transmission.In order to realize such hybrid sensing systems,the OFET needs to be operated under the same low voltage with that of Si-FET chips and in the meantime be able to achieve high sensitivity conversion of the sensed signals.Therefore,developing low voltage OFETs with high transconductance efficiency is important.However,due to material and process limitations,there are normally large sub-gap density of states at the channel and the thickness of the gate dielectric layer is difficult to be reduced for solution processed OFETs.As a result,the OFETs normally need high operation voltage.The potential detecting sensitivity of conventional single-gate OFET based ion-sensing system is determined by the performance of sensing member membrane,which could be improved through new OFET device structure.And the current conversion sensitivity of the OFET based ion-sensing system depends on the subthreshold swing and transconductance efficiency of the corresponding OFET.Combined with device structure design and process optimization,this thesis has made the following progresses to realize low voltage OFETs of high transconductance efficiency and corresponding ion-sensing systems:1)Investigated on two approaches on lowering the subthreshold swing of OFETs,including reducing sub-gap density at the channel and using high-k/low-k bilayer gate dielectric.Through device structure design and processes optimization,both the approaches were integrated into the same device,OFETs with a subthreshold swing and transconductance efficiency close to the theoretical limits were demonstrated.The fabricated OFET with solution/printing processes could achieve a subthreshold swing of 64 mV·dec^-1(theoretical limit is 59.6 mV·dec^-1)and a transconductance efficiency of 36 V^-1(theoretical limit is 38.6 V^-1).This OFET exhibited an ON/OFF current ratio larger than 10⁵ within a 0.8 V voltage range,and also excellent operational and storage stabilities.Besides,OFETs with same device structure were also demonstrated on flexible substrates.2)Developed an analytical transconductance efficiency model for OFETs,revealing the relationship among the transconductance efficiency,subthreshold swing and the top-gate to bottom gate capacitance coupling ratio of dual-gate OFETs.Designed and implemented an asymmetric structure dual-gate OFET with a transconductance efficiency over the theoretical limit of that of single-gate OFETs.Through applying two approaches of reduced sub-gap density of states channel onto the bottom gate structure and high-k/low-k bilayer dielectric onto the top gate structure,dual-gate OFETs could achieve a large top-gate to bottom gate capacitance coupling ratio?>29?,while low voltage operation being maintained.With such an asymmetric structure,solution processed low voltage OFETs with an ultra-high transconductance efficiency about 161.5 V^-1 was obtained,which was beyond the theoretical limit of that of single-gate OFETs.3)Proposed that dual-gate OFETs with large top-gate to bottom gate capacitance coupling ratio could improve the potential detecting sensitivity of the OFET based ion-sensing system.And reducing the subthreshold swing and increasing the high transconductance efficiency of OFETs could contribute to the increase of the current conversion sensitivity of the OFET based ion-sensing system.The single-gate OFET based ion-sensing system could achieve a potential detecting sensitivity of 55 mV·pH^-1,which was close to the theoretical limit(59 mV·pH^-1)calculated by Nernstian equation at room temperature.Due to the steep subthreshold swing and high tranconductance efficiency,the single-gate OFET based ion-sensing system could achieve a current conversion sensitivity of 122%pH^-1.Furthermore,due to the large top-gate to bottom gate capacitance coupling ratio?>29?,a much improved potential detecting sensitivity over 583 mV·pH^-1 could be achieved by the dual-gate OFET based ion-sensing system,which was beyond the theoretical limit(59mV·pH^-1)of the single-gate OFET based ion-sensing system.Besides,the dual-gate OFET with low subthreshold swing and high transconductance efficiency also made the corresponding ion-sensing system achieve a higher current conversion sensitivity of 1160%pH^-1.The high current conversion sensitivity enabled the corresponding ion-sensing system respond to a weaker pH change of test solution with a high detecting sensitivity of 0.1 pH while maintaining good stability (0.02 pA·min^-1).The thesis provided the basis of device design and process for achieving low voltage OFETs of high transconductance efficiency with solution/processes towards developing low power consumption high performance silicon/organic hybrid sensing systems.